GB2615309A

GB2615309A - System for detecting intermodulation distortion

Info

Publication number: GB2615309A
Application number: GB2201214.0A
Authority: GB
Inventors: Birchall James
Original assignee: Rowden Tech Ltd
Current assignee: Rowden Tech Ltd
Priority date: 2022-01-31
Filing date: 2022-01-31
Publication date: 2023-08-09
Also published as: GB202201214D0

Abstract

The present invention provides a system for detecting intermodulation distortion. The system 100 comprises a transmitter stage 102 configured to emit, via a transmitter antenna 118, an excitation signal comprising a first test signal and a second test signal, the first test signal and the second test signal being radiofrequency signals at different respective frequencies. The system further comprises a cancellation module 106 is configured to generate a cancellation signal, and a receiver stage 104. The receiver stage is configured to receive, via a receiver antenna 120, a response signal in response to emission of the excitation signal from the transmitter antenna, wherein the response signal comprises a first response component at a frequency of the first test signal and a second response component at a frequency of the second test signal. The receiver stage is further configured to combine the response signal and the cancellation signal to form an output signal, wherein the cancellation signal is arranged to at least partially cancel the first response component and the second response component. This may serve to improve a sensitivity with which intermodulation distortion components in the response signal can be detected.

Description

SYSTEM FOR DETECTING INTERMODULATION DISTORTION Field of the Invention The present invention relates to a system for detecting intermodulation distortion in a response signal received following emission of an excitation signal, which can be used for detecting the presence of a device in a target region.

Background

lntermodulation distortion (IMD) is a phenomenon that occurs when two or more radiofrequency (RF) test signals mix in a device with non-linear properties, and which results in the generation of additional signal components. The signal components resulting from IMD will depend on characteristics of the non-linear device, and will be at frequencies corresponding to sums and/or differences of multiples of the original RF test signal frequencies. For example, where IMD results from two RF test signals mixing in a non-linear device, the IMD components with have frequencies taking the form +M. F, + N. F2, where N and M are integers and F1 and F2 are the frequencies of the original RF test signals.

IMD occurs with a wide range of electronic and mechanical equipment, including both active and passive devices. In the case of an active (i.e. powered) device, IMD may result for example from non-linear active components in the device. In the case of a passive (i.e. unpowered) device, IMD may result from nonlinear properties in the device, such as at junctions between two different metals, cable connections, aged (e.g. corroded) components, or non-linear passive components. lntermodulation distortion occurring in a passive device is often referred to as passive intermodulation (PIM). IMD is generally seen as a disadvantage in RF communications, as IMD can cause interference leading to reduced sensitivity at the receiver end and in some cases prevent communication completely.

It has been shown that IMD can be used to detect the presence of a non-transmitting device, by illuminating the device with two or more of RF test signals and looking for IMD components in a reflected response signal. This technique may be used in a variety of fields, such as counter-surveillance (e.g. for detecting hidden devices) or for detecting improvised explosive devices (IEDs). Use of IMD for detecting target devices in an environment is disclosed, for example, in the article 'Compact Intermodulation Radar for Finding RF Receivers', by A. Martorell et al. (2019 16th European Radar Conference (EuRAD), 2019, pp. 109-112).

The present invention has been devised in light of the above considerations.

Summary of the Invention

The inventors have found that a difficulty with using IMD for detecting a target device is self-interference caused by the original RF test signals used to stimulate the device, which can prevent the IMD components from being detected. In particular, when a device is illuminated with two or more RF test signals via a transmitter antenna, a resulting response signal received at a receiver antenna will include self-interference components due to coupling between the transmitter and receiver antennas and reflections of the original test signals, in addition to any components resulting from IMD. As the RF test signals will have a much larger amplitude than the IMD components, the IMD components may not be discernible over the self-interference components. As a result, in practice the RF test signals tend to saturate the system, such that the much lower level IMD components cannot be detected. Moreover, the IMD components of interest for detecting presence of a device are typically odd order components (e.g. 3111 order components), meaning that the IMD components of interest will sit very near in frequency to the original RF test signals. As such, it is difficult to filter out frequencies corresponding to the RF test signals using conventional bandpass filters whilst keeping the IMD components of interest.

At its most general, the present invention provides a system for detecting IMD, where a cancellation signal is combined with the received response signal, the cancellation signal being arranged to at least partially cancel components in the response signal corresponding to the original test signals. Thus, the invention employs an active cancellation procedure to reduce effects of the self-interference caused by the RF test signals. In particular, the cancellation signal serves to reduce the amplitude of components in the response signal corresponding to the original test signals, which facilitates detection of IMD components in the response signal. Moreover, as the amplitude of components corresponding to the RF test signals is reduced by the cancellation signal, a dynamic range of a receiver stage of the system may be reduced without the receiver stage being saturated by the RF test signals. This may improve a sensitivity with which IMD components in the response signal can be detected. Accordingly, the system of the invention may increase an accuracy and reliability with which IMD components in the response signal can be detected, which in turn improves an accuracy and reliability with which presence of a device can be detected based on the response signal.

According to a first aspect of the invention, there is provided a system for detecting intermodulation distortion, the system comprising: a transmitter stage configured to emit, via a transmitter antenna, an excitation signal comprising a first test signal and a second test signal, the first test signal and the second test signal being radio frequency signals at different respective frequencies; a cancellation module configured to generate a cancellation signal; and a receiver stage configured to: receive, via a receiver antenna, a response signal in response to emission of the excitation signal from the transmitter antenna, wherein the response signal comprises a first response component at a frequency of the first test signal and a second response component at a frequency of the second test signal; and combine the response signal and the cancellation signal to form an output signal, wherein the cancellation signal is arranged to at least partially cancel the first response component and the second response component.

The transmitter stage is configured to emit (i.e. radiate) the excitation signal. The transmitter stage may include any suitable signal generation setup for generating the excitation signal and conveying the excitation signal to the transmitter antenna. For example, the transmitter stage may include a signal generator stage for generating the excitation signal, which is coupled to the transmitter antenna so that the excitation signal can be emitted from the transmitter antenna.

The excitation signal comprises a first test signal and a second test signal. In other words, the excitation signal is made up of the first test signal and the second test signal, i.e. the first and second test signals correspond to different frequency components of the excitation signal. The first test signal and the second test signal are both RF signals at different respective frequencies, i.e. the first test signal is at a first frequency and the second test signal is at a second frequency, and there is a frequency difference between the first and second frequencies. Thus, the transmitter stage may be configured to generate the first test signal and the second test signal, and to combine the first and second test signals to form the excitation signal.

Herein, a radiofrequency (RF) signal may refer to an electromagnetic signal having a frequency in the range of about 3 kHz to about 300 GHz. The frequencies of the test signals may be selected based on a type of target device to be detected, in order to improve a likelihood of detecting the target device. For example, where the target device is a radio receiver, the frequencies of the test signals may be selected such that they are within a pass band of a front end filtering stage in the radio receiver. This may typically be within a range of 300 MHz to 3 GHz. This may serve to ensure that the test signals are not suppressed by the front end filtering stage of the radio receiver, which may increase a likelihood of detection of the radio receiver. This may, for example, enable detection of a radio receiver which is used as a trigger for an IED.

