Maker Pro
Maker Pro

Need help for building a noise filter


I have a noise problem on a project that i work on that need to be adressed. I want make a noise filter (pi filter) to eliminate, or greatly attenuate the 350 khz ac that i got from my +5vdc power bloc that power my project. See the scope trace if you wanna see by yourself.

The problem is that this ac get in my oscillator circuit and make it unstable.

So i found numerous site about pi filter and how to make them. But i'm not sure about the maths there. Also i alreayd put a electrolytic caps to help filter out the low noise. I dont know if it will intefere with the filter if i place it after the caps already there.

I found this site pi low pass.htm to help me calculate the value i need. So i've done the calculation for a 300 khz cutoff and it gave me the value : C1-C2 = 6.675941nF and L1 = 25.73782uH .

Can somebody confirm that i can use those values to make my filter and eliminate my problem???




  • Oscilloscope Trace.jpg
    Oscilloscope Trace.jpg
    115 KB · Views: 172
You already have a low pass filter with the electrolytic capacitors. These are not too good at high frequencies but can be paralleled with something like 10nF to deal with high frequencies.
Any low pass filter should have cut off well below the troublesome frequency, say 10kHz, there will then be a lot of attenuation at 350kHz.

7 places of decimals is absurd, do a simulation with preferred values. You only need exact values when making tuned circuits.

The problem may be due to bad earth (ground) design where the PCB tracks do not provide a fixed earth.