A vibration transducer

Abstract

The application discloses a vibration transducer, comprising a piezoelectric sensitive element module, a measurement circuit module, a signal conditioning module, an integral conversion module, an amplification module, a transmission output module, a power module, an anti-reverse connection protection module, a temperature compensation module and an anti-interference module. The integral conversion module adopts a double-integral path structure, comprising a high-pass filter integral path and a digital integral path, and automatically selects an integral path according to an input signal frequency through a frequency detection unit and an analog switch; when the frequency is higher than 5 Hz, the analog high-pass integral is adopted; when the frequency is lower than 5 Hz, the digital integral is adopted, and the low-frequency integral drift is effectively eliminated. The temperature compensation module adopts a hybrid signal closed-loop compensation structure composed of a temperature sensor, an analog-to-digital converter, a microprocessor and a digital-to-analog converter, and performs real-time compensation on temperature drift in the integral operation and the amplification process. The application realizes wide-band, low-drift and high-precision vibration speed measurement, and is especially suitable for state monitoring and fault diagnosis of industrial mechanical equipment.

Description

Technical Field

[0001] This invention relates to the field of transmitter technology, and in particular to a vibration transmitter. Background Technology

[0002] Many industrial machinery malfunctions are related to vibration, making vibration monitoring a core method for equipment condition monitoring and fault diagnosis. Vibration intensity (effective velocity value) is a conventional indicator for evaluating equipment operating status. Piezoelectric sensors directly sense vibration acceleration, which needs to be converted into velocity through integration. Although existing integrated piezoelectric vibration transmitters have built-in integrating circuits, they suffer from severe integral drift in the low-frequency range—the input offset voltage and bias current of the operational amplifier accumulate after integration, leading to output saturation and limiting their application in monitoring low-frequency equipment such as large hydroelectric turbines and low-speed fans. Furthermore, existing products often employ simple analog temperature compensation or no compensation, resulting in sensitivity drift exceeding ±5% within the operating range of -40℃ to +85℃, which is insufficient for high-precision monitoring requirements.

[0003] For the integral conversion from acceleration to velocity, existing technologies mainly employ three schemes: pure analog integration, pure digital integration, and a simple hybrid approach. Pure analog integration circuits are simple in structure, but suffer from severe low-frequency drift, requiring large-value feedback resistors to suppress it, at the expense of low-frequency accuracy. Pure digital integration is achieved through an ADC-DSP-DAC link, eliminating drift but incurring quantization noise and sampling delay, resulting in high system complexity. The simple hybrid approach only switches between analog and digital paths within specific frequency bands, lacking optimized matching of the characteristics of the two paths, leading to discontinuous amplitude-frequency characteristics at frequency band boundaries. Furthermore, the -90° phase shift introduced by integration, combined with the phase nonlinearity of the piezoelectric sensing element itself, causes phase errors, and existing products generally lack effective phase compensation mechanisms. Additionally, the high-impedance charge signal output by piezoelectric sensors is extremely sensitive to electromagnetic interference in industrial environments, and existing anti-interference designs (filtering, shielding, grounding) are still insufficient.

[0004] In summary, existing piezoelectric vibration transmitters suffer from several technical bottlenecks, including the inability of a single integration path to simultaneously achieve wide bandwidth and low drift, inadequate temperature compensation, lack of phase compensation, and insufficient anti-interference capabilities. Therefore, there is an urgent need for a novel vibration transmitter employing a dual integration path structure, incorporating digital temperature and phase compensation, and possessing strong anti-interference capabilities to achieve wide bandwidth, low drift, and high-precision vibration velocity measurement. Summary of the Invention