Where the target device is a non-radio device (i.e. where it is not a radio receiver or transmitter), the frequencies of the test signals may be selected such that they couple to a circuit element in the device, and/or activate a resonance in an active circuit of the device. For example, the frequencies of the test signals may be selected so that the couple with a printed circuit board (PCB) trace of a particular length in the target device.

The frequencies of the first test signal and the second test signal may be selected such that IMD components of interest will lie within a detection bandwidth of spectrum analyser connected to the receiver stage, so that the IMD components can be detected. The frequency difference between the first test signal and the second test signal may similarly be selected to ensure that the IMD components of interest can be detected, e.g. taking into account a spectral resolution of the spectral analyser and linewidths of the IMD components. As noted above, the first and second test signals may be selected so as to be within the pass band of any filtering of the target device. Thus, it may be beneficial for the first and second test signals to be close in frequency. On the other hand, if the first and second test signals are too close together in frequency, then the IMD components of interest risk being masked by the self-interference components. In practice, the inventors have found that a frequency difference between the first and second test signals in the range of 0.1 MHz to 2 MHz yields good detection results, particularly where a low phase noise source is used. As an example, a frequency difference of 1 MHz between the first and second test signals may be selected.

The transmitter antenna may be any suitable RF antenna capable of emitting (i.e. radiating) the exciafion signal. In some cases, the transmitter antenna may be arranged to preferentially emit the excitation signal along a specific direction, e.g. the transmitter antenna may be a directional antenna. This may facilitate scanning a specific area for target devices, as the excitation signal can be directed at a specific area. This may also reduce signal reflections from the environment, which may reduce interference effects observed in the response signal.

The receiver stage includes a receiver antenna for receiving the response signal. The response signal is received in response to emission of the excitation signal from the transmitter antenna. The response signal comprises a first response component at the frequency of the first test signal and a second response component at the frequency of the second test signal. The first and second response components may result from reflection of the excitation signal by the environment, as well as direct transmission of the excitation signal from the transmitter antenna to the receiver antenna. The first and second response components may be referred to as self-interference components, as they relate to the original test signals and do not provide any information about the presence of a non-linear device in the environment. Where a device having non-linear properties is located in the environment, the response signal may also comprise components resulting from intermodulafion distortion (IMD) caused by the device, i.e. resulting from mixing of the first and second test signals in the device.

The cancellation module is configured to generate a cancellation signal for at least partially cancelling (i.e. suppressing) the first and second response components in the response signal. In some cases, the cancellation signal may be arranged to substantially (or completely) cancel the first and second response components in the response signal.

The cancellation module may comprise any suitable signal generating setup for generating the cancellation signal. The cancellation module is coupled to the receiver stage, so that the generated cancellation signal can be conveyed to the receiver module. The receiver stage is then configured to produce the output signal by combining the received response signal and cancellation signal. For example, the receiver stage may include a signal combiner which is configured to combine the response signal and the cancellation signal to form the output signal. Various techniques may be used for generating a suitable cancellation module. For example, the cancellation module may use the excitation signal and/or the response signal as an input for generating a cancellation module that is capable of at least partially cancelling the first and second response components.

As the cancellation signal is arranged to at least partially cancel the first and second response components, an amplitude of the first and second response components is reduced (suppressed) when the cancellation signal is combined with the response signal to form the output signal. Thus, the output signal corresponds to a version of the received response signal where the first and second response components are at least partially suppressed. As a result, the self-interference effects observed in the response signal may be reduced in the output signal, which may facilitate detecting components resulting from IMD in the output signal. In particular, reducing an amplitude of the self-interference components in the output signal may make components resulting from IMD, which are typically at a much lower power level than the self-interference components, more easily discernible. Advantageously, the cancellation signal may act to suppress the amplitudes of the self-interference components without substantially affecting the amplitudes of any IMD components in the response signal, such that relative amplitudes of any IMD components may be increased.

The output signal obtained from the receiver stage may then be analysed to determine if it contains any components resulting from IMD, e.g. by passing the output signal through a spectrum analyser. If IMD components are detected in the output signal, this may indicate that a target device (i.e. a device having non-linear properties) is located in the environment.

The cancellation signal may comprise a first cancellation component at the frequency of the first test signal and a second cancellation component at the frequency of the second cancellation signal, where the first cancellation component is arranged to at least partially cancel the first response component and the second cancellation component is arranged to at least partially cancel the second response component. Thus, the cancellation signal may comprise components at the same frequencies as the excitation signal, to ensure effective cancellation of the self-interference components when the cancellation signal is combined with the response signal. This may also avoid the cancellation signal affecting IMD components of interest in the response signal, which will be at different frequencies compared to the original test signals.

The cancellation module may be configured to generate the first and second cancellation components, and to combine the first and second cancellation components to form the cancellation signal. Various techniques may be used for generating the first and second cancellation components. For instance, the cancellation module may generate the first cancellation component using the first test signal and/or the first response component as an input, and generate the second cancellation component using the second test signal and/or the second response component as an input.

The first cancellation component may comprise a phase shifted copy of the first test signal and the second cancellation component may comprise a phase shifted copy of the second test signal. The inventors have found that using phase shifted copies of the original test signals provides an effective technique for reducing the amplitudes of the self-interference components in the output signal. In particular, the phase shifted copies of the first and second test signals may be out of phase with the first and second response components, such that they at least partially cancel the first and second response components. The phase shifted copy of the first test signal may be phase shifted relative to the first test signal, and the phase shifted copy of the second test signal may be phase shifted relative to the second test signal. The phase shifts applied to the phase shifted copies of the first and second test signals may be adjusted, in order to maximise cancellation of first and second response components.

The cancellation module may comprise a first phase shifter for controlling a phase shift of the phase shifted copy of the first test signal and a second phase shifter for controlling a phase shift of the phase shifted copy of the second test signal. In this manner, the cancellation may produce the phase shifted copies of the first and second test signals, and enable control of their phase shifts. The cancellation module may then be configured to combine the phase shifted copies of the first and second test signals to form the cancellation signal.

The phase shifted copy of the first test signal may be in antiphase with the first response component and the phase shifted copy of the second test signal may in antiphase with the second response component. In other words, the phase shifted copy of the first test signal may be 180° out of phase with the first response component, and the phase shifted copy of the second test signal may be 1800 out of phase with the second response component. This may serve to maximise cancellation of the first and second response components by the phase shifted copies of the first and second test signals. This may be achieved, for example, by adjusting the phase shifts applied to the phase shifted copies of the first and second test signals, until an antiphase relationship with the first and second response components is reached.

An amplitude of the first cancellation component may substantially match an amplitude of the first response component, and an amplitude of the second cancellation component may substantially match an amplitude of the second response component. This may enable effective and accurate cancellation of the first and second response components, by ensuring that the amplitudes of the cancellation components are adapted to the magnitudes of the self-interference components in the response signal. This may be achieved, for example, by adjusting the amplitude of the first and second cancellation components based on a detected amplitude of the first and second response components.

The cancellation module may include a first amplitude adjuster (e.g. a first attenuator) for controlling an amplitude of the first cancellation component, and a second amplitude adjuster (e.g. a second attenuator) for controlling an amplitude of the second cancellation component. In this manner, the amplitude of each of the first and second cancellation components can be controlled, e.g. to match the amplitudes of the first and second response components as mentioned above.

