GB2339917A

GB2339917A - Generating a multitone test signal

Info

Publication number: GB2339917A
Application number: GB9815889A
Authority: GB
Inventors: Harold Thomas Brown; Neil Edwin Thomas
Original assignee: IFR Ltd
Current assignee: Aeroflex Ltd
Priority date: 1998-07-21
Filing date: 1998-07-21
Publication date: 2000-02-09
Also published as: WO2000005591A1; GB9815889D0

Description

2339917 METHOD OF AND APPARATUS FOR GENERATING A MULTITONE TEST SIGNAL

This invention relates to a method of and apparatus for generating a test signal. The test signal may be used for measuring the linearity of a system.

When testing the linearity of a system, a known method involves sending a test signal into a device under test (DL7) and observing the output spectrum on a spectrum analyser. A commonly used test signal consists of two closely spaced tones at frequencies f, and f2, although test signals containing more tones are also common. Non-linearities in the DUT mix these input tones to create extra tones which can be detected by the spectrum analyser. The levels or pwxers of these extra tones may be used to characterise the degree of non-linearity in the DLT. Typically, for a two tone test signal the largest of these extra tones are the third order products at frequencies of 2frf2and 2f2-fi. Higher order products are also generated but the magnitude of these tones tends to be much smaller than the magnitude of the third order products.

Accurate measurements of non-linearities in the DUT can be made by using a clean input test sianal toaether with an accurate spectrum analyser. If the input test signal entering the DUT contains intermodulation distortion products then the dynamic range and/or accuracy of non- linearity measurements will be affected. For a two tone input signal, the individual tones are generated respectively in independent signal sources and are subsequently combined to form a composite signal. However, the process of combining the individual tones generally produces unwanted intermodulation products in the composite signal in addition to the two required tones.

Figures I to 3 illustrate three known methods for combining tone signals generated in independent signal sources S, andS2. Intermodulation products are formed in the composite test signal T when the two tone signals, f, and f2, are present simultaneously in the output of either signal source. Each method attempts to prevent the formation of intermodulation products, by minimising both the amount of f2 signal that appears in the output of the f, signal source and the amount of f, signal that appears in the output of the f2signal source.

The first method shown in Figure I is applicable only to narrow band systems and uses filters to isolate the two sources. This method is commonly used where several basestation frequencies share a common antenna. The rationale behind this is twofold - as well as reducing output intermodulation, it also reduces the power loss that occurs with other types of combiners. This is the combination method of choice for PIM (passive intermodulation) measurement systems, which operate at fixed frequency.

The second method shown in Figure 2 is for broad or medium band systems. An isolating combiner is made usin- either a Wilkinson or a MB hybrid combiner for very high frequencies, or a transformer based combiner going down to or near DC. The bandwidth of the latter depends on the ferrites used for the transformer core, but IOMHz to 2GHz are achievable.

The third method shown in Figure 3 uses only resistive combiners and is broadband but has the lowest performance of all the methods. The best resistive combiner arrangement is shown, giving the maximum source to source isolation for any given signal isolation.

2 The aim of all of these known methods is to prevent creation of intermodulation products in the desired composite test signal by effectively buffering each signal source.

According to a first aspect of the present invention there is provided a method of generating a test signal, the method comprising the steps of generating at least first and second main signals, the first main signal having a frequency component at a first predetermined frequency and the second main signal having a frequency component at a second predetermined frequency, combining said main signals to form a composite test signal containing one or more intermodulation components, wherein the method comprises the step of introducing at least one cancelling signal at a frequency substantially equal to that of one of the intermodulation components so as to suppress that intermodulation component in the composite signal.

According to a second aspect of the present invention there is provided an apparatus for generating a test signal, the apparatus comprising signal generator means for generating at C> least first and second main signals, the first main signal having a frequency component at a first predetermined frequency and the second main signal having a frequency component at a second predetermined frequency, and means for combining said main signals to form a composite test signal containing one or more intermodulation components, wherein the apparatus further comprises means for introducing at least one cancelling signal at a frequency substantially equal to that of one of the intermodulation components so as to suppress that intermodulation component in the composite signal.

