CN112615598A - Low-insertion-loss frequency-adjustable broadband pulse NMR radio frequency duplexer - Google Patents
Low-insertion-loss frequency-adjustable broadband pulse NMR radio frequency duplexer Download PDFInfo
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Abstract
The invention discloses a low-insertion-loss frequency-adjustable broadband pulse NMR radio frequency duplexer, which comprises: the probe comprises a first direct current bias module, a second direct current bias module, a first frequency selection module, a second frequency selection module and a probe coil; the first direct current bias module provides direct current bias and changes the on-off state of the input branch circuit by changing the magnitude of bias voltage; the first frequency selection module enables the duplex circuit to obtain impedance matching under different frequencies by changing the equivalent input impedance of the duplex circuit, and realizes the center frequency transfer of the passband of the duplex circuit; the second frequency selection module enables the duplex circuit to meet the impedance matching requirement in a certain range of output signal frequency by changing the equivalent output impedance of the duplex circuit, and transmits the FID signal detected by the coil to an output port; the second direct current bias module is used for providing direct current bias and changing the on and off states of the output branch circuit by changing the magnitude of bias voltage. The invention can solve the problems of small center frequency and fixed frequency band.
Description
Technical Field
The invention belongs to the technical field of nuclear magnetic resonance, and particularly relates to a low-insertion-loss adjustable-frequency broadband pulse NMR radio frequency duplexer for an ultrahigh frequency pulse nuclear magnetic resonance spectrometer system.
Background
In the Nuclear Magnetic Resonance spectrometers studied in the past and commercially used at present, most of the Magnetic fields generated by the superconducting magnet are used as the background fields of Nuclear Magnetic Resonance (NMR) experiments, because the uniformity of the Magnetic field of the superconducting magnet is good, the frequency of the obtained Free Induction Decay (FID) signals is stable, and phase dynamic fluctuation deviation is hardly generated; meanwhile, the magnetic field of the superconducting magnet has good repeatability and long duration, and the signal-to-noise ratio of a signal obtained by superposing a plurality of repeated experimental signals can also meet the requirements of most scientific experiments and commercial use. However, with the development of NMR technology, due to the limitation of the magnetic field intensity of the superconducting magnet, in an experimental scene that is greatly limited by time for biomacromolecule samples, multidimensional NMR and the like, the field intensity range which can be generated by the superconducting magnet cannot well meet the requirements of scientific research.
According to the NMR related knowledge, the signal-to-noise ratio of the FID signal is exponentially increased along with the intensity of the external magnetic field, the number of times of related NMR experiments can be greatly reduced in the NMR experiment under a high field, the time required by the experiments is shortened, the acquisition speed is accelerated, and meanwhile, for spectrum analysis, the field intensity is improved, the frequency difference of proton resonance can be increased, so that the spectrum resolution is improved, and the substance distinguishing capacity is improved. Consequently, NMR applications and related research tend to move towards higher magnetic field strengths. Relevant scholars in countries such as germany, france and japan have pioneered NMR studies under pulsed high-intensity magnetic fields, and the resonance frequency is also increased to GHz level in proportion to the magnetic field strength, and reaches up to 2.4 GHz. However, due to the magnetic field inhomogeneity of the pulsed strong magnetic field, the bandwidth of the FID signal spectrum in the primary pulse NMR experiment is larger than the bandwidth of the FID signal spectrum in the steady state field (depending on the magnetic spin ratio of the sample and the fluctuation range of the magnetic field during the experiment).
NMR experiments under a pulse strong magnetic field can effectively solve the problems of short experiment time, insufficient resolution and the like, but also bring the problems of high requirement on the insertion loss of a duplexer, increase of the difference of sample proton resonance frequency and increase of the frequency spectrum bandwidth of an FID signal. The radio frequency duplexer in a conventional NMR system under steady state conditions has a relatively low operating frequency and relatively small bandwidth. Therefore, the rf duplexer used in the conventional spectrometer system cannot be well applied to the pulsed nmr spectrometer due to the limitations of frequency and bandwidth.
Aiming at the working requirements of a pulse nuclear magnetic resonance spectrometer system, the radio frequency duplexer can meet the following requirements:
(1) the switching speed of the working state of the radio frequency duplexer is high. For NMR experiments under a pulsed strong magnetic field, the duration of each applied radio frequency magnetic field is short (probably in the order of μ s), the FID signal decays too fast, and the detection time is short (generally less than 0.05 ms). If the switching speed of the rf duplexer is too slow, the dead time is too long, so that the detected FID signal is weak, and the signal-to-noise ratio of the FID signal is reduced due to the fact that the tail noise signal and the FID signal are not very different in strength. Particularly, in a pulse NMR experiment, the flat-top time of a pulse magnet is short, and the experiment times are limited by a cooling system, so that the FID signals of each time can be ensured to be as complete as possible through the rapid switching of a duplexer, and the best acquisition effect is achieved.