Where the cancellation signal includes phase shifted copies of the test signals, the cancellation module may include a first channel including the first phase shifter and first attenuator connected in series for generating the phase shifted copy of the first test signal, and a second channel including the second phase shifter and second attenuator connected in series for generating the phase shifted copy of the second test signal.

The transmitter stage may comprise a signal generation module configured to generate the first test signal and the second test signal. For example, the transmitter stage may comprise a first signal generator (e.g. a first RF source) configured to generate the first test signal, and a second signal generator (e.g. a second RF source) configured to generate the second test signal. In other words, the first and second test signals may be generated using separate signal generators. The first and second signal generators may be any suitable RF signal generators. However, the first and second test signals need not necessarily be obtained from separate signal generators. In some cases, the first and second test signals may be derived from common RF signal obtained from a single RF source, e.g. by generating two copies of the RF signal and frequency-shifting one of the copies.

The transmitter stage may further comprise a first signal splitter configured to transmit a first portion of the first test signal to a first signal combiner and a second portion of the first test signal to the cancellation module; and a second signal splitter configured to transmit a first portion of the second test signal to the first signal combiner and a second portion of the second test signal to the cancellation module; wherein the first signal combiner is coupled to the transmitter antenna and configured to combine the first portion of the first test signal and the first portion of the second test signal to form the excitation signal. Thus, first portions of the first and second test signals may be used to form the excitation signal, whilst second portions of the first and second test signals may be used by the cancellation module to generate the cancellation signal.

The cancellation module may receive the second portion of the first test signal and the second portion of the second test signal, which the cancellation module can then use to generate the cancellation signal.

For instance, the cancellation module may apply phase shifts to the received portions of the first and second test signals to generate the phase shifted copies of the first and second test signals mentioned above. For example, the first signal splitter may be coupled to the first channel of the cancellation module mentioned above, so that the portion of the first test signal is passed through the first phase shifter and first attenuator, and the second signal splitter may be coupled to the second channel of the cancellation module mentioned above, so that the portion of the second test signal is passed through the second phase shifter and the second attenuator.

The cancellation module may comprise a second signal combiner configured to combine the first cancellation component and the second cancellation component to form the cancellation signal.

In some embodiments, the cancellation signal may comprise: two or more first cancellation components at the frequency of the first test signal and with different respective phases; and two or more second cancellation components at the frequency of the second test signal and with different respective phases. Providing multiple cancellation components at the frequency of the first test signal and with different phases may improve a cancellation of self-interference components at the frequency of the first test signal. Likewise, providing multiple cancellation components at the frequency of the second test signal and with different phases may improve a cancellation of self-interference components at the frequency of the second test signal. This is because self-interference components in the response signal may result from direct transmission of the test signals from the transmitter antenna to the receiver antenna, as well as from reflections of the test signals by the environment which are received at the receiver antenna.

Thus, the first response component may in fact comprise two or more components at the frequency of the first test signal but with different phases and amplitudes, arising from the different self-interference mechanisms (i.e. direct coupling, signal reflections) and reflection paths. Similarly, the second response component may actually comprise two or more components at the frequency of the first test signal but with different phases and amplitudes. Accordingly, each of the two or more first cancellation components may be arranged to at least partially cancel a respective first response component in the response signal which is at the frequency of the first test signal, and each of the two or more second cancellation components may be arranged to at least partially cancel a respective second component in the response signal which is at the frequency of the second test signal. In other words, using multiple cancellation components at each frequency enables self-interference components arising at each frequency from different self-interference mechanisms to be suppressed, thus improving a sensitivity with which IMD components can be detected. The phase and amplitude of each first cancellation component and each second cancellation component may be adjusted, to provide cancellation of the various self-interference components.

The two or more first cancellation components and the two or more second cancellation components may be generated using analogous techniques to those discussed above for the first cancellation component and the second cancellation component. For example, each of the two or more first cancellation components may comprise a respective phase shifted copy of the first test signal, whilst each of the two or more second cancellation components may comprise a respective phase shifted copy of the second test signal. Each phase shifted copy of the first test signal may be in antiphase with a respective first response component, and each phase shifted copy of the second test signal may be in antiphase with a respective second response component.

The cancellation module may comprise a respective first phase shifter for controlling a phase shift of each phase shifted copy of the first test signal, and a respective second phase shifter for controlling a phase shift of each phase shifted copy of the second test signal.

An amplitude of each of the two or more first cancellation components may substantially match an amplitude of a respective first response component, and an amplitude of each of the two or more second cancellation components may substantially math an amplitude of a respective second response component. The cancellation module may comprise a respective first attenuator for controlling an amplitude of each of the two or more first cancellation components, and a respective second attenuator for controlling an amplitude of each of the two or more second cancellation components.

The system may further comprise a controller configured to control generation of the cancellation signal by the cancellation module. This may enable automatic generation and control of the cancellation signal, which may facilitate detection of IMD components in the response signal. For example, the controller may be configured to control the cancellation module to set or adjust one or more properties of the cancellation signal (e.g. phases and amplitudes of the cancellation components), to maximise cancellation of the self-interference components in the response signal. Where the cancellation module includes first and second phase shifters as mentioned above, the controller may be configured to control a phase shift applied by each of the first and second phase shifters (e.g. by setting control voltages supplied to the phase shifters). Similarly, where the cancellation module includes first and second attenuators, the controller may be configured to control an attenuation factor applied by each of the first and second attenuators (e.g. by setting control voltages supplied to the attenuators).

The controller may be communicatively coupled to the cancellation module, e.g. so that the controller can transmit control signals to the cancellation module for controlling generation of the cancellation signal.

The controller may be implemented using any suitable processor or computer device having software for performing any of the control steps discussed herein. In some cases, the controller may be part of the cancellation module.

The controller may be configured to control generation of the cancellation signal by the cancellation module based on the response signal and/or the output signal. In other words, the controller may use the response signal and/or the output as an input for controlling generation of the cancellation signal. This may enable the controller to tailor the cancellation signal to the response signal received at the receiver stage, to ensure effective cancellation of the self-interference components. Where the output signal is used as an input, this may enable the controller to determine an effectiveness of the cancellation signal, so that the cancellation can be optimised to maximise cancellation of the self-interference components. This may enable the cancellation signal to be automatically adjusted in real time, e.g. to account for changing conditions in the surrounding environment.

As an example, the controller may be configured apply an optimisation algorithm which determines a phase and amplitude for each of the first cancellation component and the second cancellation component, in order to minimise an amplitude of the first and second response signals in the output signal. To achieve this, the controller may receive an amplitude of each of the first response component and the second component as an input, and in response determine a phase and amplitude of the first cancellation component and the second cancellation component, e.g. using a pre-determined look-up table or algorithm that outputs a phase and amplitude for the first cancellation component and the second cancellation component based on input amplitudes of the first and second response components. The controller may then apply an optimisation algorithm to optimise the phase and amplitude for each of first cancellation component and the second cancellation component so that a minimum amplitude of the first and second response components in the output signal is observed. Various optimisations algorithms may be used, a gradient descent algorithm being an example of a suitable algorithm. Where the cancellation signal comprises two or more first cancellation components and two or more second cancellation components, a similar process to that discussed above may be performed for each of the two or more first and second cancellation components. In particular, the controller may be configured to sequentially apply an optimisation algorithm to sequentially determine a phase and amplitude for each of the two or more first cancellation components, and to sequentially apply the optimisation algorithm to sequentially determine a phase and amplitude for each of the two or more second cancellation components. This may enable optimal phase and amplitude setting for each of the cancellation components to be determined, so as to maximise cancellation of various self-interference components.

