External biasing device for low-temperature probe and de-embedding calibration measurement method

Abstract

The invention discloses an external bias device for a low-temperature probe and further discloses a de-embedding calibration measurement method of the external bias device for the low-temperature probe, the external bias device comprises a radio frequency test instrument, an external bias unit, a bias feed network and a low-temperature magnetic resonance probe, the external bias unit is used for providing direct-current bias voltage, and the bias feed network is used for providing direct-current bias voltage; the radio frequency test instrument is used for generating a test signal in the low-temperature magnetic resonance probe, and a direct-current bias signal output by the external bias unit forces the PIN diode to be converted from a high-resistance isolation state to a low-resistance conduction state; then the radio frequency testing instrument is started, milliwatt-level signals generated by the radio frequency testing instrument can easily reach the radio frequency coil through the passive T / R switch, so that the radio frequency coil is tested, the passive T / R switch is switched on under the conditions that the packaging structure of the low-temperature magnetic resonance probe is not damaged and an internal circuit is not changed, and the low-temperature magnetic resonance probe is tested. Therefore, a radio frequency test channel is established.

Description

External biasing device and de-embedding calibration measurement method for cryogenic probes Technical Field

[0001] This invention belongs to the field of magnetic resonance radio frequency probe testing technology, specifically relating to an external bias device for low-temperature probes, and also to a de-embedding calibration measurement method for an external bias device for low-temperature probes. Background Technology

[0002] As a core component of an MRI system, the performance of the radio frequency (RF) coil directly affects the system's signal-to-noise ratio. Low-temperature probe technology significantly suppresses thermal noise and greatly improves the signal-to-noise ratio by placing the RF coil and its related circuit components in a low-temperature environment.

[0003] In the design of cryogenic probes, in order to reduce thermal load and power consumption, passive T / R switches composed of anti-parallel diodes, quarter-wavelength lines or equivalent lumped elements are usually used. The working principle is as follows: during the transmission phase, the high-power radio frequency pulse output by the magnetic resonance system can instantly forward bias the diode to make it conduct, forming a transmission path; while during the reception phase, since there is no high-power excitation, the diode is in a zero-bias or weak reverse-bias state, presenting high impedance, thereby isolating the transmitter and guiding the weak NMR signal to a low-noise preamplifier.

[0004] However, this passive design presents significant challenges in the manufacturing and quality control of the probe. Once the cryogenic probe is assembled and vacuum-sealed (encapsulated), its internal radio frequency circuitry is physically isolated from the outside world. At this point, if a low-power test signal (typically -30dBm) is input from an external port using a standard test instrument such as a network analyzer (VNA), the power of this signal is far from sufficient to turn on the diode in the passive T / R switch. In this case, the passive T / R switch is in a high blocking state, preventing the test signal from reaching the radio frequency coil. This makes it impossible for production personnel to measure and adjust key electrical performance indicators such as the probe's resonant frequency, matching status (S11 parameters), and quality factor (Q value) in the final packaged state.

[0005] To address this issue, existing technologies attempt to forcibly activate the internal switch by injecting an external DC bias. However, due to the probe's encapsulation, the DC bias must be injected via a long-distance cable or an external power supply network. The unavoidable introduction of current-limiting inductors, DC-blocking capacitors, and PCB parasitic parameters creates a complex impedance network. The drawback of existing technologies is that the parasitic capacitance of the external choke inductor at high frequencies, the insertion loss of the DC-blocking capacitor, and the phase delay of the connecting cable all directly contribute to the measurement results. Experiments show that an uncalibrated external bias circuit not only leads to a lower measured Q value but also causes a significant drift in the resonant frequency and a severe deviation in the impedance matching (S11) characteristics. If these spurious changes in frequency and matching are not corrected, they will lead to misjudgments of the probe's status by production personnel, severely impacting the actual testing performance and imaging quality of the cryogenic probe in the NMR system. Summary of the Invention

[0006] The purpose of this invention is to provide an external biasing device for cryogenic probes, and also to provide a de-embedding calibration measurement method for the external biasing device for cryogenic probes. This aims to solve the technical problems of cryogenic probes being unable to measure due to passive switching off, and the significant parasitic impedance introduced by the external biasing power supply network leading to serious distortion of measurement results and misjudgment of parameters.