[0005] In view of the above background and the problems existing in the prior art, the present invention provides a vibration transmitter, characterized in that it includes: A piezoelectric sensing element module is set at the vibration signal pickup position of the object under test, and is used to sense the vibration acceleration signal and convert the vibration acceleration signal into an electric charge signal. The measurement circuit module is electrically connected to the piezoelectric sensitive element module and is used to perform charge-to-voltage conversion and signal preprocessing on the charge signal to obtain a voltage signal proportional to the vibration acceleration. A signal conditioning module is located between the measurement circuit module and the integral conversion module, and is used to filter and impedance match the voltage signal; The integral conversion module, electrically connected to the signal conditioning module, is used to perform integral calculations on the filtered voltage signal to convert the acceleration measure into a velocity measure; the integral conversion module adopts a dual integral path structure, including a first integral path and a second integral path. An amplification module, electrically connected to the integral conversion module, is used to amplify the integrated velocity signal. The transmitter output module is electrically connected to the amplification module and is used to convert the amplified speed signal into a standard current signal output. The power supply module is used to provide operating voltage for the measurement circuit module, signal conditioning module, integral conversion module, amplification module and transmitter output module; A reverse polarity protection module is installed between the output terminal of the power supply module and the power supply input terminal of the transmitter output module to prevent damage to subsequent circuits when the power supply polarity is reversed. A temperature compensation module, located between the integral conversion module and the amplification module, includes a temperature sensor, an analog-to-digital converter (ADC), a microprocessor, and a digital-to-analog converter (DAC). The temperature sensor detects the ambient temperature and outputs a temperature voltage signal. The ADC converts the temperature voltage signal into a digital temperature signal. The microprocessor calculates the temperature compensation amount based on the digital temperature signal and a pre-stored temperature-drift compensation coefficient table and outputs a digital compensation signal. The DAC converts the digital compensation signal into an analog compensation voltage, which is then superimposed on the speed signal output by the integral conversion module to compensate for temperature drift generated during integration and amplification.

[0006] As a further explanation of the present invention, the piezoelectric sensitive element module includes a piezoelectric ceramic sheet, a base, and a housing; the piezoelectric ceramic sheet is fixed to the protrusion of the base by a preloaded spring, the lower electrode of the piezoelectric ceramic sheet is electrically connected to the base and grounded by conductive adhesive, and the upper electrode is led out by a wire as a signal output terminal.

[0007] Furthermore, the measurement circuit module includes a charge amplifier, a first-stage low-pass filter, and a second-stage low-pass filter; the charge amplifier adopts an operational amplifier for instrumentation, the first-stage low-pass filter adopts a second-order Sallen-Key topology, which is mainly used to suppress high-frequency noise and aliasing components in the output of the charge amplifier, and the second-stage low-pass filter adopts a second-order Butterworth topology.

[0008] Furthermore, the signal conditioning module includes a differential amplifier and a buffer amplifier; the differential amplifier is a fully differential operational amplifier used to suppress common-mode interference signals; the buffer amplifier is a low-output-impedance operational amplifier used to drive the input terminal of the integral conversion module.

[0009] Furthermore, the integral conversion module adopts a dual-integration path structure, including a high-pass filter integral path and a digital integral path. The high-pass filter integral path includes a cascaded high-pass filter and an analog integrator, used to process vibration signals with frequencies higher than 5Hz. The digital integral path includes an analog-to-digital converter, a digital signal processor, and a digital-to-analog converter. The analog-to-digital converter converts the vibration signal into a digital signal, the digital signal processor performs integration on the digital signal using a numerical integration algorithm, and the digital-to-analog converter converts the integrated digital signal into an analog velocity signal. The integral conversion module also includes an integral path selection circuit, which includes a frequency detection unit and an analog switch. The frequency detection unit outputs a control signal based on the comparison result between the frequency of the input signal and a preset threshold, and the analog switch activates either the high-pass filter integral path or the digital integral path according to the control signal.

[0010] Furthermore, the integral conversion module also includes a phase compensation circuit for compensating for phase errors introduced during the integration process; the phase compensation circuit includes an adjustable phase shift network and a non-volatile memory; the non-volatile memory stores phase shift compensation parameters pre-determined by an external calibration device based on the phase characteristics of the piezoelectric sensing element module, and the adjustable phase shift network performs phase compensation based on the phase shift compensation parameters stored in the non-volatile memory.

[0011] Furthermore, the amplification module includes a programmable gain amplifier and a buffer amplifier; the programmable gain amplifier is a digital programmable gain amplifier, and the buffer amplifier is a low-noise high-speed operational amplifier.

[0012] Furthermore, the transmitter output module includes a voltage-to-current conversion circuit and a current limiting protection circuit; the voltage-to-current conversion circuit adopts a four-transistor current source topology to convert the input voltage into a standard current signal output.

