Intermediate frequency pre-processing circuit for a contactless vital signs sensor

Abstract

The application discloses a kind of intermediate frequency pre-processing circuit for contactless life signal sensor, the signal of the intermediate frequency pre-processing circuit is received after the signal of radio frequency circuit is processed and is transmitted to signal acquisition and control processor;Intermediate frequency pre-processing circuit includes low noise amplifier circuit, frequency gain compensation circuit and automatic level control circuit, low noise amplifier circuit receives the signal input of radio frequency circuit, and the intermediate frequency signal in signal is amplified and then input frequency gain compensation circuit, frequency gain compensation circuit is according to the frequency distribution of intermediate frequency signal, and different gain is given to different frequency components, and the amplitude of output signal is stabilized by automatic level control circuit.The intermediate frequency pre-processing circuit proposed in the application, as part of the sensor, through the frequency gain adaptive compensation of program control, can adjust the equalization of multiple target output signals in real time according to the actual application scene, so as to improve the detection performance of the sensor and the multi-scene adaptability without modifying the hardware.

Description

Technical Field

[0001] This invention belongs to the field of vital sign signal sensor technology, and particularly relates to an intermediate frequency preprocessing circuit for a non-contact vital sign sensor. Background Technology

[0002] In the field of modern medicine and health monitoring, the detection of heart rate, respiration, and pulse has always been the most basic application. With economic and social development and increased attention to health, the demand for detecting physiological signal characteristics such as blood oxygenation, blood flow rate, limb movement, eye movement, and facial changes is gradually emerging. Traditional contact sensors (such as head-mounted stethoscopes, adhesive electrodes, and wristband smartwatches) have limitations such as restricted application scenarios, the need for cooperation from the subject being detected, and a limited range of detectable features. In recent years, non-contact vital signal sensors based on electromagnetic wave effects have attracted industry attention due to their ease of deployment, diverse application scenarios, and numerous detectable features.

[0003] To improve sensor integration and achieve miniaturization, most contactless vital sign sensors based on electromagnetic effects currently use mature commercial Si, SiGe, or CMOS radar sensor chips for hardware circuit implementation. These chips directly integrate up / down conversion, amplification, and even frequency source circuits. As chip users, at the application level, they only need to configure the corresponding clock, control processing, power management, loop filtering, and intermediate frequency preprocessing circuits on the periphery of the chip to complete the functions of signal transmission, reception, acquisition, and processing, without needing to or being able to interfere with the internal functions of the RF chip. In this case, the overall performance of the sensor is mainly affected by the following factors: (1) the design of the overall sensor scheme. This is usually related to the usage requirements and the performance of specific signal processing algorithms; (2) the performance of the sensor antenna. Currently, some sensor chips have integrated on-chip antennas, further improving the integration of the sensor; (3) the performance of the intermediate frequency preprocessing circuit. The intermediate frequency preprocessing circuit is located between the intermediate frequency output terminal of the RF integrated chip and the digital-to-analog converter (ADC). Its performance directly determines the quality of the digital signal acquired by the ADC, and also directly affects the signal processing results, ultimately affecting the accurate extraction of vital sign signals by the entire sensor.

