# Impacts of low-pass filter on micro-controller board

I have a micro-controller board, driving small DC servo motors and I control the angle via a PWM signal (50 Hz) and everything is alright.

However, there is an oil gas motor, running next to the board, causes the DC servo motor to oscillate and a noise enters to my board.

I decided to to add a low-pass filter (L-C-L) after the DC regulated voltage (5v). I do not know the frequencies of the noise which are coming from that motor. So I am thinking to put a low-pass filter with 10 Hz as a cut-off frequency.

Would my new filter attenuate that PWM 50 Hz signals that drive my DC servo motor. Furthermore, the controller is running on a 20 MHz, would the filter degrade my controller performance ?

If the filter would not work, I guess I have to know the absolute range of frequencies I should band and then use EMI beads which block only a specific range of frequencies. But the problem is that the range of noise frequencies can not be determined.

• If you put the filter in front of your regulator, you won't have to worry about any of your concerns. Any reason why you put it after? Also be aware a LCL filter can resonate easily. Commented Oct 10, 2019 at 12:30
• If I put the filter in front of my regulator, can it reject the noise of the motor from entering the PWM signals? The motor is a gas-type and the EMI can enter to any part of the circuit. That's why I was thinking to filter the 5v power supply. Commented Oct 10, 2019 at 12:39
• You can dampen the filter, with an R in parallel with the inductor. Use R o value sqrt( L / C ). Thus 1 milliHenry and 1,000 uF is sqrt(0.001 / 0.001) needing 1 ohm. You can experiment with the value; 3.3, 4.7, 6.8 10. The resistor might get hot, if lots of trash to absorb. Make sure the inductor is large enough (core volume) to not saturate with the DC current needed. Commented Oct 10, 2019 at 12:44
• The spark noise is > 10kV impulse at low frequency RPM but fast rise time > 10MHz noise fill. Driver is possible 50Hz 1~2ms pulse servo with some excess rise time <1us that can be < 1ms -- with say 100 ohm driver that would be a 0.1uF shunt cap on servo signal. Try 0.01uF Commented Oct 10, 2019 at 12:51
• Do you mean that the noise of the motor is > 10 MHz, so an RC low pass filter with 160 Hz cut-off frequency on servo signal can solve it? What if I want to put a filter for the whole micro-controller circuit, there are other driving signals who should avoid noise also. Commented Oct 10, 2019 at 13:07