(2) Low voltage standing wave ratio and low insertion loss. Because the radio frequency power output by the radio frequency amplifier at the transmitting end is high (about 200W), in order to ensure that most of the power output by the preposed radio frequency power amplifier can be transmitted and avoid power reflection, the impedance matching of the duplexer needs to be ensured to be near 50 ohms, so that the voltage standing wave ratio of the duplexer is ensured to be low when the duplexer works (generally, the voltage standing wave ratio of the duplexer is below 1.4, so that 97% of the output power can be transmitted). The higher the frequency is, the higher the requirement on the capability of the radio frequency power amplifier at the transmitting end to output power is, and the higher the cost input under the same power is, so that a duplexer with smaller insertion loss is required in a pulse NMR system to ensure that the power is transmitted to the NMR probe as completely as possible, thereby reducing the cost of the system on the radio frequency power amplifier.
(3) High isolation. Due to the complex electromagnetic environment of the pulse field and the large output power of the radio frequency power amplifier, in order to ensure the safe and stable operation of the low noise amplifier at the receiving end, it is necessary to ensure that the isolation between the output port of the radio frequency power amplifier and the input port of the low noise signal amplifier is high, and prevent the leaked radio frequency signal from damaging the low noise amplifier at the later stage.
(4) The radio frequency duplexer has high working frequency and large bandwidth. The pulse NMR system generally operates at a frequency of 1GHz or more, and the bandwidth of the FID signal increases due to the temporal and spatial non-uniformity of the pulsed strong magnetic field, which requires the rf duplexer in the pulse NMR system to have characteristics of high operating frequency and large bandwidth.
The duplexer used in wireless communication realizes the sending and receiving of signals with different frequencies through the frequency division function of the filter, and is different from the function realized by the duplexer for sending and receiving signals with the same frequency in nuclear magnetic resonance, so the different-frequency duplexer in the communication field is not suitable for the nuclear magnetic resonance field.
In chinese patent CN108680883A, the isolation of the rf duplexer can be increased to 70dB by applying high and low control levels to the high-speed rf switch, when rf excitation is added, the insertion loss can reach about 1dB at maximum in the working frequency range, the switching speed reaches 2 μ s, but its working frequency is still below GHz level, when the insertion loss is about 1dB, a large part of power cannot be transmitted to the NMR probe, and the high-power rf power amplifier has high cost, and the large insertion loss increases the cost. For a pulse NMR experiment, the radio frequency excitation adding time is shorter than the excitation applying time of the traditional NMR experiment, and most of power of a radio frequency power amplifier needs to be ensured to be transmitted to an NMR probe, so that the nuclear magnetic moment is controlled to be overturned at a target angle in an xyz coordinate system in a short time, and an FID signal with good signal-to-noise ratio is obtained, therefore, the insertion loss in the whole working frequency range of a radio frequency duplexer in a pulse NMR system is maximally about 1dB, and the maximum insertion loss of the radio frequency duplexer in the pulse NMR system is required to be less than 0.5dB, even less than 0.3 dB.
The traditional nuclear magnetic resonance radio frequency duplexer adopts a passive switching mode to realize the switching of radio frequency excitation and FID signal receiving states. In the traditional radio frequency duplexer, an 1/4 wavelength line is mainly used for realizing impedance transformation and isolation between the output end of a radio frequency power amplifier and the input end of a low-noise preamplifier when radio frequency excitation is added. It can be seen that the conventional rf duplexer is not suitable for use in a pulsed NMR system, due primarily to the narrow band nature of the 1/4 wavelength line. When radio frequency excitation is added, the radio frequency signal deviates from the central frequency of the 1/4 wavelength line at the rising edge and the falling edge, and simultaneously, due to the spatial-temporal nonuniformity of a pulse strong magnetic field, the central frequency of an FID signal obtained after each addition of the radio frequency excitation changes along with the fluctuation of the magnetic field, so that the traditional radio frequency duplexer carrying the 1/4 wavelength line is not suitable for being used in a pulse NMR system.
Secondly, because the magnetic field intensity of the pulse strong magnetic field is high and the resonance frequency is in direct proportion to the magnetic field intensity of the external pulse, when a sample is replaced each time for experiment, the corresponding resonance frequency is greatly changed, the usable frequency band range of the traditional radio frequency duplexer is not very large, and if the radio frequency duplexer needs to be replaced each time for experiment, the cost is increased.
In summary, the conventional rf duplexer cannot be applied to the pulse NMR system due to the insufficient characteristics of isolation and bandwidth of the operating frequency band, and the duplexer provided in the prior art cannot be well adapted to the pulse NMR system due to the insertion loss characteristics and the change of the operating center frequency and the shift of the center frequency band caused by the sample replacement.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a low-insertion-loss frequency-adjustable broadband pulse NMR radio frequency duplexer, aiming at solving the problems of large insertion loss, narrow frequency band and fixed frequency band of the traditional nuclear magnetic resonance radio frequency duplexer.