[0004] Current research and patent claims regarding contactless life signal sensors primarily focus on signal processing algorithms. The reason for this, as mentioned earlier, is that designers typically base sensor application systems on RF integrated chips and general-purpose solutions, with less exploration of hardware-level technologies, thus focusing more on performance improvements in software algorithms. A key development direction for contactless life signal sensors today is the so-called "high detection throughput." This means the goal is for sensors to detect as many objects as possible in the shortest possible time to improve detection efficiency. However, in practical applications, the effective detection range of a sensor is usually a range of distances. Only when the detected object is within this range can the output signal maintain sufficient strength and signal-to-noise ratio, ensuring the accuracy and validity of the measurement results. Furthermore, conventional sensors typically aim for a single, interference-free detection scenario; ideally, there should only be one detected object within the detection range. If multiple objects at different distances are present, or if there is interference from other non-detectable objects, detection accuracy may be affected. Theoretically, this is because the echo intensity of detected objects at different distances varies significantly, with closer objects exhibiting stronger echo intensity than farther ones. From the perspective of frequency-modulated continuous wave (FM-CW) signal processing, when multiple objects or even strong scattering interference sources exist in the detection scene, although distance gates can be used to distinguish between different objects, the gain of the sensor receiver is usually determined by the frequency of the signal with the highest input power—that is, by the closest object or the strong scattering interference source. In this case, the sensor's RF receiver will adaptively reduce its gain to avoid receiver link saturation due to the strong input signal. This reduction in receiver link gain inevitably affects the amplification of the echo signal from objects at greater distances, thus weakening the detection accuracy. This is extremely detrimental to high-throughput detection of non-contact vital signs. Therefore, solving this problem has become a hot topic in this field. Summary of the Invention

[0005] Purpose of the Invention: The purpose of this invention is to provide an intermediate frequency (IF) preprocessing circuit for a contactless vital sign sensor. Without modifying the radio frequency (RF) unit, an adaptive IF preprocessing circuit is proposed to achieve balanced output of echoes from multiple targets over a long distance, thereby ensuring high-quality acquisition by the subsequent ADC and the final detection accuracy. Simultaneously, the adaptive control characteristics of the IF circuit enable the vital sign sensor to have a certain degree of versatility in different scenarios, avoiding the need for hardware circuit design modifications when changing application scenarios.

[0006] Technical Solution: The present invention provides an intermediate frequency preprocessing circuit for a contactless vital sign sensor. The intermediate frequency preprocessing circuit receives signals from the radio frequency circuit, processes them, and transmits them to a signal acquisition and control processor. The intermediate frequency preprocessing circuit includes a low-noise amplifier circuit, a frequency gain compensation circuit, and an automatic level control circuit. The low-noise amplifier circuit receives the signal input from the radio frequency circuit, amplifies the intermediate frequency signal in the signal, and inputs it to the frequency gain compensation circuit. The frequency gain compensation circuit assigns different gains to different frequency components according to the frequency distribution of the intermediate frequency signal, and inputs the amplified output signal to the automatic level control circuit. The automatic level control circuit stabilizes the amplitude of the output signal.

[0007] The low-noise amplifier circuit needs to initially amplify the small intermediate frequency (IF) signal from the mixer output of the sensor receiver for further processing. It typically uses a differential input, single-ended output configuration. The frequency gain compensation circuit is the core of this invention, capable of assigning different gains to different frequency components based on the frequency distribution of the IF signal under program control. The automatic level control circuit stabilizes the output signal amplitude within a certain range to match the sampling of the subsequent ADC stage. The technical solutions for these three circuit components are described below.

[0008] Furthermore, the low-noise amplifier circuit includes an input resistor, a feedback resistor, and a low-noise operational amplifier; the input terminal of the input resistor is connected to the radio frequency receiving circuit, and the output terminal is connected to the input terminal of the feedback resistor and the input terminal of the low-noise operational amplifier, respectively; the output terminal of the feedback resistor is connected to the output terminal of the low-noise operational amplifier.

[0009] Furthermore, the frequency gain compensation circuit includes a fixed capacitor C1, an adjustable capacitor C2, an adjustable capacitor C3, a digital potentiometer R1, a digital potentiometer R2, and an operational amplifier; the input terminal of the fixed capacitor C1 is connected to the output terminal of the low-noise amplifier circuit, the output terminal of the fixed capacitor C1 is connected to the input terminal of the adjustable capacitor C3, the output terminal of the adjustable capacitor C3 is connected to the input terminal of the operational amplifier and the input terminal of the digital potentiometer R1, the output terminal of the digital potentiometer R1 is connected to the input terminal of the adjustable capacitor C2, and the output terminal of the adjustable capacitor C2 is connected to the input terminal of the digital potentiometer R2.