[0007] The above-mentioned objective of the present invention is achieved by the following technical means: an external bias device for a low-temperature probe, comprising an RF test instrument, an external bias unit, a bias feed network, and a low-temperature magnetic resonance probe, wherein the bias feed network comprises a DC input terminal, an AC input terminal, and an AC / DC output terminal; the external bias unit is connected to the DC input terminal of the bias feed network; the RF test instrument is connected to the AC input terminal of the bias feed network; and the AC / DC output terminal of the bias feed network is connected to the RF port of the low-temperature magnetic resonance probe.

[0008] As described above, a current-limiting resistor R1 and an RF choke inductor L1 are provided between the DC input terminal and the AC / DC output terminal in the bias power supply network, and a DC blocking capacitor C1 is provided between the AC input terminal and the AC / DC output terminal in the bias power supply network.

[0009] As described above, the bias power supply network is also equipped with filter capacitors C2 and C3. One end of filter capacitors C2 and C3 is grounded, and the other end is connected to the DC input terminal.

[0010] As mentioned above, the low-temperature magnetic resonance probe includes a passive T / R switch and a radio frequency coil.

[0011] A de-embedding calibration measurement method for an external bias device used in cryogenic probes includes the following steps: Step S1, constructing a feature extraction circuit for the bias feed network; Step S2, measuring the S-parameter matrix of the bias feed network based on the feature extraction circuit. And use it as the error matrix; establish the error model: use the error matrix Convert to transmission matrix Step S3: Connect the external bias unit to the DC input terminal of the bias feed network, connect the RF test instrument to the AC input terminal of the bias feed network, connect the AC / DC output terminals of the bias feed network to the RF port of the low-temperature magnetic resonance probe, and turn on the external bias unit; Step S4: Measure the total scattering parameters using the RF test instrument. and the total scattering parameters Convert to total transmission matrix Step S5: Based on the overall transmission matrix and transmission matrix Calculate the true transmission matrix of the radio frequency coil inside the cryogenic magnetic resonance probe. Step S6: Transfer the actual transmission matrix Convert to the real S-parameter matrix .

[0012] As described above, the actual transmission matrix Calculated based on the following formula: In the formula, For the transmission matrix The inverse matrix.

[0013] The feature extraction circuit described above is as follows: Using the dual-port mode of the RF test instrument, the two test ports of the dual-port RF test instrument are connected to the AC input and AC / DC output terminals of the bias feed network, respectively. The DC input terminal of the bias feed network is left floating. Specifically, port P1 of the RF test instrument is connected to the AC input terminal of the bias feed network, and port P2 of the RF test instrument is connected to the AC / DC output terminals of the bias feed network; transmission matrix Based on the following formula: In the formula, S-parameter matrix The reflection coefficient of the middle port P1, S-parameter matrix The backward transmission coefficient from port P2 to port P1. S-parameter matrix The forward transmission coefficient from port P1 to port P2. S-parameter matrix The reflection coefficient of the middle port P2.

[0014] A computer device includes a memory and a processor, the memory storing a computer program, characterized in that the processor executes the computer program to implement steps S2 and S4-S6 of the de-embedding calibration measurement method for an external bias device for a cryogenic probe as described above.

[0015] A computer-readable storage medium having a computer program stored thereon, characterized in that, when the computer program is executed by a processor, it implements steps S2 and S4-S6 of the de-embedding calibration measurement method for an external bias device for a cryogenic probe as described above.

[0016] A computer program product includes a computer program, characterized in that, when executed by a processor, the computer program implements steps S2 and S4-S6 of the de-embedding calibration measurement method for an external bias device for a cryogenic probe as described above.

[0017] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention realizes post-packaging testing: The present invention successfully solves the problem of "test blind spot" by forcibly turning on the internal passive switch by applying an external bias voltage, so that the fully packaged low temperature probe can be tested with standard instruments such as network analyzers to accurately measure electrical performance such as S11 (reflection coefficient).

[0018] (2) The present invention achieves non-destructive and non-invasive testing: The present invention only introduces a DC bias from the outside during testing, without making any physical modifications to the core radio frequency circuit (passive T / R switch, coil, preamplifier) ​​inside the low temperature probe, and completely maintains its original design and performance; it can perform comprehensive electrical performance verification of the product at the final stage of the production line, promptly identify problems, greatly reduce rework costs, and improve product yield and reliability.