[0013] Furthermore, it also includes an anti-interference module, which is disposed between the piezoelectric sensitive element module and the measurement circuit module, for suppressing external electromagnetic interference; the anti-interference module includes a common-mode choke and a differential-mode filter; the common-mode choke uses a manganese-zinc ferrite core to suppress common-mode interference; the differential-mode filter uses an LC filter topology to suppress differential-mode high-frequency noise.

[0014] The beneficial effects of this invention are: This invention employs a dual-path structure combining a high-pass filter integration path and a digital integration path. For frequencies above 5Hz, processing is performed via an analog high-pass integration path, utilizing a second-order high-pass filter to effectively remove DC bias and extremely low-frequency drift. For frequencies below 5Hz, the system switches to the digital integration path, using a DSP numerical integration algorithm to completely eliminate the drift problem of analog integration. The two paths are automatically switched via a frequency detection unit and an analog switch, achieving low-drift integration and significantly expanding the transmitter's low-frequency measurement capabilities.

[0015] The temperature compensation module adopts a hybrid signal closed-loop compensation structure consisting of a temperature sensor, ADC, microprocessor, and DAC. The microprocessor calculates the compensation amount in real time according to the pre-stored temperature-drift compensation coefficient table, and the analog compensation voltage output by the DAC is superimposed on the signal path to systematically compensate for temperature drift in multiple stages such as charge-voltage conversion, integration operation, and signal amplification. The compensation accuracy is much higher than that of traditional thermistor networks, ensuring that the transmitter maintains high measurement accuracy.

[0016] The integral conversion module is equipped with a phase compensation circuit, which compensates for the phase error introduced by the integral operation through an adjustable phase shift network. The compensation parameters are pre-calibrated according to the phase characteristics of the piezoelectric sensitive element and stored in a non-volatile memory, effectively eliminating the phase distortion caused by the resonance characteristics of the piezoelectric sensitive element and the non-ideal factors of the integral circuit, thus meeting the accuracy requirements of vibration spectrum analysis and phase synchronization monitoring.

[0017] An anti-interference module is installed between the piezoelectric sensing element and the measurement circuit. A manganese-zinc ferrite common-mode choke is used to suppress common-mode interference, and an LC differential-mode filter suppresses differential-mode high-frequency noise. The signal conditioning module uses a fully differential operational amplifier to suppress common-mode interference. A stainless steel housing provides electromagnetic shielding. These multiple anti-interference measures work synergistically to significantly improve the transmitter's signal-to-noise ratio and measurement stability in complex industrial electromagnetic environments. Attached Figure Description

[0018] Figure 1 This is a diagram of the vibration transmitter architecture according to an embodiment of the present invention; Figure 2 This is a structural diagram of the piezoelectric sensitive element module of the vibration transmitter according to an embodiment of the present invention. Detailed Implementation

[0019] Example: The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0020] In the description of this invention, it should be understood that the terms "upper", "lower", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0021] As shown in the attached figure, a vibration transmitter includes: A piezoelectric sensing element module is set at the vibration signal pickup position of the object under test, and is used to sense the vibration acceleration signal and convert the vibration acceleration signal into an electric charge signal. The measurement circuit module is electrically connected to the piezoelectric sensitive element module and is used to perform charge-to-voltage conversion and signal preprocessing on the charge signal to obtain a voltage signal proportional to the vibration acceleration. A signal conditioning module is located between the measurement circuit module and the integral conversion module, and is used to filter and impedance match the voltage signal; The integral conversion module, electrically connected to the signal conditioning module, is used to perform integral calculations on the filtered voltage signal to convert the acceleration measure into a velocity measure; the integral conversion module adopts a dual integral path structure, including a first integral path and a second integral path. An amplification module, electrically connected to the integral conversion module, is used to amplify the integrated velocity signal. The transmitter output module is electrically connected to the amplification module and is used to convert the amplified speed signal into a standard current signal output. The power supply module is used to provide operating voltage for the measurement circuit module, signal conditioning module, integral conversion module, amplification module and transmitter output module; A reverse polarity protection module is installed between the output terminal of the power supply module and the power supply input terminal of the transmitter output module to prevent damage to subsequent circuits when the power supply polarity is reversed. A temperature compensation module, located between the integral conversion module and the amplification module, includes a temperature sensor, an analog-to-digital converter (ADC), a microprocessor, and a digital-to-analog converter (DAC). The temperature sensor detects the ambient temperature and outputs a temperature voltage signal. The ADC converts the temperature voltage signal into a digital temperature signal. The microprocessor calculates the temperature compensation amount based on the digital temperature signal and a pre-stored temperature-drift compensation coefficient table and outputs a digital compensation signal. The DAC converts the digital compensation signal into an analog compensation voltage, which is then superimposed on the speed signal output by the integral conversion module to compensate for temperature drift generated during integration and amplification.