As another example, the controller may be configured to determine a phase and amplitude of each of the first response component and the second response component, and to adjust the first cancellation component and the second cancellation component based on the determined phases and amplitudes. For instance, the controller may adjust the phases of the first and second cancellation components such that they are in anfiphase with the first and second response components, respectively, and the controller may adjust the amplitudes of the first and second cancellation components such that they substantially match the amplitudes of the first and second response components, respectively.

Additionally or alternatively to using a controller as discussed above, generation of the cancellation signal may be controlled manually by a user. For example, a user may manually adjust properties of the cancellation signal (e.g. phases and amplitudes of the cancellation components) to obtain a suitable suppression of the self-interference components.

The system may further comprise an analyser stage configured to determine a spectrum of the output signal. In this manner, the spectrum of the output signal can be analysed, to determine if there are any components resulting from IMD in the signal, and which may be indicative of the presence of a device in the environment around the system. In particular, the spectrum of the output signal provides information of the frequency components of the output signal, i.e. after the self-interference components have been at least partially cancelled by the cancellation signal. Thus, components resulting from IMD may be more easily detectable in the spectrum of the output signal than in a spectrum of the raw response signal. The analyser stage may comprise a spectrum analyser that is coupled to the receiver stage in order to receive the output signal. Any suitable spectrum analyser capable of determining a spectrum of an RF signal may be used. The obtained spectrum of the output signal may be indicative of signal amplitude as a function of signal frequency.

The system may be configured to determine, based on the spectrum of the output signal, if a target device is located in an environment of the system. In this manner, the system may automatically determine if a target device is located in the environment. For example, the system may be configured to analyse the spectrum of the output signal to determine if the output signal includes any components at frequencies corresponding to IMD of the original test signals and, if so, that there is a target device in the environment. The system may comprise any suitable computing device for analysing the spectrum of the output signal.

The system may be configured to look for specific IMD components in the output signal. For example, the system may be configured to look for third order IMD components at frequencies 2F1 -F2 and 2F2 -F1, where F1 and F2 are the frequencies of the first test signal and the second test signal, respectively. Then, if these third order IMD components are detected in the spectrum of the output signal, the system may determine that there is a target device in the environment.

Upon detecting that a target device is in the environment, the system may be configured to generate an alert (or notification), e.g. to alert a user to the presence of the target device in the environment. Such an alert may be a visual alert (e.g. displayed on a screen) and/or an audio alert (e.g. played on a speaker).

Herein, a target device may refer to any device that can cause IMD, i.e. any device having a non-linear property. A target device may be an electrical device, an electronic device and/or a mechanical device. A target device may be an active (i.e. powered) device or a passive (i.e. unpowered) device.

It should be noted that the invention is not limited to using two test signals, and that in some cases more than two test signals may be used. In other words, the excitation signal may comprise two or more test signals, each test signal being at a respective radiofrequency. Then, the principles discussed above in relation to the first test signal and the second test signal may be applied in a context where more than two test signals are used. In particular, the response signal may comprise response components (i.e. self-interference components) at the frequencies of each of the two or more test signals, and the cancellation signal may be arranged to at least partially cancel each of the response components. For example, the cancellation signal may comprise a respective cancellation component adapted to at least partially cancel each response component in the response signal. Each cancellation component may correspond to a phase shifted copy a respective one of the two or more test signals, in line with the discussion above.

According to a second aspect of the invention, there is provided a method of detecting intermodulation distortion, the method comprising: transmitting, via a transmitter antenna, an excitation signal comprising a first test signal and a second test signal, the first test signal and the second test signal being radiofrequency signals at different respective frequencies; receiving, via a receiver antenna, a response signal in response to emission of the excitation signal from the transmitter antenna, wherein the response signal comprises a first response component at a frequency corresponding to the first test signal and a second response component at a frequency corresponding to the second test signal; generating, with a cancellation module, a cancellation signal arranged to at least partially cancel the first response component and the second response component; and combining the response signal and the cancellation signal to form an output signal.

The method of the second aspect of the invention may be used with the system of the first aspect of the invention. Thus, any of the features discussed above in relation to the first aspect of the invention may be shared with the second aspect of the invention.

The cancellation signal may comprise a first cancellation component at the frequency of the first test signal and a second cancellation component at the frequency of the second cancellation signal, where the first cancellation component is arranged to at least partially cancel the first response component and the second cancellation component is arranged to at least partially cancel the second response component.

The first cancellation component may comprise a phase shifted copy of the first test signal and the second cancellation component may comprise a phase shifted copy of the second test signal.

The method may comprise controlling a first phase shifter for controlling a phase shift of the phase shifted copy of the first test signal, and controlling a second phase shifter for controlling a phase shift of the phase shifted copy of the second test signal.

The phase shifted copy of the first test signal may be in antiphase with the first response component and the phase shifted copy of the second test signal is in antiphase with the second response component.

An amplitude of the first cancellation component may substantially match an amplitude of the first response component, and an amplitude of the second cancellation component may substantially match an amplitude of the second response component.

The method may comprise controlling a first attenuator for controlling an amplitude of the first cancellation component, and controlling a second attenuator for controlling an amplitude of the second cancellation component.

The cancellation signal may comprise: two or more first cancellation components at the frequency of the first test signal and with different respective phases; and two or more second cancellation components at the frequency of the second test signal and with different respective phases.

The method may comprise generating the cancellation signal based on the response signal. In other words, the response signal may be used as an input for generating the cancellation signal, as discussed above in relation to the first aspect of the invention.

The method may further comprise determining a spectrum of the output signal.

The method may further comprise determining, based on the spectrum of the output signal, if a target device is located in an environment of the receiver stage.

Summary of the Figures

Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: Fig. 1 is a schematic diagram of a system according to an embodiment of the invention; Fig. 2a is a schematic representation of a spectrum of a response signal detected by a receiver antenna of the system of Fig. 1; Fig. 2b is a schematic representation of a spectrum of an output signal produced by a receiver stage of the system of Fig. 1; Fig. 3 is a schematic representation of a cancellation module that may form part of a system according to an embodiment of the invention; Figs. 4 and 5 show experimental data obtained using a variation of the system of Fig. 1, where no cancellation signal was used; Figs. 6 and 7 show experimental data obtained using the system of Fig. 1; and Fig. 8 is a schematic diagram of a system according to an embodiment of the invention.

Detailed Description; Further Optional Features

Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art.

Fig. 1 shows a schematic diagram of a system 100 according to an embodiment of the invention. The system 100 includes a transmitter stage 102, a receiver stage 104, and a cancellation module 106. The transmitter stage 102 is configured to emit an RF excitation signal comprising a first test signal and a second test signal. In more detail, the transmitter stage 102 includes a first signal generator 108 configured to generate the first test signal, and a second signal generator 110 configured to generate the second test signal. The first signal generator 108 and the second signal generator 110 are RF sources which generate RF signals at different respective frequencies, such that the first and second test signals are different frequency RF signals. Herein, the frequency of the first test signal may be referred to as F, and the frequency of the second test signal may be referred to as F2.

