# What is the noise in this PWM signal?

I'm in the process of calibrating servos while constructing a new hobbyist robot arm. When testing the base rotation servo I noticed some unexpected noise in the PWM signal while the servo is in motion. There is no noise in the PWM signal when the servo is stationary. What causes the PWM noise? Is it likely to be significant? If so how would I filter it? The motion behaviour of the servo is exactly as expected, so the likely answer I'm expecting is to just ignore it, but I wanted to learn more about what I am seeing.

The equipment in use is:

• PCA9685 I2C PWM controller with a 470$$\mu$$F 16V electrolytic filter capacitor
• an inline breakout in the servo connection for multimeter and oscilloscope connection
• external bench power supply 6V limited to 2A
• only one servo is connected to PCA9685 port 0
• the servo is a GoBilda 2000-0025-0002

Update: Link to video of oscilloscope trace (as per the question description - no bypass capacitor between PWM signal and ground).

• So after some more reading, I’m thinking it’s back EMF from the servo and that a bypass capacitor from signal to ground near the servo would suppress it. A 0.1 micro Farads ceramic bypass capacitor was suggested in the linked question, is this suitable for my situation? electronics.stackexchange.com/questions/275441/… Oct 16 '21 at 20:38
• A 100nF ceramic capacitor from PWM signal to ground did not appreciably change the result. The shoulder joint servo (under no physical load) has similar results, but the PWM noise is less. I'll update my question with a video of the trace. Oct 16 '21 at 23:12
• Would you mind share data in txt file or post them here? From the picture, a common assumption about this kind of noise would be Gaussian noise (i.e. zero mean and some variance) and you should not ignore it. Kalman filter would be a perfect choice here. Oct 20 '21 at 7:49
• @CroCo I’ll work on exporting the data from the oscilloscope, may take me a while as I’ve not done that before. Can you tell me more about your thought? What makes you think the noise is Gaussian (i.e., normally distributed)? Why should it not be ignored (what are consequences you foresee)? Oct 20 '21 at 11:36