[0010] The frequency gain compensation circuit is the core of this invention and a major component of the intermediate frequency preprocessing circuit. It is designed to eliminate the problem of large differences in echo signal power caused by path attenuation of targets at different distances during the same measurement.

[0011] Furthermore, the frequency gain compensation circuit also includes a digital-to-analog converter; the output terminal of the digital-to-analog converter is connected to adjustable capacitors C2 and C3, and the input terminal is connected to a signal acquisition and control processor.

[0012] Furthermore, the automatic level control circuit includes a variable gain amplifier, a detector circuit, and an integrator circuit;

[0013] The function of the automatic level control circuit is to output a signal with a basically fixed amplitude for signals of different input amplitudes, so that the subsequent ADC can perform stable sampling. It is usually implemented using a detector circuit and a variable gain amplifier. The amplification factor of the variable gain amplifier can be controlled by the control voltage Vagc, and the amplification factor decreases as Vagc increases. The output level of the detector circuit is proportional to the difference between the output voltage of the variable gain amplifier and the set external reference voltage, and is converted into the control voltage Vagc by an integrator circuit. When the input increases, the voltage output by the variable gain amplifier at its current gain is greater than the set reference voltage. The difference in the output of the detector circuit, after being integrated by the integrator circuit, increases the output Vagc, decreases the gain, and thus decreases the output voltage of the variable gain amplifier until it approaches equal to the reference voltage. The reverse is also true.

[0014] Furthermore, the radio frequency circuit includes a frequency source, a quadrature downmixer one, a quadrature downmixer two, a transmitting antenna, a receiving antenna, a receiving low-noise amplifier, and a 90° phase shifter; one output of the frequency source is connected to the transmitting antenna, and the other output serves as the local oscillator signal and is connected to the input terminals of quadrature downmixer one and the 90° phase shifter respectively; the receiving antenna is connected to the input terminal of the receiving low-noise amplifier, the output of the receiving low-noise amplifier is connected to the input terminals of quadrature downmixer one and quadrature downmixer two respectively, the 90° phase shifter is connected to the input terminal of quadrature downmixer two, and the output terminals of quadrature downmixer one and quadrature downmixer two are connected to the input of the intermediate frequency preprocessing circuit.

[0015] This invention also discloses a method for adjusting the operating frequency range and gain of an intermediate frequency preprocessing circuit for a contactless vital sign sensor, comprising the following steps:

[0016] Step 1: Determine the required gain K and maximum intermediate frequency fc of the circuit based on the measurement scenario;

[0017] Step 2: Calculate the value of the adjustable capacitor C2 based on the value of the fixed capacitor C1 and the required gain K of the circuit;

[0018] Step 3: Let C3 = mC2, R2 = nR1, according to the formula Determine the values ​​of m and n;

[0019] Step 4: Determine the values ​​of adjustable capacitors C3 and C2, and then, based on the maximum intermediate frequency fc and the formula... Calculate the values ​​of digital potentiometers R1 and R2;

[0020] Step 5: Write a program to modify the control word from the signal acquisition and control processor to the digital potentiometer and digital-to-analog converter, changing the values ​​of adjustable capacitors C2 and C3 and digital potentiometers R1 and R2;

[0021] Step 6: The calculation methods for the maximum intermediate frequency fc and gain K are as follows:

[0022]

[0023] According to radar equations: The target's echo signal power P r The amplitude of the echo signal is inversely proportional to the fourth power of the target distance R. Assuming the echo signal is a sinusoidal voltage signal, meaning its power is directly proportional to the square of its amplitude, then the amplitude of the echo signal is inversely proportional to the square of the target distance. In a zero-IF receiver architecture, the amplitude of the intermediate frequency (IF) signal output from the mixer is proportional to the amplitude of the echo signal. The frequency of the IF signal is directly proportional to the target distance, meaning its amplitude is inversely proportional to the square of its frequency. The power of the echo IF signal attenuates rapidly with frequency, decreasing by a factor of 100 every ten octaves. This necessitates a large dynamic range for the receiver's preamplifier. Therefore, to eliminate the amplitude attenuation of the IF signal caused by distance and prevent the weak signal from a distant target from being blocked by the strong signal from a nearby target, a frequency gain compensation circuit can compensate for the frequency gain of the IF signal within a specified operating frequency range. This allows the amplitude to increase with increasing frequency to compensate for attenuation, while simultaneously providing a certain gain to the signal.