The invention provides a low-insertion-loss frequency-adjustable broadband pulse NMR radio frequency duplexer, which comprises: the probe comprises a first direct current bias module, a second direct current bias module, a first frequency selection module, a second frequency selection module and a probe coil; the input end of the first direct current bias module is used as the radio frequency input end of the duplexer, and the first direct current bias module is used for providing direct current bias and changing the on-off state of an input branch circuit by changing the magnitude of bias voltage; the input end of the first frequency selection module is connected to the output end of the first direct current bias module, and the first frequency selection module is used for enabling the duplex circuit to obtain impedance matching under different frequencies by changing the equivalent input impedance of the duplex circuit, so that the transfer of the center frequency of the passband of the duplex circuit is realized; the input end of the probe coil is connected to the output end of the first frequency selection module, and the probe coil is used for receiving an input radio frequency signal and detecting a generated FID signal; the input end of the second frequency selection module is connected to the output end of the probe coil, and the second frequency selection module is used for enabling the duplex circuit to meet the impedance matching requirement in a certain range of output signal frequency by changing the equivalent output impedance of the duplex circuit and transmitting the FID signal detected by the coil to an output port; the input end of the second direct current bias module is connected to the output end of the second frequency selection module, the output end of the second direct current bias module is used as the output end of the duplexer, and the second direct current bias module is used for providing direct current bias and changing the on and off states of the output branch circuit by changing the magnitude of bias voltage.
Still further, the first dc bias module includes: a capacitor C2, a bypass capacitor C9, an inductor L3, an inductor L4 and a first BIAS voltage BIAS 1; one end of the inductor L3 and one end of the inductor L4 are respectively connected to two ends of the capacitor C2, and the other end of the inductor L3 and the other end of the inductor L4 are both connected with a first direct current BIAS voltage BIAS 1; one end of a bypass capacitor C9 is connected with the first direct current BIAS voltage BIAS1, and the other end of the bypass capacitor C9 is grounded; turning on the input branch when the first dc BIAS voltage BIAS1 is high; turning off the input branch when the first dc BIAS voltage BIAS1 is low; the bypass capacitor C9 is used to filter out harmonics.
Further, the second dc bias module includes: a capacitor C6, a capacitor C10, an inductor L9, an inductor L10 and a second BIAS voltage BIAS 2; one end of an inductor L9 and one end of an inductor L10 are respectively connected to two ends of the capacitor C6, and the other end of the inductor L9 and the other end of the inductor L10 are both connected with a second direct-current BIAS voltage BIAS 2; one end of the bypass capacitor C10 is connected with a second direct current BIAS voltage BIAS2, and the other end of the bypass capacitor C10 is grounded; when the second dc BIAS voltage BIAS2 is at a high level, turning on the output branch; when the second dc BIAS voltage BIAS2 is at a low level, turning off the output branch; the bypass capacitor C10 is used to filter out harmonics.
Further, the first BIAS voltage BIAS1 and the second dc BIAS voltage BIAS2 have opposite level signs. That is, when BIAS1 is a positive voltage, BIAS2 is a negative voltage, the input branch is turned on and the output branch is turned off, and vice versa.
Further, the first frequency selecting module comprises: a frequency-selecting inductor L1 and a frequency-selecting inductor L2; the inductance values of the frequency-selecting inductor L1 and the frequency-selecting inductor L2 are the same; the frequency-selecting inductor L1 is connected between one end of the capacitor C1 and ground, and the frequency-selecting inductor L2 is connected between the other end of the capacitor C1 and ground.
The input impedance of the duplex circuit is changed under different frequencies by changing the sizes of the frequency-selecting inductor L1 and the frequency-selecting inductor L2, so that the impedance matching condition is changed under different frequency conditions, the center frequency of a passband working under the input state of the duplex circuit is changed, the purpose of center frequency transfer is achieved, and the problem of fixed frequency band is solved.
Further, the second frequency selecting module comprises: the inductance values of the frequency-selecting inductor L7 and the frequency-selecting inductor L8, the inductance values of the frequency-selecting inductor L7 and the frequency-selecting inductor L8 are the same; the frequency-selecting inductor L7 is connected between one end of the capacitor C5 and ground, and the frequency-selecting inductor L8 is connected between the other end of the capacitor C5 and ground.
The output impedance of the duplex circuit changes under different frequencies by changing the sizes of the frequency-selecting inductor L7 and the frequency-selecting inductor L8, so that the impedance matching condition changes under different frequencies, the center frequency of a passband of the duplex circuit working under the output state changes, the purpose of center frequency transfer is achieved, and the problem of fixed frequency band is solved.
The inductance values of the frequency-selecting inductor L1, the frequency-selecting inductor L2, the frequency-selecting inductor L7 and the frequency-selecting inductor L8 are the same.
Still further, the radio frequency duplexer further includes: two pairs of oppositely disposed PIN diodes D1, D2, D3, D4, and a pair of oppositely disposed PIN diodes D5 and D6; after being connected in parallel, the PIN diodes D1 and D2 are connected in series with the parallel connection PIN diodes D3 and D4 in reverse direction at two ends of the capacitor C2, and the PIN diodes D5 and D6 are connected in reverse direction at two ends of the capacitor C6 in series.