The first signal generator 108 is coupled to an input port of a first signal splitter 112, whilst the second signal generator 110 is coupled to an input port of a second signal splitter 114. A first output port of the first signal splitter 112 is coupled to a first signal combiner 116, whilst a second output port of the first signal splitter 112 is coupled to the cancellation module 106. Thus, the first signal splitter 112 is arranged to receive the first test signal from the first signal generator 108, and transmit a first portion of the first test signal to the first signal combiner 116 and a second portion of the first test signal to the cancellation module 106. In a similar manner, a first output port of the second signal splitter 114 is coupled to the first signal combiner 116, whilst a second output port of the second signal splitter 114 is coupled to the cancellation module 106. Thus, the second signal splitter 114 is arranged to receive the second test signal from the second signal generator 110, and transmit a first portion of the second test signal to the first signal combiner 116 and a second portion of the second test signal to the cancellation module 106. The first and second signal splitters 112 and 114 may each be implemented by a directional coupler or other suitable power splitter. As an example, each of the first and second signal splitters 112 and 114 may be implemented using a Minicircuits ZGDC 10-372HP+.

The first signal combiner 116 is configured to combine the received portions of the first test signal and the second signal to form an excitation signal, which is conveyed to a transmitter antenna 118 that emits (i.e. radiates) the excitation signal into an environment of the system 100. The first signal combiner 116 may be implemented, for example, by a low passive intermodulation (PIM) hybrid combiner. Typically a hybrid combiner may have two output ports. The unused output port of the hybrid combiner (i.e. the one which is not connected to the transmitter antenna 118) may be terminated with a low PIM load (e.g. a 50 Ohm load). The transmitter antenna 118 may be any suitable RF antenna for radiating the excitation signal into the surrounding environment. In some cases, the transmitter antenna 118 may be implemented using a directional antenna, so that the excitation signal can be preferentially emitted in a specific direction. This may enable a particular area in the environment to be probed, which may facilitate locating target devices in the environment.

The first signal splitter 112 may be arranged to transmit only a small portion of the first test signal to the cancellation module 106, with most of the first test signal being conveyed to the first signal combiner 116. Likewise, the second signal splitter 114 may be arranged to transmit only a small portion of the second test signal to the cancellation module 106, with most of the second test signal being conveyed to the first signal combiner 116. In other words, most of the power of the first and second test signals may be conveyed to the first signal combiner 116 to form the excitation signal emitted by the transmitter antenna 118, with only a small amount of the power of the first and second test signals being conveyed to the cancellation module 106. This may ensure that the excitation has a sufficient power level for stimulating any devices in the environment. For example, the first signal splitter 112 and the second signal splitter 114 may be configured to transmit 90% of the first and second test signals (respectively) to the first signal combiner 116, and 10% of the first and second test signals (respectively) to the cancellation module 106.

The receiver stage 104 includes a receiver antenna 120, which receives a response signal in response to emission of the excitation signal by the transmitter antenna 118. The response signal may result from direct transmission of the excitation signal from the transmitter antenna 118 to the receiver antenna 120, as well as reflections of the excitation signal from the environment. Thus, the response signal will include frequency components which are at the frequencies of the first test signal and the second test signal. The components of the response signal at the frequencies of the first and second test signals are referred to herein as a first response component and a second response component, respectively, and they may also be referred to as self-interference components. Where a device 122 having non-linear properties is located in the environment, the response signal may also include components resulting from mixing of the test signal frequencies in the device 122. This corresponds to intermodulation distortion (IMD), and results in the response signal having components with frequencies taking the form +M. F, + N. F2, where N and M are integers.

The cancellation stage 106 is configured to generate a cancellation signal which is arranged to suppress the first and second response components (i.e. the self-interference components) in the response signal, to facilitate detection of components resulting from IMD. The cancellation stage 106 generates the cancellation signal based on the received portions of the first and second test signals. In particular, the cancellation module 106 includes a first signal modulator 124 configured to generate a phase shifted copy of the first test signal, and a second signal modulator 126 configured to generate a phase shifted copy of the second test signal. The first signal modulator 124 receives the second portion of the first test signal from the first signal splitter 112, and applies a phase shift 0, to the second portion of the first test signal to generate the phase shifted copy of the first test signal. The first signal modulator 124 can also set an amplitude A1 of the phase shifted copy of the first test signal. Similarly, the second signal modulator 126 receives the second portion of the second test signal from the second signal splitter 114, and applies a phase shift 02 to the second portion of the second test signal to generate the phase shifted copy of the second test signal. The second signal modulator 126 can also set an amplitude A2 of the phase shifted copy of the second test signal. The cancellation module 106 further includes a second signal combiner 128, which is arranged to combine the phase shifted copies of the first and second test signals to form the cancellation signal. Thus, the phase shifted copy of the first test signal forms a first component of the cancellation signal, which is at frequency F1, and the phase shifted copy of the second test signal forms a second component of the cancellation signal, which is at frequency F2. Similarly to the first signal combiner 116, the second signal combiner may be implemented by a low PIM hybrid combiner whose unused output port is terminated by a low PIM 50 Ohm load. A more detailed implementation of the cancellation module 106 is discussed below in relation to Fig. 3.

The receiver antenna 120 and an output of the second signal combiner 128 are connected to respective input ports of a third signal combiner 130, which is configured to combine the response signal from the receiver antenna 120 and the cancellation signal from the cancellation module 106 to form an output signal. As noted above, the cancellation signal is arranged to suppress the self-interference components in the response signal. Thus, following combination of the response signal and the cancellation signal by the third signal combiner 130, the self-interference components are suppressed in the resulting output signal compared with in the raw response signal. In the present embodiment, this is achieved by setting the phase shifts Awl and Ar,02 applied by the cancellation module 106 such that the phase shifted copy of the first test signal is in anfiphase (i.e. 180° out of phase) with the first response component, and such that the phase shifted copy of the second test signal is in antiphase with the second response component. Additionally, the amplitude A1 is set so that it substantially matches an amplitude of the first response component, and the amplitude A2 is set so that it substantially matches an amplitude of the second response component. The inventors have found that setting Awl, 4(1)2, A1 and A2 in this manner may maximise cancellation of the first and second response components in the response signal. The values LIT", Acp2, A, and A2 may be determined by a user, e.g. by adjusting them until maximal cancellation of the self-interference components is observed. Alternatively, the values of AT,, 4(1)2, A1 and A2 may be controlled automatically, e.g. using a suitably programmed controller. For example, the controller may be configured to determine a phase an amplitude of each of the first response component and second response component, and set the values of LIcp,, *92, A, and A2 accordingly. For example, where the first response component has a phase 01, then the phase shift Aco, may be set such that the phase shifted copy of the first test signal has a phase equal to 01 + 1800. Likewise, where the second response component has a phase 02, then the phase shift 11T2 may be set such that the phase shifted copy of the second test signal has a phase equal to 02 + 1800.