[0024] The operating frequency range of the circuit is 0 to f. c Maximum intermediate frequency f c The circuit gain K is determined by the values ​​of capacitors C1 and C2, which are jointly determined by capacitors C2, C3, R1, and R2. C2 and C3 are adjustable capacitors, their values ​​linearly controlled by an externally applied control voltage. R1 and R2 are digital potentiometers, typically using I2C or SPI interfaces, and their resistance values ​​are controlled by an external control word. By programming the signal acquisition and control processor to drive the digital-to-analog converter to output the control voltage, the capacitance values ​​of C2 and C3 can be adjusted. Similarly, by writing control words to R1 and R2 using the signal acquisition and control processor, their resistance values ​​can be adjusted, thereby dynamically adjusting the circuit's operating frequency range and gain. When the life signal sensor is applied to different scenarios and requires adjustments to the circuit characteristics, modifications can be made through software rather than custom hardware.

[0025] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0026] (1) The intermediate frequency preprocessing circuit proposed in this invention, as part of the sensor, can adjust the equalization of multi-target output signals in real time according to the actual application scenario through programmable frequency gain adaptive compensation. Experimental testing has verified that the proposed intermediate frequency preprocessing circuit can reduce the difference in multi-target output signals from 36.4% to less than 14.3%. This improves the sensor's detection performance and adaptability to multiple scenarios without hardware modifications.

[0027] (2) Without modifying the radio frequency unit, this invention proposes an intermediate frequency adaptive preprocessing circuit to achieve balanced output of echoes from multiple targets over a large distance range, thereby ensuring high-quality acquisition of the subsequent ADC and the final detection accuracy. At the same time, through the adaptive control characteristics of the intermediate frequency circuit, the life signal sensor can have a certain degree of versatility in different scenarios, avoiding the need to design and modify the sensor's hardware circuit when the application scenario changes. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of a conventional radar sensor structure provided by the present invention;

[0029] Figure 2 This is a schematic diagram of the intermediate frequency circuit structure provided by the present invention;

[0030] Figure 3 This is a schematic diagram of the low-noise amplifier circuit structure provided by the present invention;

[0031] Figure 4 This is a schematic diagram of the frequency gain compensation circuit structure provided by the present invention;

[0032] Figure 5 This is a schematic diagram of the automatic level control circuit structure provided by the present invention;

[0033] Figure 6 This is a schematic diagram of a low-noise amplifier circuit provided by the present invention;

[0034] Figure 7 This is a schematic diagram of the automatic level control circuit provided by the present invention.

[0035] Figure 8 These are the test results of the examples provided by this invention. Detailed Implementation

[0036] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0037] like Figure 1As shown, the intermediate frequency preprocessing circuit for a contactless vital sign sensor of the present invention includes a frequency source 1, quadrature downmixers 2 and 6, a transmitting antenna 3, a receiving antenna 4, a receiving low-noise amplifier 5, a 90° phase shifter 7, an intermediate frequency preprocessing circuit 8, and a signal acquisition and control processor 9. The radio frequency circuit 10 typically uses a commercially available integrated chip, and even the antenna is integrated on-chip, meaning that 1 to 7 are implemented on the same chip.

[0038] The intermediate frequency preprocessing circuit proposed in this invention mainly includes an intermediate frequency low-noise amplifier circuit 8-1, a frequency gain compensation circuit 8-2, and an automatic level control circuit 8-3. These circuits are directly coupled, such as... Figure 2 As shown. The low-noise amplifier circuit 8-1 provides preliminary amplification of the intermediate frequency (IF) small signal from the mixer output of the sensor receiver for further processing. It typically uses a differential input, single-ended output configuration. The frequency gain compensation circuit 8-2 is the core of this invention, capable of assigning different gains to different frequency components based on the frequency distribution of the IF signal under program control. The automatic level control circuit 8-3 stabilizes the output signal amplitude within a certain range to match the sampling of the subsequent ADC stage. The technical solutions for these three circuits are described below.