[0022] The piezoelectric sensing element module includes a piezoelectric ceramic sheet 101, a base 102, and a housing 103. The housing and base are connected by threads and sealed with an O-ring. The inner wall of the housing is coated with a conductive shielding coating, forming a Faraday cage structure with the base to achieve electromagnetic shielding. The piezoelectric ceramic sheet is fixed to the base protrusion by a pre-compression spring, ensuring stable and reliable force transmission between the piezoelectric ceramic sheet and the object being measured. The lower electrode of the piezoelectric ceramic sheet is electrically connected to the base and grounded via conductive adhesive, while the upper electrode is led out via a wire as a signal output terminal.

[0023] The measurement circuit module includes a charge amplifier, a first-stage low-pass filter, and a second-stage low-pass filter. The first-stage low-pass filter employs a second-order Sallen-Key topology, primarily used to suppress high-frequency noise and aliasing components in the charge amplifier output. The second-stage low-pass filter employs a second-order Butterworth topology, exhibiting the flattest amplitude-frequency response within the passband, ensuring amplitude accuracy for signals up to 10kHz. The cascaded two-stage low-pass filter effectively suppresses high-frequency interference and aliasing effects.

[0024] The signal conditioning module includes a differential amplifier and a buffer amplifier. The differential amplifier converts the single-ended signal output from the measurement circuit module into a differential signal while suppressing common-mode interference. The buffer amplifier is a voltage follower structure, providing low-impedance drive capability to drive the input of the subsequent integration and conversion module, ensuring the ability to track rapidly changing signals.

[0025] The integral conversion module adopts a dual-integration path structure, including a high-pass filter integral path 401, a digital integral path 402, an integral path selection circuit 403, and a phase compensation circuit 404. The high-pass filter integral path 401 includes a cascaded second-order high-pass filter and an analog integrator. The second-order high-pass filter uses a Sallen-Key topology to filter out DC bias and extremely low-frequency drift in the signal, while introducing a +90° phase shift. The analog integrator uses an active integrator structure. After the high-pass filter and analog integrator are cascaded, the +90° phase shift introduced by the high-pass filter cancels out the -90° phase shift introduced by the integrator, achieving a near-zero net phase shift within the passband and converting the acceleration signal into a velocity signal. The digital integral path 402 includes an analog-to-digital converter (ADC), a digital signal processor (DSP), and a digital-to-analog converter (DAC). To eliminate DC bias, the DSP performs a digital high-pass filter on the acceleration signal before integration, and the DAC converts the integrated digital velocity signal into an analog signal output. The integration path selection circuit 403 includes a frequency detection unit and an analog switch. When the input signal frequency is higher than 5Hz, the output is low; when the input signal frequency is lower than 5Hz, the output is high. The analog switch automatically switches on or off the high-pass filter integration path or the digital integration path based on the output level. The phase compensation circuit 404 includes an adjustable phase shift network and a non-volatile memory. The adjustable phase shift network adopts an all-pass filter structure, implemented by cascading two first-order all-pass filters.

[0026] The temperature compensation module is located between the integral conversion module and the amplification module, and includes a temperature sensor, an analog-to-digital converter (ADC), a microprocessor, and a digital-to-analog converter (DAC). The temperature sensor is mounted near the analog integrator of the integral conversion module to monitor the ambient temperature in real time. During operation, the microprocessor looks up the current temperature value in a table and performs linear interpolation to calculate the temperature compensation amount. The DAC converts the digital compensation signal calculated by the microprocessor into an analog compensation voltage, which is then superimposed on the speed signal through an adder circuit to achieve real-time compensation for temperature drift.