[0019] (3) The solution of the present invention is simple and low cost: it can be realized with only an external DC power supply and a standard bias feed network. The technology is mature, the cost is low, and it is easy to deploy in production and maintenance. Attached Figure Description

[0020] Figure 1 is a schematic diagram of the radio frequency front end of a cryogenic probe with a built-in passive T / R switch in the prior art; Figure 2 is a schematic diagram of the connection of the external bias unit, bias feed network, cryogenic magnetic resonance probe, and network analyzer VNA in Embodiment 1 of the present invention; Figure 3 is a flowchart of the de-embedding calibration measurement method in Embodiment 2 of the present invention; Figure 4 is a schematic diagram of the connection of the feature extraction circuit of the bias feed network in Embodiment 2 of the present invention; Reference numerals and corresponding component names: 10-Cryogenic vacuum chamber; 11-Passive T / R switch; 20-RF port; 30-External bias unit; 40-Bias feed network. Detailed Implementation

[0021] To facilitate understanding and implementation of the present invention by those skilled in the art, the present invention will be further described in detail below with reference to embodiments. The embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0022] Example 1: As shown in Figure 2, the external bias device for the cryogenic probe includes an RF test instrument (such as a network analyzer VNA), an external bias unit 30, a bias feed network 40, and a cryogenic magnetic resonance probe (i.e., a cryogenic probe). The bias feed network 40 includes a DC input terminal (DC port), an AC input terminal (RF port), and an AC / DC output terminal (DC+RF port). The external bias unit 30 is connected to the DC input terminal of the bias feed network 40, the RF test instrument is connected to the AC input terminal of the bias feed network 40, and the AC / DC output terminal of the bias feed network 40 is connected to the RF port 20 of the cryogenic magnetic resonance probe.

[0023] The external bias unit 30 is used to provide a DC bias voltage, and can be a DC power supply or a bias module with voltage scanning capability; the radio frequency test instrument is used to generate the test signal inside the low-temperature magnetic resonance probe, and can be a network analyzer VNA; as shown in Figure 1, in this embodiment, the low-temperature magnetic resonance probe includes a low-temperature vacuum chamber 10 and an internal radio frequency coil and a passive T / R switch 11.

[0024] The bias feed network 40 is located outside the cryogenic magnetic resonance probe and is a key component for realizing radio frequency and DC composite transmission. In one possible implementation, the bias feed network 40 includes a current-limiting resistor R1, a radio frequency choke inductor L1, two filter capacitors (C2 and C3), and a DC blocking capacitor C1. The filter capacitors C2 and C3, the current-limiting resistor R1, and the radio frequency choke inductor L1 are all located between the DC input terminal and the AC / DC output terminal of the bias feed network 40. The filter capacitors C2 and C3, the current-limiting resistor R1, and the radio frequency choke inductor L1 are arranged sequentially along the direction from the DC input terminal to the AC / DC output terminal of the bias feed network 40. One end of the filter capacitors C2 and C3 is grounded (GND), and the other end is connected to the DC input terminal of the bias feed network 40.

[0025] Filter capacitors C2 and C3 are used to suppress DC power supply ripple and transient signals, ensuring stable bias voltage and suppressing bias noise; current-limiting resistor R1 is used to protect the internal diodes of the low-temperature magnetic resonance probe from overcurrent, improving test safety; the impedance of RF choke inductor L1 is high enough in the RF band to block high-frequency signals (such as high-frequency AC test signals output by RF test instruments) from entering the bias voltage path; at the same time, it has low impedance to DC, allowing the bias voltage to be effectively injected into the low-temperature magnetic resonance probe; DC blocking capacitor C1 is used to block DC bias voltage from entering the RF test instrument, protecting the test instrument.

[0026] Because the diodes in the traditional passive T / R switch 11 require a high-power RF signal (tens of watts) emitted by the magnetic resonance imaging (MRI) equipment to conduct, while the network analyzer VNA only outputs a milliwatt-level signal, it cannot drive the diodes.

[0027] The testing device of the present invention first turns on the external bias unit 30. The DC signal output by the external bias unit 30 is filtered by two filter capacitors, and then passes through the current limiting resistor R1 and the RF choke inductor L1 to the AC / DC output terminal of the bias feed network 40. It is then input from the AC / DC output terminal and applied to the passive T / R switch 11 in the low-temperature vacuum cavity 10 inside the low-temperature magnetic resonance probe, forcing the PIN diode to change from a "high-impedance isolation" state to a "low-impedance conduction" state. Then, the RF test instrument is turned on. At this time, the milliwatt-level signal generated by the RF test instrument can easily reach the RF coil through the passive T / R switch 11, thereby testing the RF coil. This allows the passive T / R switch 11 to be turned on without damaging the packaging structure of the low-temperature magnetic resonance probe or modifying the internal circuit, thus establishing an RF test path.