[0039] (1) Low-noise amplifier circuit

[0040] Currently, most sensors use RF integrated chips with differential I and Q outputs, each outputting differentially. Because RF integrated chips have limited receiver gain, their IF power is relatively low. Therefore, the differential IF signals need to be combined and preliminarily amplified externally. To minimize noise, this part of the circuit uses a low-noise operational amplifier (op-amp) for differential amplification. The basic topology is as follows: Figure 3 As shown, 8-1-3 is a low-noise operational amplifier, and the circuit gain is determined by the ratio of the feedback resistor 8-1-2 to the input resistor 8-1-1.

[0041] (2) Frequency gain compensation circuit

[0042] The frequency gain compensation circuit is the core of this invention and a major component of the intermediate frequency preprocessing circuit. It is designed to eliminate the problem of large differences in echo signal power caused by path attenuation of targets at different distances during the same measurement.

[0043] Circuit topology such as Figure 4 As shown, it consists of an operational amplifier 8-2-6, fixed capacitors C1 (8-2-3), adjustable capacitors C2 (8-2-2) and C3 (8-2-4), and digital potentiometers R1 (8-2-7) and R2 (8-2-5). It also includes a digital-to-analog converter 8-2-1 and a signal acquisition and control processor 9.

[0044] According to radar equations: The target's echo signal power P r The amplitude of the echo signal is inversely proportional to the fourth power of the target distance R. Assuming the echo signal is a sinusoidal voltage signal, meaning its power is directly proportional to the square of its amplitude, then the amplitude of the echo signal is inversely proportional to the square of the target distance. In a zero-IF receiver architecture, the amplitude of the intermediate frequency (IF) signal output from the mixer is proportional to the amplitude of the echo signal. The frequency of the IF signal is directly proportional to the target distance, meaning its amplitude is inversely proportional to the square of its frequency. The power of the echo IF signal attenuates rapidly with frequency, decreasing by a factor of 100 every ten octaves. This necessitates a large dynamic range for the receiver's preamplifier. Therefore, to eliminate the amplitude attenuation of the IF signal caused by distance and prevent the weak signal from a distant target from being blocked by the strong signal from a nearby target, a frequency gain compensation circuit can compensate for the frequency gain of the IF signal within a specified operating frequency range. This allows the amplitude to increase with increasing frequency to compensate for attenuation, while simultaneously providing a certain gain to the signal.

[0045] The operating frequency range of the circuit is 0 to f. c Maximum intermediate frequency f c The circuit gain K is determined by the values ​​of capacitors C1 and C2, which are jointly determined by capacitors C2, C3, R1, and R2. C2 and C3 are adjustable capacitors, their values ​​linearly controlled by an externally applied control voltage. R1 and R2 are digital potentiometers, typically using I2C or SPI interfaces, and their resistance values ​​are controlled by an external control word. By programming the signal acquisition and control processor to drive the digital-to-analog converter to output the control voltage, the capacitance values ​​of C2 and C3 can be adjusted. The signal acquisition and control processor can also write control words to R1 and R2 to adjust their resistance values, thereby dynamically adjusting the circuit's operating frequency range and gain. When the vital sign sensor is applied to different scenarios and requires adjustments to the circuit characteristics, modifications can be made through software rather than custom hardware. The specific adjustment steps are as follows:

[0046] Step 1: Determine the required gain K and maximum intermediate frequency fc of the circuit based on the measurement scenario;

[0047] Step 2: Calculate the value of the adjustable capacitor C2 based on the value of the fixed capacitor C1 and the required gain K of the circuit;