The invention has the following technical advantages:
(1) the invention realizes the performance requirements of high central frequency and wide frequency band, and improves the compatibility of the pulse nuclear magnetic resonance experiment. Compared with the traditional nuclear magnetic resonance duplexer, the inductor and capacitor coupling frequency selection is adopted, the defect of narrow bandwidth of 1/4 wavelength lines is overcome, the large-bandwidth radio frequency duplexer with the center frequency above 1GHz and the frequency band around 700MHz is obtained, and the pulse nuclear magnetic resonance experiment with the frequency band becoming large due to large magnetic field fluctuation has good compatibility.
(2) The invention realizes the performance requirement of low input insertion loss and adjustable center frequency, and has good compatibility to various sample experiments. Compared with the traditional nuclear magnetic resonance duplexer, the PIN diode is used as a switching device for switching the circuit state, the input insertion loss is optimized to be below 0.3dB from below 1dB, more than 93% of input radio frequency can reach the Coil of the probe, and more effective energy is added to a sample in a short time. In addition, the resonance center frequency can be changed by changing the size of an inductance access circuit in the frequency selection module in the schematic diagram, so that the working center frequency of the duplexer can be adjusted, the working center frequency of the duplexer can be conveniently adjusted when the resonance frequency is changed due to the replacement of an experimental sample, and the input insertion loss near the resonance center frequency can be ensured to be kept at a lower value.
(3) The invention can effectively reduce the cost of the pulse NMR system and improve the cost performance. By reducing the input insertion loss of the duplexer, the output power capacity of the transmitting end radio frequency power amplifier can be reduced under the condition that the power transmitted to the probe coil is the same, so that the cost is saved to a certain extent. In addition, through the frequency selection function of the frequency selection module, the trouble caused by the fact that the duplexer needs to be replaced when a sample is replaced and the cost of the duplexer is increased are avoided.
Drawings
Fig. 1 is a schematic block diagram of a low insertion loss tunable wideband pulse NMR radio frequency duplexer provided in an embodiment of the present invention;
fig. 2 is a schematic circuit diagram of a low insertion loss tunable wideband pulse NMR radio frequency duplexer according to an embodiment of the present invention;
fig. 3 is a schematic diagram illustrating a comparison between a voltage standing wave ratio and a frequency relationship curve of a radio frequency input port when radio frequency excitation is added and inductance values in a frequency selection module are different according to an embodiment of the present invention;
fig. 4 is a comparison graph of relationship curves between insertion loss and frequency of the probe coil and the radio frequency input port when the frequency selection module has different inductance values when the radio frequency excitation is added according to the embodiment of the present invention;
fig. 5 is a graph comparing the isolation of the output and rf input ports with the frequency relationship curve when the inductance values of the frequency-selective modules are different when rf excitation is added according to the embodiment of the present invention;
fig. 6 is a comparison graph of insertion loss and frequency relationship between a probe coil and an output port when an NMR probe receives an FID signal under the condition that inductance values in the frequency selection module are different according to the embodiment of the present invention.
The names of each element and each port in the figure are respectively: l1, L2, L7 and L8 are frequency-selective inductors, L5, L6, L11 and L12 are tuning inductors, L3, L4, L9 and L10 are BIAS loop choke inductors, C1, C2, C3, C5, C6 and C7 are all dc blocking capacitors, C4 and C8 are port impedance parallel matching capacitors, C8 and C8 are powered by bypass capacitors, S8 and S8 are radio frequency MOS transistor switches, D8 are PIN diodes, BIAS 8 and BIAS voltage of the PIN diodes, BIAS 8 and BIAS 8 are a pair of opposite control signals, MLIN 8 and mtin 72 are microstrip lines with European character type characteristics of microstrip lines 50; the RFin port is an output port of the radio frequency amplifier, namely an input port of the duplexer, the Coil is a probe Coil, and the OUT port is an output port of the duplexer, namely an input port of the low noise amplifier; the device comprises a first direct current bias module 1, a second direct current bias module 2, a first frequency selection module 3, a second frequency selection module 4 and a probe coil 5.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The invention can solve the problems of small center frequency and fixed frequency band through the first frequency selection module and the second frequency selection module; the problems of narrow frequency band and large insertion loss can be solved by designing inductance and capacitance parameters in the circuit and utilizing two pairs of PIN diodes to be connected in parallel in an opposite direction. The problem of fixing the frequency band is that the center frequency of the pass band of the circuit is shifted by changing the value of the frequency-selecting inductor and changing the impedance value of the circuit at different frequencies (for example, increasing the value of the frequency-selecting inductor can make the impedance value of the circuit meet the impedance matching requirement at a place with a small frequency, and then VSWR and insertion loss move relatively to a place with a small frequency). The problem of small center frequency is mainly that the impedance of the duplex circuit meets the requirement of impedance matching under the condition of high frequency by selecting small capacitance and proper inductance, so that the center frequency of the circuit operation is at the high frequency.