[0028] The core switching element of the passive T / R switch 11 can be a PIN diode or a common high-speed switching diode (such as a PN junction diode like 1N4148); regardless of the type of diode used, the biasing mechanism of this invention is effective; the measuring device of this invention can turn on the passive T / R switch 11 without damaging the packaging structure of the low-temperature magnetic resonance probe or altering the internal circuitry, thereby establishing an RF test path.

[0029] Example 2: As shown in Figure 3, the de-embedding calibration measurement method for the external bias device of the low temperature probe includes the following steps: The test device in Example 1 successfully established a physical path, but due to the introduction of L1 (parallel shunt) and C1 (series voltage divider), the data measured by the VNA is actually a mixed response of the "bias feed network 40 + RF coil". At this time, the measured resonant frequency deviates from the true value by several MHz, and the amplitude of S11 is degraded. If this data is directly accepted, it will lead to misjudgment of quality. Therefore, as shown in Figure 3, this example proposes a de-embedding calibration measurement method to eliminate the measurement distortion introduced by the hardware circuit in the external bias device of Example 1.

[0030] Step S1: Construction of the feature extraction circuit for the bias feed network 40: As shown in Figure 4, before connecting the cryogenic probe, the bias feed network 40 is considered as a two-port device (DUT). Using the two-port mode of the RF test instrument, the two test ports of the two-port RF test instrument are connected to the AC input terminal and AC / DC output terminal of the bias feed network 40, respectively. The DC input terminal of the bias feed network 40 is left floating. The S-parameter matrix of the bias feed network 40 is then measured. As the error matrix, the S-parameter matrix This includes all non-ideal characteristics of L1, C1, and PCB traces.

[0031] The bias feed network 40 includes filter capacitors (C2, C3). The filter capacitors short-circuit the DC line to ground in the RF band. At this time, the external connection state (open circuit or short circuit) of the DC port is isolated by the capacitors and will not affect the measurement of RF S-parameters.

[0032] Step S2: Feature extraction of the bias feed network 40: Based on the feature extraction circuit built in step S1, the S-parameter matrix of the bias feed network 40 is measured and obtained. As the error matrix, the S-parameter matrix This includes all non-ideal characteristics of L1, C1, and PCB traces.

[0033] Establish an error model: [This involves] setting the error matrix... Convert to transmission matrix The conversion is based on the following formula: (1); where, S-parameter matrix The reflection coefficient of the middle port P1, S-parameter matrix The backward transmission coefficient from port P2 to port P1. S-parameter matrix The forward transmission coefficient from port P1 to port P2. S-parameter matrix The reflection coefficient of port P2; as shown in Figure 4, in this embodiment, port P1 of the RF test instrument is connected to the AC input terminal of the bias feed network 40, and port P2 of the RF test instrument is connected to the AC / DC output terminal of the bias feed network 40.

[0034] Step S3: Connect the external bias unit 30 to the DC input terminal of the bias feed network 40, connect the RF test instrument to the AC input terminal of the bias feed network 40, connect the AC / DC output terminals of the bias feed network 40 to the RF port 20 of the low-temperature magnetic resonance probe, and turn on the external bias unit 30. At this time, the diode is turned on, and an RF path is established.

[0035] Step S4: Measure the total scattering parameters using an RF testing instrument. and the total scattering parameters Convert to total transmission matrix Total transmission matrix This includes the error of the radio frequency coil response inside the cryogenic magnetic resonance probe and the error of the external bias feed network 40.

[0036] Step S5, De-embedding computation: Based on microwave cascade theory: Perform matrix inverse operation, remove error terms, and transmit the actual matrix. Calculated based on the following formula: (2); where, For the transmission matrix The inverse matrix.

[0037] Step S5 completely removes the physical interference of the external bias feed network 40 (especially the RF choke inductor L1 and DC blocking capacitor C1) on the measurement results numerically.

[0038] Step S6, Parameter Inversion: Convert the actual transmission matrix Convert to the real S-parameter matrix The resonant frequency, S11 curve, and Q value obtained at this time are the true performance indicators of the low-temperature magnetic resonance probe after eliminating external interference.

[0039] In one embodiment, a computer device is also provided, including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement steps S2 and S4-S6 in the above method embodiments.

