High frequency noise from Duet board
I am seeing a lot of noise coming from the Duet board, severe enough to interfere with external circuits. If I put the z_probe_in pin on an oscilloscope, I see a ~75Khz sinusoidal signal that reaches almost +/- 1 volt (shouldn't this pin be dead, as it's just for reading input?). This is without the motors on. When the motors turn on (even if idle – movement versus idle seems to make no difference), this changes to about 180Khz at the z_probe_in pin, and noise of 75Khz - 116Khz is measured on external circuits that I have hooked up for z probing. It seems that some of this noise is coming straight from the board, and some is EMI from the motors or stepper drivers, since other circuits do not have to be physically connected to be affected.
I note that none of these frequencies are anywhere near the clock speed of the processor (120 Mhz) or the stepper drivers (Trinamic TMC2660 at 15 Mhz). Do the frequencies I am seeing ring any bells? Should the board pins have this noise on them? Any idea why stopped (but energized) motors would give off this type of EMI?
I'm thinking about putting a low pass filter in front of the circuit that is experiencing the problems, but I'd like to better understand what is going on first.
I tried a low pass filter in front of my external circuit using a 2K resistor and a 22 pF capacitor.
This does seem to cut down the noise a bit. That's actually a little strange to me because those values are calculated to have a frequency cutoff of 3.45 MHz – far higher than either the signal or the noise. But, if I up the capacitor to 104 pF, which theoretically has a cutoff of 765 KHz (still far higher than the signal or the noise), then the circuit doesn't work at all.
Any thoughts on how to cut down the noise more effectively would be great. As would knowing if this noise should be there at all!
The other source of high frequency noise on the board is the 5V BUCK circuit. It might be worth a test diabling this using the jumper and powering the 5V on the board externally or via USB (just to test).
I am very surprised you are seeing enough electrical noise to cause problems away from the board though.
Thanks for the input – I'll try disabling that if I end up needing to. With some changes to the resistor and capacitor values, I have the noise down to about +/- 0.2V, which I think is ok. I can easily discriminate the piezo signal from the noise with my eye -- now I just need to make sure the circuit can.
Your "104pF" capacitor is actually a 0.1uF capacitor. The 104 marking means 10e4.
Yeah… How do I make an embarrassed face lol.
I figured the 104 thing out last night. Once you choose proper values (I ended up using a 1 nF capacitor), such a simple circuit is quite effective at filtering the high frequency noise with almost no impact on the signal.