[0027] The amplification module includes a programmable gain amplifier and a buffer amplifier. The programmable gain amplifier is a digitally programmable gain amplifier, and the buffer amplifier is a voltage follower structure, providing low output impedance and sufficient drive capability to drive the input of the subsequent transmitter output module.

[0028] The transmitter output module includes a voltage-to-current conversion circuit and a current-limiting protection circuit. When the output current exceeds 25mA, the comparator triggers the MOSFET to turn off, limiting the output current to within 25mA to prevent damage to the transmitter in the event of an accidental short circuit.

[0029] The anti-interference module, located between the piezoelectric sensing element module and the measurement circuit module, includes a common-mode choke and a differential-mode filter. The common-mode choke uses a manganese-zinc ferrite core common-mode inductor, exhibiting high impedance to common-mode interference (such as power supply noise and electromagnetic radiation) and low impedance to differential-mode signals, effectively suppressing common-mode interference. The differential-mode filter uses an LCπ-type filter topology, exhibiting attenuation characteristics for differential-mode high-frequency noise (such as switching power supply harmonics and digital circuit clock interference). The anti-interference module, working in conjunction with the common-mode choke and differential-mode filter, effectively suppresses complex electromagnetic interference in industrial environments.

[0030] The overall working process of the vibration transmitter of this invention is as follows: (1) The piezoelectric sensing element module senses the vibration acceleration of the object under test and converts the mechanical vibration into an electric charge signal; (2) The charge signal is preprocessed by the common-mode choke and differential-mode filter of the anti-interference module before entering the measurement circuit module; (3) The charge amplifier of the measurement circuit module converts the charge signal into a voltage signal, and high-frequency noise is filtered out by two-stage low-pass filters; (4) The signal conditioning module performs differential amplification and buffering of the voltage signal to improve signal quality and drive subsequent circuits; (5) The integral conversion module automatically selects the integration path according to the frequency of the input signal: when the frequency is higher than 5Hz, it performs analog integration through the high-pass filter integration path; when the frequency is lower than 5Hz, it performs numerical integration through the digital integration path; at the same time, the phase compensation circuit compensates for the phase error according to the pre-stored parameters. (6) The temperature compensation module detects the ambient temperature in real time, calculates the compensation amount according to the pre-stored temperature-drift compensation coefficient table, and outputs an analog compensation voltage through the DAC to be superimposed on the speed signal; (7) The programmable gain amplifier of the amplification module automatically adjusts the gain according to the external range configuration signal, and the buffer amplifier provides low impedance drive; (8) The transmitter output module converts the amplified speed signal into a 4~20mA standard current signal output, and the current limiting protection circuit prevents short circuit damage.

[0031] The above description only illustrates preferred embodiments of the present invention and should not be construed as limiting the scope of the claims. The present invention is not limited to the above embodiments, and variations in its specific structure are permitted. In short, all variations made within the scope of the independent claims of the present invention are within the scope of protection of the present invention.

Claims

1. A vibration transmitter, characterized in that, include: A piezoelectric sensing element module is set at the vibration signal pickup position of the object under test, and is used to sense the vibration acceleration signal and convert the vibration acceleration signal into an electric charge signal. The measurement circuit module is electrically connected to the piezoelectric sensitive element module and is used to perform charge-to-voltage conversion and signal preprocessing on the charge signal to obtain a voltage signal proportional to the vibration acceleration. A signal conditioning module is located between the measurement circuit module and the integral conversion module, and is used to filter and impedance match the voltage signal; The integral conversion module, electrically connected to the signal conditioning module, is used to perform integral calculations on the filtered voltage signal to convert the acceleration measure into a velocity measure; the integral conversion module adopts a dual integral path structure, including a first integral path and a second integral path. An amplification module, electrically connected to the integral conversion module, is used to amplify the integrated velocity signal. The transmitter output module is electrically connected to the amplification module and is used to convert the amplified speed signal into a standard current signal output. The power supply module is used to provide operating voltage for the measurement circuit module, signal conditioning module, integral conversion module, amplification module and transmitter output module; A reverse polarity protection module is installed between the output terminal of the power supply module and the power supply input terminal of the transmitter output module to prevent damage to subsequent circuits when the power supply polarity is reversed. A temperature compensation module, located between the integral conversion module and the amplification module, includes a temperature sensor, an analog-to-digital converter (ADC), a microprocessor, and a digital-to-analog converter (DAC). The temperature sensor detects the ambient temperature and outputs a temperature voltage signal. The ADC converts the temperature voltage signal into a digital temperature signal. The microprocessor calculates the temperature compensation amount based on the digital temperature signal and a pre-stored temperature-drift compensation coefficient table and outputs a digital compensation signal. The DAC converts the digital compensation signal into an analog compensation voltage, which is then superimposed on the speed signal output by the integral conversion module to compensate for temperature drift generated during integration and amplification.