In the embodiment of the invention, the value of the equivalent resistance accessed by the PIN diode in the circuit can be reduced through the parallel connection of the PIN diode, so that the insertion loss is reduced; meanwhile, because the duplex circuit does not adopt the conventional 1/4 wavelength line, the frequency band of the duplex circuit is not limited by the 1/4 wavelength line, and the obtained duplex frequency band is larger than that of the conventional duplex circuit. In addition, the access of the 1/4 wavelength line also introduces an increase in insertion loss. In a word, the duplex circuit avoids the influence brought by the 1/4 wavelength line, and the inductor and the capacitor with proper sizes are selected, so that the impedance matching effect of the duplex circuit obtained in a wider frequency band is better, and the VSWR and the insertion loss of the duplex circuit are relatively smaller.
Fig. 1 shows a schematic block diagram of a low-insertion-loss tunable wideband pulse NMR radio frequency duplexer provided by an embodiment of the present invention, and for convenience of illustration, only the parts related to the embodiment of the present invention are shown, which is detailed as follows:
the low insertion loss adjustable frequency formula broadband pulse NMR radio frequency duplexer includes: the duplexer comprises a first direct current bias module 1, a second direct current bias module 2, a first frequency selection module 3, a second frequency selection module 4 and a probe coil 5, wherein the input end of the first direct current bias module 1 is used as the radio frequency input end of the duplexer, the first direct current bias module 1 is used for providing direct current bias for PIN diodes D1-D4, and the on and off states of an input branch circuit are changed by changing the magnitude of bias voltage; the input end of the first frequency selection module 3 is connected to the output end of the first direct current bias module 1, and the first frequency selection module 3 is used for changing the equivalent input impedance of the duplex circuit, so that the duplex circuit obtains impedance matching under different frequencies, and the purpose of transferring the center frequency of the passband of the duplex circuit is achieved; the input end of the probe coil 5 is connected to the output end of the first frequency selection module 3, and the probe coil 5 is used for receiving an input radio frequency signal and detecting a generated FID signal; the input end of the second frequency selection module 4 is connected to the output end of the probe coil 5, the second frequency selection module 4 is used for changing the equivalent output impedance of the duplex circuit, and the equivalent output impedance is the same as the element parameters in the first frequency selection module 3, so that the duplex circuit meets the impedance matching requirement in a certain range of output signal frequency, and the FID signal detected by the coil is transmitted to an output port as far as possible; the input end of the second dc bias module 2 is connected to the output end of the second frequency selecting module 4, the output end of the second dc bias module 2 serves as the output end of the duplexer, the second dc bias module 2 is used for providing dc bias for the PIN diodes D5-D6, the function of the second dc bias module is the same as that of the first dc bias module 1, and the on and off states of the output branches are changed by changing the magnitude of the bias voltage.
The radio frequency input end of the duplexer is used for being connected with the front radio frequency power amplifier, and the output end of the duplexer is an output end of the probe coil after detecting the FID signal and is used for being connected with the input end of the rear-stage signal amplifier.
Fig. 2 shows a specific circuit structure of a low insertion loss frequency-tunable broadband pulse NMR radio frequency duplexer provided in an embodiment of the present invention, wherein the first dc bias module 1 and the second dc bias module 2 both implement switching on and off of radio frequency input to a probe coil branch and a probe coil to output end branch by adding a dc bias control signal; specifically, the method comprises the following steps:
the first dc bias module 1 includes: capacitor C2, capacitor C9, inductor L3, inductor L4 and first BIAS voltage BIAS1, one end of inductor L3 and one end of inductor L4 are respectively connected with capacitor C2, the other ends are mutually connected to first BIAS voltage BIAS1, one end of bypass capacitor C9 is connected with direct current BIAS, and the other end is connected with ground. When the first BIAS voltage BIAS1 is at a high level, the dc BIAS current reaches the PIN diodes D1, D2, D3, and D4 through the inductor L3 and the inductor L4, so that the PIN diodes D1, D2, D3, and D4 are forward conducted, and the input branch is conducted; conversely, when the dc bias voltage is low, the low level is applied across the capacitor C2 to reverse the blocking of the PIN diodes D1, D2, D3, D4, thereby turning off the input branch. The bypass capacitor C9 mainly functions to filter out harmonics.
The second dc bias module 2 includes: the capacitor C6, the capacitor C10, the inductor L9, the inductor L10 and a second BIAS voltage BIAS2, one end of the inductor L9 and one end of the inductor L10 are respectively connected with the capacitor C6, the other ends of the inductor L9 and the inductor L10 are mutually connected with the second BIAS voltage BIAS2, one end of the bypass capacitor C10 is connected with direct current BIAS, and the other end of the bypass capacitor C10 is connected with the ground. When the second BIAS voltage BIAS2 is at a high level, the dc BIAS current reaches the PIN diodes D5 and D6 through the inductor L9 and the inductor L10, so that the PIN diodes D5 and D6 are forward conducted, and the output branch is conducted; conversely, when the dc bias voltage is low, a low level is applied across the capacitor C6 to reverse the PIN diodes D5 and D6, thereby turning off the output branch. The bypass capacitor C10 mainly functions to filter out harmonics. The given levels of the second dc BIAS voltage BIAS2 and the first dc BIAS voltage BIAS1 have opposite signs, that is, when the first dc BIAS voltage BIAS1 is a forward voltage, the second dc BIAS voltage BIAS2 is a negative voltage, the input branch is turned on, the output branch is turned off, otherwise, the input branch is turned off, and the output branch is turned on.