The principle of self-interference cancellation used in the invention is illustrated in Figs. 2a-2b. Fig. 2a shows an example spectrum of a response signal received by the receiver antenna 120. As can be seen, the response signal includes components at the frequencies F1 and F2 of the first and second test signals, corresponding to self-interference. Additionally, the response signal includes components resulting from IMD caused by the device 122 at frequencies 2F1 -F2 and 2F2 -F1, corresponding to third order IMD components. In practice, the response signal may include further IMD components, however these are not shown for illustration purposes. As shown in Fig. 2a, the first response component with frequency F, has an amplitude A, and a phase 01, whilst the second response component with frequency F2 has an amplitude A2 and a phase 02. The cancellation module 106 may then be controlled to set the amplitude and phase shift of the phase shifted copy of the first test signal to Al and 01 + 180°, respectively, and to set the amplitude and phase shift of the phase shifted copy of the second test signal to A2 and 82 + 180°, respectively. Fig. 2b shows an example spectrum of an output signal obtained by combining the response signal of Fig. 2a with the cancellation signal. Fig. 2b illustrates an ideal scenario, where the first and second response components have been completely cancelled by the cancellation signal, leaving only the IMD components. Thus, detection of the IMD components is facilitated, and a sensitivity with which the IMD components can be detected is increased. In practice, the self-interference components may not be completely cancelled by the cancellation signal, such that their amplitudes may only be partially suppressed.

Returning to Fig. 1, the system 100 further includes a spectrum analyser 132 which connected to an output port of the third signal combiner 130, to receive the output signal. The spectrum analyser 132 is configured to determine a spectrum of the output signal, e.g. as shown in Fig. 2b. The spectrum analyser 132 may be implemented using any suitable hardware and/or software components. The spectrum of the output signal may be used to determine if there is a target device (i.e. a device having non-linear properties) in the environment. In particular, the presence of IMD components in the spectrum of the output signal may be indicative of the presence of a device 122 in the environment. Accordingly, the spectrum of the output signal can be analysed to determine if it contains any IMD components to determine if there is a target device in the environment. The spectrum analyser 132 may be configured to display the spectrum of the output signal, to enable a user of the system 100 to look for IMD components in the spectrum, in order to determine if a target device is located in the environment. Additionally or alternatively, the system 100 may be configured to automatically analyse the spectrum of the output signal (e.g. using spectrum analysis software), to determine if there are any components at frequencies corresponding to IMD. If the system determines that IMD components are present in the output signal, then the system may determine that a target device is located in the environment.

In practice, odd order IMD components may be of particular interest as they will tend to sit near to the original test signal frequencies, and may be less likely to be suppressed by an internal front end architecture of the device 122. Thus, odd order IMD components may provide a higher probability for detecting the device 122. Specifically, the third order IMD components at frequencies 2F1 -F2 and 2F2 -F1 are of interest, as these will lie close in frequency to the original test signal frequencies and so yield a higher probability of detection. The other third order IMD components are at frequencies 3F1, 3F2, 2F1 + F2, 2E72 Fj., however these are further in frequency from the original test signal frequencies and so may be less of interest. A detection bandwidth of the spectrum analyser 132 may be set to ensure that IMD components of interest can be detected. Thus, the detection bandwidth of the spectrum analyser 132 may be set to ensure that components at frequencies 2F1 -F2 and 2F2 -F, can be detected.

Fig. 3 shows a schematic diagram of a cancellation module 300 that may form part of a system according to an embodiment of the invention. For example, the cancellation module 300 may correspond to the cancellation module 106 of system 100 discussed above. The cancellation module 300 includes a first channel 302 for generating the first cancellation component of the cancellation signal, and a second channel 304 for generating the second cancellation component of the cancellation signal. The first channel 302 includes an input port 306 connected in series with an attenuator 308 and a pair of phase shifters 310a, 310b. The input port 306 is arranged to receive a portion of the first test signal. For example, the input port 306 may be connected to the first signal splitter 112 of the system 100. The received portion of the first test signal is passed through the attenuator 308 which controls an amplitude of the signal. The amplitude of the signal may be controlled by setting a control voltage V1 supplied to the attenuator 308. As an example, a Minicircuits EVA-3000+ voltage variable attenuator may be used, which provides a variable attenuation between 40 dB and 3 dB with a 0-7 V control signal. The signal is then sequentially passed through phase shifter 310a and phase shifter 310b, such that the phase shifts applied to the signal by each phase shifter are added together. The phase shifters 310a and 310b are controlled with a common control voltage V2, such that the phase shifters 310a and 310b both phase shift the signal by the same amount. The first channel 302 thus produces an amplitude-adjusted phase shifted copy of the first test signal. The reason for using two phase shifters is that in some cases a large phase shift may need to be applied to the phase shifted copy of the test signal, e.g. up to 3590 in some cases, which may not be achievable using a single phase shifter. For example, each of the phase shifters 310a, 310b may be implemented by a Minicircuits JSPHS-661+ phase shifter which provides 0-180° phase shift with a 0-9 V control voltage. In other cases however, a single phase shifter may be used, e.g. where the single phase shifter is capable of applying sufficiently large phase shifts. The second channel 304 of the cancellation module 300 is configured in an analogous manner to the first channel 302. In particular, the second channel 304 includes an input port 312 arranged to receive a portion of the second test signal (e.g. from the second signal splitter 114), and an attenuator 312 and pair of phase shifters 314a, 314b connected in series. The attenuator 312 is controlled with a control voltage 173, whilst the phase shifters 314a, 314b are controlled with a control voltage V,. Thus, the second channel 304 is configured to produce an amplitude-adjusted phase shifted copy of the second test signal.

The control voltages VI, V2, V3 and V4 may be set in order to adjust the phase and amplitude of the phase shifted copies of the first and second test signals, in line with the discussion above. In particular, the control voltages V3 and V4 may be set such that the phase shifted copies of the first and second test signals are in antiphase with the first and second response signals, whilst the voltages V, and V2 may be set such that the amplitudes of the phase shifted copies of the first and second test signals match those of the first and second response signals. The voltages V1, V,, V, and V4 may be set manually by a user, or automatically via a controller.

The cancellation module 300 further includes a signal combiner 316, having a first input port which is connected to an output of the first channel 302, and a second input port which is connected to an output of the second channel 304. The signal combiner 316 is configured to combine the phase shifted copies of the first and second test signals received from the first and second channels 302, 304 to form a cancellation signal. The cancellation signal is output via an output port 318 of the signal combiner 316, so that it can be conveyed to the receiver stage of the system. The signal combiner 316 may, for example, correspond to the second signal combiner 128 of system 100 discussed above. The signal combiner 316 is implemented by a low PIM hybrid combiner (such as a Microlab CA-14N hybrid combiner), whose unused port is terminated by a low PIM load 320 (e.g. a 50 Ohm load such as Microlab TK-210MN 50 c2).

For completeness, it is noted that connections between the various components shown in Figs. 1 and 3 by full lines may be provided by coaxial cables or other suitable connectors.

Figs. 4-7 show experimental data demonstrating an effect of the invention. Figs. 4 and 5 correspond to tests performed using the system 100 of Fig. 1, but where no cancellation signal is combined with the response signal. Figs. 6 and 7 correspond to tests performed using the system 100 of Fig. 1, including the cancellation module 300 of Fig. 3. Each of Figs. 4-7 shows a measured spectrum of the output signal received from the third signal combiner 130, showing amplitude of the signal as a function of frequency of the signal. The table below the spectrum in each of Figs. 4-7 shows the frequency and amplitude of various points indicated in the spectrum. The tests for Figs. 4-7 were performed in a shielded environment, which was a shielded 3mx3mx5m chamber with radiation absorbent material (RAM) to provide multipath damping (the chamber was not anechoic or semi-anechoic). For each of the tests, an excitation signal including a first test signal at F, = 445.5 MHz and a second test signal at F2 = 446.5 MHz was used.