A method of or an apparatus for generating a test signal in accordance with the first or second ZP aspect of the invention has an advantage that it can generate a test signal with reduced 3 intermodulation distortion, which in turn makes the test signal cleaner. The method in accordance with the invention is thus able to generate a test signal which may be used in test systems with improved performance.

Ideally, the test signal is used for measuring the linearity of a system. Suitably, the first main signal is generated in a first signal path, the second main signal is generated in a second signal path, and the composite signal contains frequency components at the first and second predetermined frequenzies. Preferably, the cancelling signal is introduced at a suitable amplitude and phase so as to reduce or in some cases cancel one of the intermodulation components in the composite signal.

In one embodiment of the invention, the cancelling signal is introduced into the main signals before they are combined to form the composite test. In an alternative embodiment, the cancelling signal is introduced into the composite signal. In a further embodiment, the cancelling is introduced into the main si-nals in the combining step to produce the composite signal.

I In a preferred embodiment of the invention, the step of introducing the cancelling signal comprises modulating a main signal with a modulating signal. The step of modulating the t) C 0 main signal may also generate a sideband signal at substantially the frequency of another C, 0 main signal.

Preferably, the modulating signal has a frequency substantially equal to the difference between the first and second predetermined frequencies.

4 Suitably, the intermodulation components are third-order intennodulation components.

The modulation of the main signal may be achieved using amplitude modulation (AM). In an alternative embodiment, modulation of the main signal is achieved using angle modulation which may involve frequency or phase modulation techniques. Typically, the angle modulation is low-index angle modulation.

Preferably, the method comprises a calibration step in which the composite test signal is analysed to determine the residual signal levels at the frequency of the intermodulation components, and the amplitude and phase of the additional frequency component is adjusted in dependence on the determined residual signal levels. Preferably, the calibration step optimises suppression of the intermodulation components in the composite test signal.

Further features and advantages of the invention will be apparent from the description below.

I Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figgre I is a block diagram of a known system for producing a test signal T from two signal sources S, and S2; Figure 2 is a block diagram of alternative known system for producing a test signal T from two signal sources S, andS2; Figggre 3 is a block diagram of another known system for producing a test signal T from two signal sources S, andS2; Figure 4 is a block diagram of a system for producing a test signal T' in accordance with the invention incorporating two modulators; Figgre 5 is a block diagram of a system for producing a test signal T' in accordance with the invention incorporating two amplitude modulators and two phase angle modulators; Figure 6 is a block diagram of a system for producing a test signal T' in accordance with the invention incorporating two IQ modulators; Figure 7 is a diagram illustrating the process by which a test signal T is produced in the known systems shown in Figures 1 to 3, and the process by which a test signal T' is produced in the modulating systems shown in Figures 4 to 6; Figgure 8 is a block diagram of an alternative system for producing a test signal T' in accordance with the invention incorporating two frequency synthesisers; Figure 9 is a block diagram iof a system for producing a test signal T' in accordance with the invention incorporating a composite modulation of one of the tones; and Fi,gure 10 is a diagram illustrating the process by which a test signal T' is produced in the system of Figure 9.

C) Referring to Figure 4 there is shown a block diagram of a system 40 for producing a relatively C clean two-tone test signal T'. The system shares some of the basic elements of the prior art

6 systems shown in Figure I to 3 such as the two independent signal generators SI, S2 and a combiner 4 1.

The signal generator S, generates a tone signal at a frequency f, which is modulated by a modulator 42. The resultant modulated signal is fed to a first input of the combiner 41. In a similar way, the signal generator S2 generates a tone signal of similar amplitude and at a frequency f2 slightly higher than frequency fi. This tone signal is modulated by a second modulator 43 and the resultant modulated signal is fed to a second input of the combiner 41. The combiner 41 adds the two modulated signals together to produce a test signal T' containing frequency components at the frequencies f, and f2. This test signal T' may be used to test the linearity of a system or a device under test.