The first frequency selection module 3 and the second frequency selection module 4 can realize the transfer of the center frequency and solve the problem of fixed frequency band by changing the parameter values of elements in the frequency selection modules; specifically, the method comprises the following steps:
the first frequency selection module 3 includes: frequency-selecting inductor L1 and frequency-selecting inductor L2, and the two inductance values are the same, wherein one end of frequency-selecting inductor L1 and one end of frequency-selecting inductor L2 are connected with capacitor C1 respectively, and the other end is grounded. By changing the size of the frequency-selecting inductor L1 and the frequency-selecting inductor L2, the input impedance of the duplex circuit is changed under different frequencies, so that the impedance matching condition is changed under different frequencies, the center frequency of a working passband of the duplex circuit under the input state is changed, the purpose of center frequency transfer is achieved, and the problem of fixed frequency band is solved.
The second frequency selection module 4 includes: frequency-selecting inductor L7 and frequency-selecting inductor L8, and the inductance values are the same as frequency-selecting inductor L1 and frequency-selecting inductor L2, i.e. symmetrical to first frequency-selecting module 3. One end of the frequency-selecting inductor L7 and one end of the frequency-selecting inductor L8 are respectively connected with the capacitor C5, and the other end of the frequency-selecting inductor L8 is grounded. By changing the size of the frequency-selecting inductor L7 and the frequency-selecting inductor L8, the output impedance of the duplex circuit changes under different frequencies, so that the impedance matching condition changes under different frequencies, the center frequency of a passband of the duplex circuit working under the output state changes, the purpose of center frequency transfer is achieved, and the problem of fixed frequency band is solved. Since the first frequency selecting module 3 and the second frequency selecting module 4 are symmetrical, the center frequency of the pass band and the operating frequency band are substantially identical in the input/output operating state, as shown in fig. 4 and 6.
The (Coil) port of the probe Coil 5 can change the state of the Coil for receiving and detecting the generated signal through the on and off of the input and output branches. In NMR, a probe coil can apply a radio frequency signal to a sample atomic nucleus when an input branch is conducted; when the input branch is turned off and the output branch is turned on, an FID signal can be induced in the coil, and the FID signal can be transmitted to the output end.
In the embodiment of the invention, the switching of the on and off states of the input and output branches is changed by diode BIAS voltages BIAS1 and BIAS2, so that the diodes are conducted when the high level is provided, and the corresponding branches are also conducted; for low levels the diodes are turned off and the corresponding branch is turned off. BIAS1 is high, BIAS2 is low, the input branch is on, the coil receives the input rf signal, and conversely, the output branch is on, and the coil outputs the detected FID signal.
In the embodiment of the invention, two pairs of PIN diodes D1, D2, D3 and D4 placed in opposite directions are utilized in the first direct current BIAS module 1 to control the switch switching state of the branch circuit by changing the size of the first BIAS voltage BIAS1 in the first direct current BIAS module 1; in the second dc BIAS block 2, the switching state of the legs is controlled by varying the magnitude of the second BIAS voltage BIAS2 in the dc BIAS block 2 using a pair of oppositely placed PIN diodes D5 and D6. The two pairs of PIN diodes are connected in parallel, so that the insertion loss from the radio frequency input port to the probe coil port is reduced to a certain extent, and most of radio frequency power is ensured to be applied to a sample; a pair of PIN diodes is adopted from the probe coil port to the output port, so that the isolation degree of the radio frequency input port is increased to a certain degree, and the low-noise amplifier at the output port is prevented from being damaged by leaked power.
The input impedance of the port is adjusted at the radio frequency input end and the radio frequency output end through pF-level small capacitors C4 and C8, so that the impedance of the port is matched to be close to 50 ohms as much as possible, a low voltage standing wave ratio is kept, meanwhile, the leaked energy is transferred by using the switch S1 and the switch S2 in parallel connection with the radio frequency input port and the radio frequency output port, and the duplex isolation degree is further optimized. And the DC blocking capacitor C3 and the DC blocking capacitor C7 prevent the DC bias signal from damaging the preposed power amplifier of the radio frequency input end and the post-stage signal amplifier of the output end at the radio frequency input end and the output end. In the first frequency selection module 3 and the second frequency selection module 4, the dc blocking capacitor C1 and the dc blocking capacitor C5 prevent a dc bias signal from entering the probe coil, and the transfer of the operating center frequency of the duplexer can be achieved by changing the values of the frequency selection inductor L1, the frequency selection inductor L2, the frequency selection inductor L7, and the frequency selection inductor L8.
In the duplex circuit, the inductors L1, L2, L5, L6, L7, L8, L11 and L12 not only provide a loop for a direct current bias signal, but also are coupled with the capacitors C1, C3, C5 and C7 to determine the center frequency and frequency band of duplex. The microstrip lines MLIN 1-MLIN 6 and MTEE provide spacing for placement of components, facilitate soldering of components and ensure heat dissipation from diodes and other components when rf power signals are added.