Fig. 4 shows the spectrum of the output signal for a test where no target device was located in the environment, and where no cancellation signal was combined with the response signal. Thus, in the test of Fig. 4, the output signal is the same as the raw response signal received at the receiver antenna. As can be seen, the output signal includes two frequency components, at frequencies F1 and F2. This results from direct transmission of the excitation signal from the transmitter antenna to the receiver antenna, as well as reflections of the excitation signal by the environment. These components are referred to herein as the first and second response components, or as the self-interference components. The self-interference components have an average signal power of -28.82 dBm.

Fig. 5 shows the spectrum of the output signal for a test where a target device is located in the environment, and where no cancellation signal was combined with the response signal. As can be seen, introducing the target device into the environment results in additional frequency components appearing in the spectrum, resulting from IMD. The average power of the third order IMD components (peaks indicated by numerals 3 and 5 in Fig. 5) is -81.8 dBm. The target device used was a Private Mobile Radio (PMR), in particular a Kenwood TK-3501.

Fig. 6 shows the spectrum of the output signal for a test where no target device was located in the environment, and where a cancellation signal was combined with the response signal. The cancellation signal included phase shifted copies of the first and second test signals in antiphase with the self-interference components, and having amplitudes matching those of the self-interference components, as discussed above. As can be seen, there is a significant reduction in amplitude of the first and second response components, of approximately 36.5 dBm compared with the test in Fig. 4 where no cancellation signal was applied. Additionally, the signal has been reduced in power such that phase noise artefacts are now below a receiver noise floor, which my increase frequency resolution of the measurement and enable detection of more closely spaced excitation frequencies.

Fig. 7 shows the spectrum of the output signal for a test where the target device (the same as that used for Fig. 5) was located in the environment, and where the cancellation signal (the same as for the test of Fig. 6) was combined with the response signal. As can be seen, application of the cancellation signal suppresses the amplitude of the first and second response components by about 14.7 dBm compared to the test of Fig. 5. This is slightly less than the reduction in amplitude observed for the test where there was no target device in the environment (Fig. 6), however it is still a significant reduction. Importantly, the average power of the third order IMD components (peaks indicated by numerals 3 and Sin Fig. 7) is - 82.28 dBm, which is about the same as when no cancellation was applied (Fig. 5). This means that the cancellation signal acts to suppress the self-interference components without significantly affecting the IMD components of interest, thus facilitating the detection of the IMD components.

As can be seen in Fig. 7, the first and second response components are not completely suppressed by application of the cancellation signal. This is likely due to the presence of self-interference components in the response signal which result from reflections of the test signals by the environment, and which are not completely cancelled by the cancellation signal. As noted above, self-interference components in the response signal may result from direct coupling between the transmitter and receiver antennas, as well as reflections of the test signals by the environment. Self-interference components resulting from different self-interference paths will typically have different phases and amplitudes. In other words, the response signal may comprise multiple self-interference components at F1 having different phases and amplitudes, and multiple self-interference components at F2 having different phases and amplitudes.

Fig. 8 shows a schematic diagram of a system 800 according to an embodiment of the invention, where multiple cancellation components with different phases are generated at each of frequencies F1 and F2, in order to increase cancellation of the self-interference components. The system 800 is based on a similar architecture to that of system 100 discussed above, and functions based on similar principles. For convenience, components of the system 800 corresponding to components of the system 100 described above are indicated in Fig. 8 using the same reference numerals as in Fig. 1, and are not described again.

The cancellation module 806 of the system 800 is modified compared with that of the system 100. In particular, whereas the cancellation module 106 of the system 100 only had a first signal modulator 124 and a second signal modulator 126, the cancellation module 806 has three pairs of signal modulators: a first pair of signal modulators 124a, 126a, a second pair of signal modulators 124b, 126b, and a third pair of signal modulators 124c, 126c. The first signal splitter 112 of the system 800 includes four outputs, with one output connected to the first signal combiner 116 and a respective output connected to each of the signal modulators 124a, 124b, 124c (i.e. to a first signal modulator in each pair). Thus, a first portion of the first test signal is transmitted to the first signal combiner 116, and a respective portion of the first test signal is transmitted to each of the signal modulators 124a, 124b, 124c. Similarly, the second signal splitter 114 of the system 800 includes four outputs, with one output connected to the first signal combiner 116 and a respective output connected to each of the signal modulators 126a, 126b, 126c (i.e. to a second signal modulator in each pair). Thus, a first portion of the second test signal is transmitted to the first signal combiner 116, and a respective portion of the first second signal is transmitted to each of the signal modulators 126a, 126b, 126c. Each of the signal modulators 124a, 124b, 124c functions in a similar manner to the first signal modulator 124 discussed above, and is configured to generate a respective phase shifted copy of the first test signal. In particular, each of the signal modulators 124a, 124b, 124c can respectively set phase shifts Awl, ,Z1(p3 and Acps and amplitudes A,, A3 and A5 of the phase shifted copies of the first test signal. Likewise, each of the signal modulators 126a, 126b, 126c functions in a similar manner to the second signal modulator 126 discussed above, and is configured to generate a respective phase shifted copy of the second test signal, with each of the signal modulators 126a, 126b, 126c respectively setting phase shifts Ac02, d(p4 and.th46 and amplitudes A2, A4 and A6 of the phase shifted copies of the second test signal. In practice, each of the signal modulators 124a, 124b, 124c, 126a, 126b, 126c may be implemented as the channel 302 or 304 discussed above in relation to Fig. 3.

The cancellation module 806 has a second signal combiner 828, which fulfils a similar purpose to the second signal combiner 128 discussed above. The second signal combiner 828 is configured to combine the outputs from each of the signal modulators 124a, 124b, 124c, 126a, 126b, 126c, to form the cancellation signal. The second signal combiner 828 then transmits the cancellation signal to the third signal combiner, where the cancellation signal is combined with the response signal as discussed above for the system 100. Thus, the cancellation signal includes three phase shifted copies of the first test signal, and three phase shifted copies of the second test signal.

The phases and amplitudes of the phase shifted copies may be set (either manually or automatically) in order to maximise cancellation of the self-interference components in the response signal. For example, the response signal may include three self-interference components at F having amplitude and phase (AA, OA), (A8,09) and (Ac,i9c), respectively. Then, the signal modulators 124a, 124b and 124c may be controlled to set the amplitudes Al= AA, A3 = AB and A5 = AG., and to set the phase shifts Awl= OA + 180°, .43 = OB + 1800, and 4(pr, = OB + 180°. Thus, the phase shifted copies of the first test signal will be in antiphase with the self-interference components at F1, and will have amplitudes matching the self-interference components at Fi. In this manner, the phase shifted copies of the first test signal will suppress the self-interference components at F1. Similarly, the response signal may include three self-interference components at F2, having amplitude and phase (AD,OD), (AE, OE) and (AF,OF), respectively. Then, the signal modulators 126a, 126b and 126c may be controlled to set the amplitudes A2 = AD, A4= AE and A6= AF, and to set the phase shifts Ay, = OD+ 180°, ,L1Q4 = BE + 1800, and.,8(p6 = OF + 180°.

