Interference source current spectrum test of current probe

Electromagnetic interference suppression is the key problem to improve the low-speed sensitivity of light and small fiber optic gyroscope . To reduce electromagnetic interference , The interference characteristics of the interference source in the three elements of electromagnetic compatibility must be analyzed 、 The transmission characteristics of the coupling channel and the anti-interference characteristics of the sensitive equipment are deeply analyzed .

Fiber optic gyroscope (FOG) is based on Sagnac Effect angular velocity sensor , With its high sensitivity 、 Many advantages such as wide dynamic range are widely used in the field of inertial measurement . Fiber optic gyroscope is an extremely precise measuring instrument , With 0.01°/h Take typical inertial navigation products for example , Rely on relevant detection principles , Can achieve nV Weak signal detection of stage , But it is also very sensitive to some interference signals . for example , A major technical problem of fiber optic gyroscope ———“ dead zone ”, An important reason for this is the crosstalk between signals of the same frequency . Fiber optic gyroscope is a typical digital analog hybrid circuit , The electromagnetic interference is complex . With the development of fiber optic gyroscope towards the integration of light and small components and circuits , Problems caused by electromagnetic interference, especially power supply interference, have become increasingly prominent , It becomes the key problem to limit the low-speed sensitivity of fiber optic gyroscope .

In order to effectively evaluate the rationality of the power distribution network design of fog detection circuit , It is necessary to analyze the typical power interference propagation channel of fog from the perspective of electromagnetic compatibility . This paper selects DAC（DigitaltoAnalogConverter） To photodetector （PINFET） The power interference propagation channel is the research object , Use a current probe to DA Convert chip power pin interference current spectrum for measurement , Get the interference characteristics of the interference source , Thus, the interference size is quantized for the follow-up 、 Improve the design of power distribution network .

Analysis of power interference propagation characteristics from the perspective of electromagnetic compatibility , If it is necessary to ensure that sensitive equipment can still work normally under the interference of interference sources , Should meet ： Interference source strength × Channel attenuation < Anti interference capability of sensitive equipment . In the process of power interference propagation ,DAC It is the interference source , The detector is a sensitive device , The transmission channel is the power distribution network between the two .

The interference goes from... Along the power channel DAC Schematic diagram of propagation from chip to detector signal output 1 Shown . because DA The power distribution network between the conversion chip and the detector is not ideal 0 Impedance plane , therefore DAC Chip generated

Interference current I（ 覼 ） Flow through PDN The impedance of Z（ 覼 ） A certain voltage fluctuation will be generated at the power input of the detector V（ 覼 ）, This interference voltage enters the detector through the power supply terminal , Finally, it is coupled to the signal output of the detector , Become part of modulation crosstalk , After the subsequent signal conditioning circuit amplification, the performance and accuracy of fog will be affected .

therefore , The interference characteristics of the interference source must be obtained 、 Coupling characteristics of transmission channel and anti-interference characteristics of sensitive equipment , Only by reasonable means can DAC The power interference transmitted to the detector shall be controlled within the range that does not affect the gyro accuracy .

3 Interference source current spectrum measurement

The maximum detection current of the probe is 5A, The current voltage conversion relationship is 1V/A, The bandwidth is 120MHz, The amplitude can be measured at 5mA AC current below , Meet the measurement requirements . At the same time, it acts as a ring current clamp , It is not in contact with the conductors of the power supply , It is not necessary to change the original circuit structure , Convenient for experimental operation . Connect... With wires DA Conversion chip +5V Current limiting at the power pin 0 Ohmic resistance R64 Make a short circuit , Clip the current probe onto the wire , That is, the conductor passes through the annular closed area of the current clamp , The experimental hardware platform is shown in the figure 2 Shown .

The output data of gyroscope are collected at high speed and low speed respectively , After the gyro is judged to be in normal working state, the current probe is used for measurement , One end of the probe is stuck on the wire , The other end is connected to the oscilloscope . Set the oscilloscope time-domain acquisition bandwidth to 0～3MHz, choice AC coupling , Remove the DC bias , After transient capture FFT Transformation , Set the transform bandwidth to 10kHz～3MHz, The sampling rate is 10GHz/s,RBW=5kHz.

Set the oscilloscope to harmonic search mode , obtain DAC chip +5V The interference current spectrum waveform of the power pin is shown in the figure 3 Shown

The figure can show the power spectrum of the interference signal with the same frequency as the demodulated square wave harmonic , After a certain conversion, the actual current spectrum can be obtained . It can be seen from the measured interference current spectrum ,DA The supply current of the power pin of the conversion chip is mixed with the interference current with the same frequency as each harmonic of the demodulated square wave . According to the principle of correlation detection , Fiber optic gyroscope is only sensitive to the interference signal at the odd frequency multiplication of the demodulated square wave , Therefore, in the follow-up study, it is necessary to integrate the data from DA The interference with the same frequency as the demodulation square wave odd doubling frequency in the conversion chip interference signal is suppressed .

Through to DA The interference current spectrum of the power pin of the conversion chip is measured , It can be seen that the interference mainly comes from the signal with the same frequency as each harmonic of the gyro demodulated square wave . After the interference characteristics of the interference source are obtained, the interference at the special frequency can be suppressed , Thus, the anti-interference ability of the detection circuit of fog is improved .