The combiner 41 is a standard component used in the art and may, for example, be equivalent to one of the combiners used in the prior art systems shown in Figures I to 3.

A modulating signal which is supplied to the modulators 42, 43 is generated in an oscillator 44. The oscillator 44 is adjusted to generate a tone signal at a frequency equal to the difference in frequency of the tone signals generated by the signal generators S, andS2 ie. f2 fi. The modulating signal is supplied along a first signal path via a variable phase adjuster 45 and a variable attenuator 46 to an input of the modulator 42. The modulating signal is also supplied along a second signal path via a variable phase adjuster 47 and a variable attenuator 48 to an input of the modulator 43.

Referring to Figure 7, there is shown a frequency domain plot i) representing the tone signals I ZD at f, and f,- generated by the signal generators S, andS2in the prior art systems of Figures I to

7 3 and in the system 40 shown in Figure 4. The tone signals are shown as vertical peaks whose height is proportional to the magnitude of the tone signal. The prior art method for combining the tone signals generated in the independent signal sources S, and S2 is symbolised by the arrow C. The resultant test signal T is shown in the frequency domain plot ii) and contains in addition to the frequency components f, and f2, third order intermodulation products at the frequencies 2f,- f2 and 2f2 - fl. Contamination of the two-tone test signal T with these intermod,ulation products is undesirable and can result in a degradation of performance when used in a linearity test system.

The process shown in Figure 4 for producing a two-tone test signal T' is represented by the arrows M and C' in Figure 7. The arrow M symbolises the modulation stage and the arrow C' symbolises the combination stage. Referring to Figures 4 and 7, modulation of the tone signal f, by the modulating signal f2-f, in the modulator 42 generates a modulated signal containina additional frequency components at the frequencies f, (f2-fl) ie at 2fi-f, and f2. Modulation of the tone signal f2by the modulating signal f2-f i in the modulator 43 generates a modulated signal containing additional frequency components at the frequencies f2 (f2 - fl) ie at f, and Z2-fi. The modulated signals are represented in the frequency domain plot iii). Combination C' of these modulated signals in the combiner 41 produces a test signal T' represented by the frequency domain plot iv). Third order intermodulation products which would normally be generated in the test signal T' during the combination step are cancelled by the additional frequency components 2fi-f2and 2f2-f, in the modulated signals.

Cancellation of the intermodulation products requires the additional frequency components in the modulated signals to be set to an appropriate phase and amplitude. This is achieved by independent adjustment of phase and amplitude of the modulating signals using respectively 8 the variable phase adjusters 45, 47 and the variable attenuators 46, 48. Optimal adjustment of the modulating signals produces a test signal T' with two tones at f, and f2, and suppressed frequency components at the intermodulation frequencies.

Optimal adjustment or calibration of the test signal device of Figure 4 is achieved in practice by sending the two-tone test signal T' directly to a signal analyser. The analyser measures the levels of the frequency components at the third order intermodulation frequencies and either displays the results for manual adjustment of the test signal device or automatically implements the test signal device adjustments on the basis of a feedback algorithm. As the third order intermodulation frequencies comprise an upper and a lower sideband, each with an associated amplitude and phase, 4 parameters are required to be adjusted in the test signal device.

A basic technique for calibration relies on independent amplitude measurements of the upper and lower side bands and performs iterative adjustments of the two modulating signals in phase and amplitude by means of the variable phase adjusters 45, 47 and the variable attenuators 46, 48. Calibration is achieved when amplitude nulls are detected for the upper and lower sidebands corresponding to the intermodulation frequencies. According to one algorithm, each parameter (amplitude or phase) of the modulating signals is systematically varied until a minimum is observed for signal components in the corresponding upper or lower sideband. This systematic variation is then repeated to determine a minimum for the signal components in the upper or lower sideband with respect to both adjustment parameters (amplitude and phase).

9 In another embodiment, the signal analyser measures both the amplitude and phase of the upper and lower side bands and calculates a single adjustment for each parameter in the test signal device to achieve optimum cancellation of the intermodulation products.