To further illustrate the low insertion loss tunable wideband pulse NMR rf duplexer provided by the embodiments of the present invention, the operation of the duplexer will be detailed as follows with reference to the accompanying drawings:
firstly, a PIN diode is controlled by a first BIAS voltage BIAS1 and a second BIAS voltage BIAS2 to connect a radio frequency input port to a probe coil port branch and cut off the radio frequency input port to an output port branch, meanwhile, a switch S1 is opened, a switch S2 is turned off, and the switch S1 is connected with an output port in parallel, so that a power radio frequency signal input by the radio frequency input port can be applied to the probe coil. The first frequency selection module 3 and the second frequency selection module 4 are formed by connecting a plurality of inductors in series, and the effective values of the inductors L1, L2, L7 and L8 in the first frequency selection module 3 and the second frequency selection module 4 which are connected into a duplex circuit can be changed by changing the node positions of the series inductors connected into the circuit ground, so that the resonance frequency is changed to obtain the high-frequency and large-bandwidth radio frequency duplexer with different center frequencies.
As shown in fig. 3, the simulation diagram is obtained when equivalent inductance values of the inductor L1, the inductor L2, the inductor L7 and the inductor L8 are different when the circuit is connected to the circuit, wherein a solid line indicates that values of the inductor L1, the inductor L2, the inductor L7 and the inductor L8 are first inductance values, and a dotted line indicates that values of the inductor L1, the inductor L2, the inductor L7 and the inductor L8 are second inductance values; and the first inductance value is smaller than the second inductance value, because the accessed inductance value is increased, the center frequency of the passband is reduced, the center frequency of the duplexer is converted from about 1.3GHz to about 1.1GHz, and the change of the center frequency in a certain range can ensure that the voltage standing wave ratio of the radio frequency input port is below 1.3 (the transmitting power accounts for 98 percent), and the covered bandwidth is 700 MHz.
Insertion losses of the radio frequency input port and the probe coil port are below 0.3dB when radio frequency excitation is added as shown in fig. 4, wherein a solid line indicates that values of the inductance L1, the inductance L2, the inductance L7, and the inductance L8 are first inductance values, and a dotted line indicates that values of the inductance L1, the inductance L2, the inductance L7, and the inductance L8 are second inductance values; and the first inductance value is less than the second inductance value.
As shown in fig. 5, the isolation between the output port and the rf input port can be below 52dB when the rf excitation is added, and it can be ensured that the peak voltage of the rf input port is limited below 1V when the rf excitation is added, where the solid line represents that the values of the inductor L1, the inductor L2, the inductor L7, and the inductor L8 are the first inductance value, the dotted line represents that the values of the inductor L1, the inductor L2, the inductor L7, and the inductor L8 are the second inductance value, and the first inductance value is smaller than the second inductance value. When the radio frequency excitation stops adding, the switch S1 is turned off, the switch S2 is turned on, meanwhile, the branch from the probe coil port to the output port is conducted through the first BIAS voltage BIAS1 and the second BIAS voltage BIAS2, the branch from the probe coil port and the output port is turned off, and the output port outputs an FID signal.
As shown in fig. 6, when outputting the FID signal, the insertion loss of the probe coil port and the output port is below 1dB, and the FID signal with strength of 80% or more can be ensured to be input to the low noise amplifier, where the solid line indicates that the values of the inductance L1, the inductance L2, the inductance L7, and the inductance L8 are the first inductance value, and the dotted line indicates that the values of the inductance L1, the inductance L2, the inductance L7, and the inductance L8 are the second inductance value; and the first inductance value is less than the second inductance value. In addition, since the switching speed of the PIN diode depends on the carrier lifetime and the magnitude of the dc bias current, the switching time of the duplexer can be controlled within 1 μ s by selecting the proper PIN diode and dc bias current.
Therefore, the radio frequency duplexer realizes the performances that the switching speed is within 1 mu s, the standing wave ratio of the radio frequency input voltage is below 1.3, the input insertion loss is below 0.3dB, the isolation is above 52dB, the working center frequency can be adjusted within the range of 1GHz to 1.6GHz (only two conditions are given in the drawing), the working frequency band is about 700MHz and the like, meets the requirements of the radio frequency duplexer in a pulse NMR system, and effectively reduces the cost of the pulse NMR system.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (10)
1. A low insertion loss frequency-tunable broadband pulse NMR radio frequency duplexer is characterized by comprising: the probe comprises a first direct current bias module (1), a second direct current bias module (2), a first frequency selection module (3), a second frequency selection module (4) and a probe coil (5);
the input end of the first direct current bias module (1) is used as the radio frequency input end of the duplexer, the first direct current bias module (1) is used for providing direct current bias and changing the on-off state of an input branch by changing the magnitude of bias voltage;
the input end of the first frequency selection module (3) is connected to the output end of the first direct current bias module (1), and the first frequency selection module (3) is used for enabling the duplex circuit to obtain impedance matching under different frequencies by changing the equivalent input impedance of the duplex circuit, so that the transfer of the center frequency of the passband of the duplex circuit is realized;
the input end of the probe coil (5) is connected to the output end of the first frequency selection module (3), and the probe coil (5) is used for receiving an input radio frequency signal and detecting a generated FID signal;
the input end of the second frequency selection module (4) is connected to the output end of the probe coil (5), the second frequency selection module (4) is used for enabling the duplex circuit to meet the impedance matching requirement in a certain range of output signal frequency by changing the equivalent output impedance of the duplex circuit, and transmitting the FID signal detected by the coil to an output port;
the input end of the second direct current bias module (2) is connected to the output end of the second frequency selection module (4), the output end of the second direct current bias module (2) serves as the output end of the duplexer, and the second direct current bias module (2) is used for providing direct current bias and changing the on and off states of the output branch circuit by changing the magnitude of bias voltage.