Accordingly, the phase shifted copies of the second test signal will be in antiphase with the self-interference components at F2, and will have amplitudes matching the self-interference components at F2, such that the self-interference components at F2 will be suppressed.

In one example, this may be achieved by sequentially setting the phases and amplitudes for the pairs of signal modulators. For example, starting with the first pair of signal modulators 124a, 126a, their amplitudes and phases may be adjusted in order to minimise amplitudes of the self-interference components at RI and F2 observed in the output signal. This process should effectively cancel the dominant self-interference path, which is likely to be direct coupling between the transmitter and receiver antennas. While the first pair of signal modulators 124a, 126a is being adjusted, the other signal modulators may be switched off or disconnected from the signal combiner 128, i.e. so that no cancellation components from the other signal modulators are combined with the response signal. Then, keeping the previously set amplitudes and phases for the first pair of signal modulators 124a, 126b, the second pair of signal modulators 124b, 126b may be activated (e.g. by switching them on and/or connecting them to the signal combiner 128) and their amplitudes and phases adjusted to further minimise the amplitudes of the self-interference components at F, and F2 observed in the output signal. Finally, keeping the previously set amplitudes and phases for the first and second pairs of signal modulators 124a, 126a, 124b, 126b, the third pair of signal modulators 124c, 126c is activated (e.g. by switching them on and/or connecting them to the signal combiner 128) and their amplitudes and phases are adjusted to further minimise the amplitudes of the self-interference components at F1 and F2 observed in the output signal. This process should result in suppression of self-interference components arising from different self-interference paths.

This process may be performed manually, or automatically using any suitable optimisation algorithm.

In the example shown, the cancellation module 806 includes three pairs of signal modulators, such that it can be used to suppress three self-interference components at each of the frequencies Fi and F2. However, in other examples, different numbers of pairs of signal modulators may be used, e.g. there may be two or more pairs of signal modulators. The sequential selling of amplitudes and phases discussed above may be adapted to the number of pairs of signal modulators used.

The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

Claims

Claims: 1. A system for detecting intermodulation distortion, the system comprising: a transmitter stage configured to emit, via a transmitter antenna, an excitation signal comprising a first test signal and a second test signal, the first test signal and the second test signal being radiofrequency signals at different respective frequencies; a cancellation module configured to generate a cancellation signal; and a receiver stage configured to: receive, via a receiver antenna, a response signal in response to emission of the excitation signal from the transmitter antenna, wherein the response signal comprises a first response component at a frequency of the first test signal and a second response component at a frequency of the second test signal; and combine the response signal and the cancellation signal to form an output signal, wherein the cancellation signal is arranged to at least partially cancel the first response component and the second response component.
2. A system according to claim 1, wherein the cancellation signal comprises a first cancellation component at the frequency of the first test signal and a second cancellation component at the frequency of the second cancellation signal, where the first cancellation component is arranged to at least partially cancel the first response component and the second cancellation component is arranged to at least partially cancel the second response component.
3. A system according to claim 2, wherein the first cancellation component comprises a phase shifted copy of the first test signal and the second cancellation component comprises a phase shifted copy of the second test signal.
4. A system according to claim 3, wherein the cancellation module comprises a first phase shifter for controlling a phase shift of the phase shifted copy of the first test signal and a second phase shifter for controlling a phase shift of the phase shifted copy of the second test signal.
5. A system according to claim 3 or 4, wherein the phase shifted copy of the first test signal is in antiphase with the first response component and the phase shifted copy of the second test signal is in antiphase with the second response component.
6. A system according to one of claims 2 to 5, wherein an amplitude of the first cancellation component substantially matches an amplitude of the first response component, and an amplitude of the second cancellation component substantially matches an amplitude of the second response component.
7. A system according to one of claims 2 to 6, wherein the cancellation module comprises a first attenuator for controlling an amplitude of the first cancellation component and a second attenuator for controlling an amplitude of second cancellation component.
8. A system according to one of claims 2 to 7, wherein the transmitter stage comprises: a signal generation module configured to generate the first test signal and the second test signal; a first signal splitter configured to transmit a first portion of the first test signal to a first signal combiner and a second portion of the first test signal to the cancellation module; a second signal splitter configured to transmit a first portion of the second test signal to the first signal combiner and a second portion of the second test signal to the cancellation module; wherein the first signal combiner is coupled to the transmitter antenna and configured to combine the first portion of the first test signal and the first portion of the second test signal to form the excitation signal.
9. A system according to one of claims 2 to 8, wherein the cancellation module comprises a second signal combiner configured to combine the first cancellation component and the second cancellation component to form the cancellation signal.
10. A system according to any preceding claim, wherein the cancellation signal comprises: two or more first cancellation components at the frequency of the first test signal and with different respective phases; and two or more second cancellation components at the frequency of the second test signal and with different respective phases.
11. A system according to any preceding claim, further comprising a controller configured to control generation of the cancellation signal by the cancellation module.
12. A system according to any preceding claim, further comprising an analyser stage configured to determine a spectrum of the output signal.
13. A system according to claim 12, wherein system is configured to determine, based on the spectrum of the output signal, if a target device is located in an environment of the system.
14. A method of detecting intermodulation distortion, the method comprising: transmitting, via a transmitter antenna, an excitation signal comprising a first test signal and a second test signal, the first test signal and the second test signal being radiofrequency signals at different respective frequencies; receiving, via a receiver antenna, a response signal in response to emission of the excitation signal from the transmitter antenna, wherein the response signal comprises a first response component at a frequency corresponding to the first test signal and a second response component at a frequency corresponding to the second test signal; generating, with a cancellation module, a cancellation signal arranged to at least partially cancel the first response component and the second response component; and combining the response signal and the cancellation signal to form an output signal.
15. A method according to claim 14, wherein the cancellation signal comprises a first cancellation component at the frequency of the first test signal and a second cancellation component at the frequency of the second cancellation signal, where the first cancellation component is arranged to at least partially cancel the first response component and the second cancellation component is arranged to at least partially cancel the second response component.
16. A method according to claim 15, wherein the first cancellation component comprises a phase shifted copy of the first test signal and the second cancellation component comprises a phase shifted copy of the second test signal.
17. A method according to claim 16, wherein the phase shifted copy of the first test signal is in antiphase with the first response component and the phase shifted copy of the second test signal is in antiphase with the second response component.
18. A method according to one of claims 15 to 17, wherein an amplitude of the first cancellation component substantially matches an amplitude of the first response component, and an amplitude of the second cancellation component substantially matches an amplitude of the second response component. 20
19. A method according to one of claims 14 to 18, wherein the cancellation signal comprises: two or more first cancellation components at the frequency of the first test signal and with different respective phases; and two or more second cancellation components at the frequency of the second test signal and with different respective phases.
20. A method according to one of claims 14 to 19, wherein the cancellation signal is generated based on the response signal.
21. A method according to one of claims 14 to 20, further comprising determining a spectrum of the output signal.
22. A method according to claim 21 further comprising determining, based on the spectrum of the output signal, if a target device is located in an environment of the receiver stage