I've got one of those super cheap ($10) signal generator kits, and despite the fact that I'm almost positive the XR2206 chip it's using is a counterfeit knockoff (it gets
hot, and I had to bodge some traces on the PCB to get it to output anything at all), it does produce a basic sine wave signal that I can use as an input as I'm playing around with guitar/bass effect pedal circuits. However, despite the fact that it is definitely producing the overall sine wave I expect, it is also outputting some very high frequency noise on top of the expected signal, at almost double the amplitude of the expected signal!
![Why is my low-pass filter not filtering out this high-frequency noise? 1032430-0](https://www.eevblog.com/forum/index.php?action=dlattach;topic=248270.0;attach=1032432;image)
The signal I'm expecting is roughly 1kHz, and since the circuits I'm building are designed to operate in the audio range, I assumed that adding a low pass filter at roughly 20kHz should do something to reduce the noise. To this end, I added a 820 ohm resistor in series with a 0.1uF capacitor to ground. When the noise remained, I tried adding another low pass filter at ~1kHz (150ohm, 1uF). The noise yet remained!
![Why is my low-pass filter not filtering out this high-frequency noise? 1032434-1](https://www.eevblog.com/forum/index.php?action=dlattach;topic=248270.0;attach=1032436;image)
Channel 1 is the original signal, channel 2 is after the 1kHz filter, channel 3 is after the 20kHz filter (I have the filters reversed - 1kHz first, followed by 20kHz - in the circuit; the order does not change the result).
Zooming in on one of the spikes, I see that there is a slight bit of attenuation on the noise, but not nearly as much as I would expect there to be:
![Why is my low-pass filter not filtering out this high-frequency noise? 1032438-2](https://www.eevblog.com/forum/index.php?action=dlattach;topic=248270.0;attach=1032440;image)
Am I misunderstanding how an RC filter is supposed to work? Is there a better way to clean up the signal I'm getting out of this cheap-ass signal generator?
Also, the signal generator is able to output a pretty clean square wave with a ~100us rise time. Would I be better off trying to filter that square wave into a sine wave instead?