[0048] Step 3: Let C3 = mC2, R2 = nR1, according to the formula Determine the values ​​of m and n;

[0049] Step 4: Determine the values ​​of adjustable capacitors C3 and C2, and then, based on the maximum intermediate frequency fc and the formula... Calculate the values ​​of digital potentiometers R1 and R2;

[0050] Step 5: Write a program to modify the control word from the signal acquisition and control processor to the digital potentiometer and digital-to-analog converter, changing the values ​​of adjustable capacitors C2 and C3 and digital potentiometers R1 and R2;

[0051] Step 6: Maximum intermediate frequency f c The method for calculating the gain K is as follows:

[0052]

[0053] By adjusting the capacitance and resistance values ​​through the built-in program, the gain and operating frequency range of the frequency gain compensation circuit can be dynamically adjusted without modifying the circuit hardware. This maximizes the compensation for the attenuation of the intermediate frequency signal amplitude caused by distance. At the same time, since the operating frequency range can be dynamically adjusted according to different application scenarios, it can reduce interference from objects outside the distance range of the tested object and improve measurement accuracy.

[0054] (3) Automatic level control circuit

[0055] The automatic level control circuit outputs a signal with a relatively fixed amplitude for signals of varying input amplitudes, ensuring stable sampling by the subsequent ADC. This is typically achieved using a detector circuit and a variable gain amplifier, as shown in the circuit diagram below. Figure 5 As shown, the amplification factor of the variable gain amplifier 8-3-1 can be controlled by the control voltage Vagc, and the amplification factor decreases as Vagc increases. The output level of the detector circuit 8-3-2 is proportional to the difference between the output voltage of the variable gain amplifier and the set external reference voltage, and is converted into the control voltage Vagc by the integrator circuit 8-3-3. When the input increases, the voltage output by the variable gain amplifier at its current gain is greater than the set reference voltage. The difference in the output of the detector circuit is integrated by the integrator circuit, causing the output Vagc to increase as well, and the gain to decrease. Therefore, the output voltage of the variable gain amplifier decreases until it approaches equal to the reference voltage, and vice versa.

[0056] Example

[0057] This embodiment provides a non-contact life signal sensor, including a frequency modulated continuous wave radar front-end operating in the F-band and an intermediate frequency preprocessing circuit;

[0058] The radar front-end includes a radar chip and a phase-locked loop (PLL) chip. It transmits a frequency-modulated continuous wave, which is modulated by the PLL chip. The radar chip integrates a VCO, an on-chip antenna, and a mixer, and outputs two orthogonal intermediate frequency (IF) signals, I and Q, in differential form. The transmitted waveform has a starting frequency of 120 GHz, a bandwidth of 5 GHz, and a continuous sawtooth waveform.

[0059] The relationship between the intermediate frequency (IF) of a frequency-modulated continuous wave radar and the target distance is as follows: Where f b Here, B is the intermediate frequency (IF), T is the sweep bandwidth, d is the sweep period, and c is the speed of light. In this example, the radar sweep bandwidth is 5 GHz, the sweep period is 500 μs, and the sensor operating range is set to 0–10 m, so the IF frequency range is 0–667 kHz.

[0060] The intermediate frequency preprocessing circuit consists of three parts: a low-noise amplifier circuit, a frequency gain compensation circuit, and an automatic level control circuit. The schematic diagram of the low-noise amplifier circuit is shown below. Figure 6 As shown, R1 and R4 are input resistors, and R2 and R3 are feedback resistors. The amplification factor is... C1 and C2 are DC blocking capacitors, filtering out the small DC component of the intermediate frequency output. VREF is the DC bias of VCC / 2, raising the signal to near VCC / 2 to prevent signal distortion when powered by a single power supply. In this example, the amplification factor is 20x. The frequency gain compensation circuit in this example, based on the above specifications, sets fc = 700kHz, K = 10dB, and R1 and R2 are selected from TI's 256-bit... 2 This is a 100k ohm digital potentiometer with a USB-C interface. C1, C2, and C3 are adjustable capacitors from Murata Manufacturing Co., Ltd., with a modulation voltage range of 0–3V. C2 has a capacitance range of 16.5–33pF, and C1 and C3 have a capacitance range of 45–90pF. It can be modulated by a three-way digital-to-analog converter. Based on the above adjustment steps and design specifications, adjust the resistor values ​​as follows: R1 = 22.66kΩ (control word 0x3Ah), R2 = 2.73kΩ (control word 0x07h), and adjust the capacitor values ​​as follows: C1 = 45pF, C2 = 16.5pF, and C3 = 68pF.