The modulators 42, 43 in the system 40 may be amplitude modulators or angle modulators. Amplitude modulation will form modulated signals containing the two sideband signals shown in plot iii) in Figure 7 together with the main signal at f, or f2. For angle modulation, however, low index modulation techniques have to be used to form a modulated signal containing only the two sideband signals shown in plot iii) in Figure 7 together with the main signal at f, or f2. Low index modulation, requires the magnitude of the angle variation of the main signal f, or f2 to be small. This occurs when the modulating signal is at a low level or when the modulation factor of the modulator is small. Low index modulation generates a modulated signal in which the sideband signals are at a relatively low level compared to the main signals. This restriction is acceptable in the test signal device of Figure 4 as the intermodulation products which require cancellation are also at a relatively low level.

In an alternative embodiment of the invention, the signal source S 1 and modulator 42 of Figure 4 can be replaced by a first frequency modulated source that generates a modulated signal with the frequency components fi, f2and 2f,42, and the signal source S2 and modulator 43 can be replaced by a second frequency modulated source that generates a modulated signal with the frequency component fi, f,_ and 2f,-fi, thus producing the same result as the embodiment of Figure 4.

C:1 Referrina to Fi-ure 5, there is shown a block diagram of a system 50 in accordance with the C:- t C) invention for producing a relatively clean two-tone test signal T'. In this embodiment, the modulating signal previously generated by the oscillator 44 in Figure 4 is now generated by mixing the signals f 1 and f2from the signal sources S, andS2 in a mixer 54. The phase of the modulating signal generated by the mixer 54 is not directly controllable. Therefore, the arrangement of the two modulators 42, 43 in Figure 4 has been replaced by an arrangement of two amplitude modulators 52, 53 and two angle modulators 92, 93. Each modulator 52, 53, 92, 93 is fed with the modulating signal from the mixer 54 via a respective variable attenuator 55, 56, 57, 58. The two variable attenuators 55, 57 together control the amplitude and phase of the lower sideband frequency component 2frf2whilst the two variable attenuators 56, 58 together control the amplitude and phase of the higher sideband frequency component X241. The process of calibrating the test signal device is equivalent to calibration process described with reference to Figure 4.

Referring to Figure 6, there is shown a block diagram of another system 60 in accordance with the invention for producing a relatively clean twotone test signal T'. In this embodiment, the amplitude and angle modulators 52, 92 shown in Figure 5 have been Z replaced by an IQ modulator 62, and the amplitude and angle modulators 53, 93 have similarly been replaced by an IQ modulator 63. Again the two variable attenuators 55, 57 together control the amplitude and phase of the lower sideband frequency component 2fl 42 whilst the two variable attenuators 56, 58 together control the amplitude and phase of the higher sideband frequency component 2f2-fl.

Referring to Figure 8, there is shown a block diagram of a system 80 in accordance with the ZP invention for producing a relatively clean two-tone test signal T'. In this embodiment, the sideband components 2f,42 and 2f2-fj are produced respectively by two synthesisers 81, 82 and are added to the main sianals f, and f2by the adders 83, 84. The output of synthesiser 81 is fed to the adder 83 via a variable phase adjuster 86 and a variable attenuator 88, and the output of the synthesiser 82 is fed to the adder 84 via a variable phase adjuster 87 and a variable attenuator 89. The variable phase adjusters 86, 87 and variable attenuators 88, 89 are adjusted as in the previous embodiments to provide optimum cancellation of intermodulation products produced during the step of combining the signals f, and f2.