2. The radio frequency duplexer according to claim 1, characterized in that the first dc bias module (1) comprises: a capacitor C2, a bypass capacitor C9, an inductor L3, an inductor L4 and a first BIAS voltage BIAS 1;
one end of the inductor L3 and one end of the inductor L4 are respectively connected to two ends of the capacitor C2, and the other end of the inductor L3 and the other end of the inductor L4 are both connected to the first dc BIAS voltage BIAS 1; one end of the bypass capacitor C9 is connected to the first DC BIAS voltage BIAS1, and the other end of the bypass capacitor C9 is grounded;
turning on the input branch when the first dc BIAS voltage BIAS1 is high; turning off the input branch when the first dc BIAS voltage BIAS1 is low; the bypass capacitor C9 is used to filter out harmonics.
3. The radio frequency duplexer of claim 1 or 2, wherein the second direct current biasing module (2) comprises: a capacitor C6, a capacitor C10, an inductor L9, an inductor L10 and a second BIAS voltage BIAS 2;
one end of the inductor L9 and one end of the inductor L10 are respectively connected to two ends of the capacitor C6, and the other end of the inductor L9 and the other end of the inductor L10 are both connected to the second dc BIAS voltage BIAS 2;
one end of the bypass capacitor C10 is connected to the second dc BIAS voltage BIAS2, and the other end of the bypass capacitor C10 is grounded;
when the second dc BIAS voltage BIAS2 is at a high level, turning on the output branch; when the second dc BIAS voltage BIAS2 is at a low level, turning off the output branch; the bypass capacitor C10 is used to filter out harmonics.
4. The radio frequency duplexer of claim 3, wherein the levels of the first BIAS voltage BIAS1 and the second DC BIAS voltage BIAS2 are opposite in sign.
5. A radio frequency duplexer according to any one of claims 1-4, wherein the first frequency-selective module (3) comprises: a frequency-selecting inductor L1 and a frequency-selecting inductor L2;
the inductance values of the frequency-selecting inductor L1 and the frequency-selecting inductor L2 are the same;
the frequency-selecting inductor L1 is connected between one end of the capacitor C1 and the ground, and the frequency-selecting inductor L2 is connected between the other end of the capacitor C1 and the ground.
6. The radio frequency duplexer of claim 5, wherein the input impedance of the duplexer circuit is changed at different frequencies by changing the sizes of the frequency-selecting inductor L1 and the frequency-selecting inductor L2, so that the impedance matching condition is changed at different frequencies, thereby changing the center frequency of the passband operating at the input state of the duplexer circuit, achieving the purpose of center frequency shift and solving the problem of frequency band fixation.
7. A radio frequency duplexer according to any one of claims 1-6, wherein the second frequency-selective module (4) comprises: a frequency-selecting inductor L7 and a frequency-selecting inductor L8,
the inductance values of the frequency-selecting inductor L7 and the frequency-selecting inductor L8 are the same;
the frequency-selecting inductor L7 is connected between one end of the capacitor C5 and the ground, and the frequency-selecting inductor L8 is connected between the other end of the capacitor C5 and the ground.
8. The radio frequency duplexer of claim 7, wherein the output impedance of the duplexer circuit is changed at different frequencies by changing the sizes of the frequency-selecting inductor L7 and the frequency-selecting inductor L8, so that the impedance matching condition is changed at different frequencies, thereby changing the center frequency of the passband of the duplexer circuit operating in the output state, achieving the purpose of center frequency shift and solving the problem of frequency band fixation.
9. The radio frequency duplexer of claim 7 or 8, wherein inductance values of the frequency-selecting inductor L1, the frequency-selecting inductor L2, the frequency-selecting inductor L7 and the frequency-selecting inductor L8 are the same in magnitude.
10. The radio frequency duplexer of any one of claims 1-9, further comprising: two pairs of oppositely disposed PIN diodes D1, D2, D3, D4, and a pair of oppositely disposed PIN diodes D5 and D6;
after being connected in parallel, the PIN diodes D1 and D2 are connected in series with the parallel connection PIN diodes D3 and D4 in reverse direction at two ends of the capacitor C2, and the PIN diodes D5 and D6 are connected in reverse direction at two ends of the capacitor C6 in series.
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