[0061] In this example, the automatic level control circuit is as follows: Figure 7 As shown, the AD603 variable gain amplifier chip from Analog Devices (ADI) is used. The gain is controlled by VPOS and VNEG. As shown in the figure, VNEG is a fixed level, so the gain control voltage Vagc is actually adjusted only through the VPOS pin. Q2 and R7 form a half-wave detector. The current entering capacitor CAV is the difference between the collector current of Q2 (300μA at 300K and 27℃) and the collector current of Q1. This difference increases with the amplitude of the output signal. The gain control voltage Vagc is the time integral of this error current. For Vagc to be equivalent to the gain of the variable gain amplifier and to remain stable against short-term amplitude changes in the output signal, the rectified current in Q1 must be precisely balanced with that in Q2. Therefore, when the output of the variable gain amplifier is too small to achieve balance, Vagc increases, causing the gain to increase until Q2 is fully turned on, stabilizing the gain. Conversely, the same applies when the input increases.

[0062] This example demonstrates the effectiveness of the present invention through a test experiment using an experimental prototype fabricated according to the technical solution of the present invention. The experimental results are as follows: Figure 8 As shown. Figure 8 The difference in echo intensity was demonstrated for the same test target, with and without the intermediate frequency circuit provided by the present invention. The difference was characterized by the ratio of the difference between the amplitude of the different targets and their overall mean to the overall mean. The difference in echo intensity was reduced from 36.4% to 14.3%.

Claims

1. A mid-frequency preprocessing circuit for a contactless vital sign sensor, characterized in that, The intermediate frequency preprocessing circuit (8) receives the signal from the radio frequency circuit (10), processes it, and then transmits it to the signal acquisition and control processor (9). The intermediate frequency preprocessing circuit (8) includes a low noise amplifier circuit (8-1), a frequency gain compensation circuit (8-2), and an automatic level control circuit (8-3). The low noise amplifier circuit (8-1) receives the signal input from the radio frequency circuit (10), amplifies the intermediate frequency signal in the signal, and inputs it into the frequency gain compensation circuit (8-2). The frequency gain compensation circuit (8-2) assigns different gains to different frequency components according to the frequency distribution of the intermediate frequency signal, and inputs the amplified output signal into the automatic level control circuit (8-3). The automatic level control circuit (8-3) stabilizes the amplitude of the output signal. The frequency gain compensation circuit (8-2) includes a fixed capacitor C1 (8-2-3), an adjustable capacitor C2 (8-2-2), an adjustable capacitor C3 (8-2-4), a digital potentiometer R1 (8-2-7), a digital potentiometer R2 (8-2-5), and an operational amplifier (8-2-6). The input terminal of the fixed capacitor C1 (8-2-3) is connected to the output terminal of the low-noise amplifier circuit (8-1), the output terminal of the fixed capacitor C1 (8-2-3) is connected to the input terminal of the adjustable capacitor C3 (8-2-4), the output terminal of the adjustable capacitor C3 (8-2-4) is connected to the input terminal of the operational amplifier (8-2-6) and the input terminal of the digital potentiometer R1 (8-2-7), the output terminal of the digital potentiometer R1 (8-2-7) is connected to the input terminal of the adjustable capacitor C2 (8-2-2), and the output terminal of the adjustable capacitor C2 (8-2-2) is connected to the input terminal of the digital potentiometer R2 (8-2-5). The frequency gain compensation circuit (8-2) also includes a digital-to-analog converter (8-2-1); the output terminal of the digital-to-analog converter (8-2-1) is connected to the adjustable capacitor C2 (8-2-2) and the adjustable capacitor C3 (8-2-4), and the input terminal is connected to the signal acquisition and control processor (9).