It will be evident in view of the foregoing description that various modifications may be made within the scope of the present invention. For example, the two-tone test signal may be a three, four or multiple tone test signal. Also, although the embodiments have been described with reference to discrete components, it is equally applicable for the components to be embodied as functional elements of a digital signal processing (DSP) chip or as programmed functions in a programmable device. In the DSP implementation the adders 83 and 84 and the combiner 41 may be integrated as one functional element

Figure 9 shows a block diagram of a system in which two signal generators S andS2 generate signals f, and f,, respectively, one of these signals F, being modulated in a modulator 90 before both signals are combined in a combiner 91 to produce a two-tone test signal T'. The modulating signal comprises two modulating frequency components f, and 2f, combined in a combiner 92. The effect of the modulator of the signal f, is illustrated in Figure 10, which shows the resulting two sets of sidebands at &-f, and 2f,1-2f, The modulating signal f. is selected to be equal frequency to the difference between the frequencies F, and f2 so that the intermodulation components of F, and f2 are substantially cancelled by the sidebands -f, as shown by broken lines in Figure 10. The lower sideband component -2t remains in the composite test signal T', but does not cause a problem because it is removed from the two tones F, and F2.

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Claims

I A method of generating a test signal, the method comprising the steps of generating at least first and second main signals, the first main signal having a frequency component at a first predetermined frequency, and the second main signal having a frequency component at a second predetermined frequency, combining said main signals to form a composite test signal containing one or more intermodulation components, characterised in that the method further comprises the step of introducing at least one cancelling signal at a frequency substantially equal to that of one of the inicrmodulation components so as to suppress that inter-modulation component in the comTx), ite,ignal.
2. A method as claimed in claim 1, wherein the test signal is used for measuring the linearity of a system
3. A method as claimed in claim I or claim 2, wherein each main signal is generated in a respective signal path, and the composite test signal contains at least frequency components C of the first and second predetermined frequencies.
4. A method as claimed in any one of the preceding claims, wherein each cancelling signal is introduced at a suitable amplitude and phase so as to reduce or cancel a respective one of the intermodulation components in the composite signal.
5. A method as claimed in any one of the preceding claims, wherein the step of introducing each cancelling signal comprises generating a signal at a frequency substantially ID 4D:P 13 equal to that of one of the intermodulation components, and combining this signal with at least one of the main signals.
6. A method as claimed in claim 5 wherein at least one cancelling signal is combined with each of two or more of the main signals to suppress a respective intermodulation component.
7. A method as claimed in any one of claims I to 4, wherein the step of introducing each cancelling signal comprises generating a signal at a frequency substantially equal to that of 0 0 one of the intermodulation components, and combining this with the composite test signal.
8. A method as claimed in any one of claims I to 4, wherein each cancelling signal is introduced by modulating one of said main signals with a modulating signal.

0
9. A method as claimed in claim 8, wherein one of said main signals is modulated with a composite modulating signal containing at least two frequency components so as to generate two or more cancelling signals
10. A method as claimed claim 8 wherein each of two or more of said main signals is modulated with a respective modulating signal to generate one or more cancelling signals.
11. A method as claimed in any one of claims 8 to 10, wherein the step of modulating one main signal generates a cancelling signal and an additional sideband signal at substantially the 0 0 frequency of another of said main signals.

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12. A method as claimed in any of claims 8 to I I wherein each of said first and second main signals is modulated by a modulating signal substantially equal to the difference in frequency between them.
13. A method as claimed in any one of the preceding claims, wherein the intermodulation components comprise third-order intermodulation components.
14. A method as claimed in any one of claims 8 to 12, wherein modulation of the main signal is achieved using amplitude modulation (AM).
15. A method as claimed in any one of claims 8 to 12, wherein modulation of the main signal is achieved using angle modulation.
16. A method as claimed in claim 14, wherein the angle modulation is achieved through frequency modulation.
17. A method as claimed in any one of claims 8 to 12, wherein modulation of the main signal is achieved using IQ modulation.
18. An apparatus for generating a test signal, the apparatus comprising signal generator means for -enerating at least first and second main signals, the first main signal having a frequency component at a first predetermined frequency, and the second main signal having a frequency component at a second predetermined frequency, and means for combining said main signals to form a composite test signal containing one or more intermodulation components, wherein the apparatus further comprises means for introducing at least one cancelling signal at a frequency substantially equal to that of one of the intermodulation components so as to suppress that intermodulation component in the composite signal.
19. An apparatus for generating a test signal substantially as herein described with reference to any of Figures 4 to 8.

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