2. The vibration transmitter according to claim 1, characterized in that, The piezoelectric sensitive element module includes a piezoelectric ceramic sheet, a base, and a housing. The piezoelectric ceramic sheet is fixed to the protrusion of the base by a pre-compression spring. The lower electrode of the piezoelectric ceramic sheet is electrically connected to the base and grounded by conductive adhesive. The upper electrode is led out by a wire as a signal output terminal.

3. The vibration transmitter according to claim 1, characterized in that, The measurement circuit module includes a charge amplifier, a first-stage low-pass filter, and a second-stage low-pass filter. The charge amplifier is an operational amplifier for instrumentation. The first-stage low-pass filter adopts a second-order Sallen-Key topology, which is mainly used to suppress high-frequency noise and aliasing components in the output of the charge amplifier. The second-stage low-pass filter adopts a second-order Butterworth topology.

4. The vibration transmitter according to claim 1, characterized in that, The signal conditioning module includes a differential amplifier and a buffer amplifier; the differential amplifier is a fully differential operational amplifier used to suppress common-mode interference signals; the buffer amplifier is a low-output-impedance operational amplifier used to drive the input terminal of the integral conversion module.

5. The vibration transmitter according to claim 1, characterized in that, The integral conversion module adopts a dual-integration path structure, including a high-pass filter integral path and a digital integral path. The high-pass filter integral path includes a cascaded high-pass filter and an analog integrator, used to process vibration signals with frequencies higher than 5Hz. The digital integral path includes an analog-to-digital converter, a digital signal processor, and a digital-to-analog converter. The analog-to-digital converter converts the vibration signal into a digital signal, the digital signal processor performs integration on the digital signal using a numerical integration algorithm, and the digital-to-analog converter converts the integrated digital signal into an analog velocity signal. The integral conversion module also includes an integral path selection circuit, which includes a frequency detection unit and an analog switch. The frequency detection unit outputs a control signal based on the comparison result between the frequency of the input signal and a preset threshold, and the analog switch activates either the high-pass filter integral path or the digital integral path according to the control signal.

6. The vibration transmitter according to claim 5, characterized in that, The integral conversion module further includes a phase compensation circuit for compensating for phase errors introduced during the integration process. The phase compensation circuit includes an adjustable phase shift network and a non-volatile memory. The non-volatile memory stores phase shift compensation parameters that are pre-determined by an external calibration device based on the phase characteristics of the piezoelectric sensing element module. The adjustable phase shift network performs phase compensation based on the phase shift compensation parameters stored in the non-volatile memory.

7. The vibration transmitter according to claim 1, characterized in that, The amplification module includes a programmable gain amplifier and a buffer amplifier; the programmable gain amplifier is a digital programmable gain amplifier, and the buffer amplifier is a low-noise high-speed operational amplifier.

8. The vibration transmitter according to claim 1, characterized in that, The transmitter output module includes a voltage-to-current conversion circuit and a current limiting protection circuit; the voltage-to-current conversion circuit adopts a four-transistor current source topology to convert the input voltage into a standard current signal output.

9. The vibration transmitter according to claim 1, characterized in that, It also includes an anti-interference module, which is disposed between the piezoelectric sensitive element module and the measurement circuit module, for suppressing external electromagnetic interference; the anti-interference module includes a common-mode choke and a differential-mode filter; the common-mode choke uses a manganese-zinc ferrite core to suppress common-mode interference; the differential-mode filter uses an LC filter topology to suppress differential-mode high-frequency noise.

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