2. The intermediate frequency preprocessing circuit for a contactless vital sign sensor according to claim 1, characterized in that, The low-noise amplifier circuit (8-1) includes an input resistor (8-1-1), a feedback resistor (8-1-2), and a low-noise operational amplifier (8-1-3). The input terminal of the input resistor (8-1-1) is connected to the radio frequency receiving circuit (10), and the output terminal is connected to the input terminal of the feedback resistor (8-1-2) and the input terminal of the low-noise operational amplifier (8-1-3), respectively. The output terminal of the feedback resistor (8-1-2) is connected to the output terminal of the low-noise operational amplifier (8-1-3).

3. The intermediate frequency preprocessing circuit for a contactless vital sign sensor according to claim 1, characterized in that, The automatic level control circuit (8-3) includes a variable gain amplifier (8-3-1), a detector circuit (8-3-2), and an integrator circuit (8-3-3). The output level of the detector circuit (8-3-2) is proportional to the difference between the output voltage of the variable gain amplifier (8-3-1) and the set external reference voltage. The integrator circuit (8-3-3) converts the difference into a control voltage Vac to control the amplification factor of the variable gain amplifier (8-3-1).

4. The intermediate frequency preprocessing circuit for a contactless vital sign sensor according to claim 1, characterized in that, The radio frequency circuit (10) includes a frequency source (1), a quadrature downmixer one (2), a quadrature downmixer two (6), a transmitting antenna (3), a receiving antenna (4), a receiving low-noise amplifier (5), and a 90° phase shifter (7). One output of the frequency source (1) is connected to the transmitting antenna (3), and the other output is connected as a local oscillator signal to the input terminals of the quadrature downmixer one (2) and the 90° phase shifter (7). The receiving antenna (4) is connected to the input terminal of the receiving low-noise amplifier (5). The output terminal of the receiving low-noise amplifier (5) is connected to the input terminals of the quadrature downmixer one (2) and the quadrature downmixer two (6). The 90° phase shifter (7) is connected to the input terminal of the quadrature downmixer two (6). The output terminals of the quadrature downmixer one (2) and the quadrature downmixer two (6) are connected to the input of the intermediate frequency preprocessing circuit.

5. A method for adjusting the operating frequency range and gain of an intermediate frequency preprocessing circuit for a contactless vital sign sensor based on claim 1, characterized in that, Includes the following steps: Step 1: Determine the required gain K and maximum intermediate frequency f of the circuit based on the measurement scenario. c The operating frequency range of the circuit is 0 to f. c Maximum intermediate frequency f c The circuit gain K is determined by the values ​​of capacitors C1 and C2, which are jointly determined by C2, C3, R1, and R2. Among them, C2 and C3 are adjustable capacitors, whose capacitance values ​​are linearly controlled by the externally applied control voltage, and R1 and R2 are digital potentiometers. Step 2: Calculate the value of the adjustable capacitor C2 based on the value of the fixed capacitor C1 and the required gain K of the circuit; Step 3: Let C3 = mC2, R2 = nR1, according to the formula Determine the values ​​of m and n; Step 4: Determine the values ​​of adjustable capacitors C3 and C2, and then, based on the maximum intermediate frequency fc and the formula... Calculate the values ​​of digital potentiometers R1 and R2; Step 5: Write a program to modify the control word from the signal acquisition and control processor to the digital potentiometer and digital-to-analog converter, changing the values ​​of adjustable capacitors C2 and C3 and digital potentiometers R1 and R2; Step 6: Maximum Intermediate Frequency The method for calculating the gain K is as follows: 。 6. A contactless vital sign sensor comprising the intermediate frequency preprocessing circuit according to any one of claims 1-4.

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