[0040] In one embodiment, a computer-readable storage medium is provided having a computer program stored thereon, which, when executed by a processor, implements steps S2 and S4-S6 in the above method embodiments.

[0041] In one embodiment, a computer program product is provided, including a computer program that, when executed by a processor, implements steps S2 and S4-S6 in the above method embodiments.

[0042] It should be noted that the embodiments described in this invention are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains can make various modifications or additions to the described embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. An external biasing device for a cryogenic probe, comprising a radio frequency test instrument, an external biasing unit (30), a biasing feed network (40), and a cryogenic magnetic resonance probe, characterized in that, The bias feed network (40) includes a DC input terminal, an AC input terminal, and an AC / DC output terminal. An external bias unit (30) is connected to the DC input terminal of the bias feed network (40). The radio frequency test instrument is connected to the AC input terminal of the bias feed network (40). The AC / DC output terminal of the bias feed network (40) is connected to the radio frequency port (20) of the low-temperature magnetic resonance probe.

2. The external biasing device for a cryogenic probe according to claim 1, characterized in that, In the bias power supply network (40), a current-limiting resistor R1 and an RF choke inductor L1 are provided between the DC input terminal and the AC / DC output terminal, and a DC blocking capacitor C1 is provided between the AC input terminal and the AC / DC output terminal.

3. The external biasing device for a cryogenic probe according to claim 2, characterized in that, The bias power supply network (40) is also equipped with filter capacitor C2 and filter capacitor C3. One end of filter capacitor C2 and filter capacitor C3 is grounded, and the other end is connected to the DC input terminal.

4. The external biasing device for a cryogenic probe according to claim 3, characterized in that, The cryogenic magnetic resonance probe includes a passive T / R switch and a radio frequency coil.

5. A method for de-embedding calibration measurement of an external bias device for a cryogenic probe, characterized in that, Includes the following steps: Step S1: Construct the feature extraction circuit of the bias feed network (40); Step S2: Measure the S-parameter matrix of the bias feed network (40) based on the feature extraction circuit of the bias feed network (40). And use it as the error matrix; establish the error model: use the error matrix Convert to transmission matrix Step S3: Connect the external bias unit (30) to the DC input terminal of the bias feed network (40), connect the RF test instrument to the AC input terminal of the bias feed network (40), connect the AC / DC output terminals of the bias feed network (40) to the RF port (20) of the low-temperature magnetic resonance probe, and turn on the external bias unit (30); Step S4: Measure the total scattering parameters using the RF test instrument. and the total scattering parameters Convert to total transmission matrix Step S5: Based on the overall transmission matrix and transmission matrix Calculate the true transmission matrix of the radio frequency coil inside the cryogenic magnetic resonance probe. Step S6: Transfer the actual transmission matrix Convert to the real S-parameter matrix 。 6. The de-embedding calibration measurement method for an external bias device for a cryogenic probe according to claim 5, characterized in that, The real transmission matrix Calculated based on the following formula: In the formula, For the transmission matrix The inverse matrix.

7. The de-embedding calibration measurement method for an external bias device for a cryogenic probe according to claim 6, characterized in that, The feature extraction circuit is as follows: Using the dual-port mode of the RF test instrument, the two test ports of the dual-port RF test instrument are respectively connected to the AC input and AC / DC output terminals of the bias feed network (40), with the DC input terminal of the bias feed network (40) left floating. Specifically, port P1 of the RF test instrument is connected to the AC input terminal of the bias feed network (40), and port P2 of the RF test instrument is connected to the AC / DC output terminals of the bias feed network (40); transmission matrix Based on the following formula: In the formula, S-parameter matrix The reflection coefficient of the middle port P1, S-parameter matrix The backward transmission coefficient from port P2 to port P1. S-parameter matrix The forward transmission coefficient from port P1 to port P2. S-parameter matrix The reflection coefficient of the middle port P2.

8. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that, When the processor executes the computer program, it implements steps S2 and S4-S6 of the de-embedding calibration measurement method for an external bias device for a cryogenic probe as described in any one of claims 5 to 7.

9. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, it implements steps S2 and S4-S6 of the de-embedding calibration measurement method for an external bias device for a cryogenic probe as described in any one of claims 5 to 7.

10. A computer program product, comprising a computer program, characterized in that, When the computer program is executed by the processor, it implements steps S2 and S4-S6 of the de-embedding calibration measurement method for an external bias device for a cryogenic probe as described in any one of claims 5 to 7.

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