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With a simple filter circuit (either high pass or low pass), I'm seeing unexpected behavior in the o-scope output when I use a resistor with a low ohm value (e.g., 10 Ohms). Since a picture is worth 1000 words, here's my o-scope readout at the cutoff frequency (with a waveform generator creating sin waves at 160 KHz, w/amplitude of 11 V): Picture of o-scope

Here's the high pass filter circuit design I'm using:

enter image description here

There are probably several issues, but from my POV at least two issues appear relative to other circuits I've built and tested: 1) The dual sin waves offset by half a cycle, one faint and one 'blurry'. Note: the 'faint' wave flickers rapidly and is not always there. 2) The amplitude is a fraction of what I would expect (and what it is with other circuits I build w/higher ohm resistors). In this case I was expecting a voltage of ~8 V.

If I try other higher ohm resistors, they seem to work fine and I get readouts that have Bode plots in-line with expectations (e.g., 22k Ohm, 500pF for a cutoff of 14 KHz has a clean readout with a drop in signal that matches expectations).

Question: Why does this behavior occur when using a low ohm resistor, while higher ohm ones work fine?

Any help or reference links that will help me understand would be greatly appreciated. Apologies if I missed something basic, new to hobby electronics.

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  • \$\begingroup\$ What is your capacitor value? \$\endgroup\$ – WhatRoughBeast Dec 4 '18 at 6:57
  • \$\begingroup\$ 500pF is the capacitor, and resistor is 10 ohm. \$\endgroup\$ – Thomas Dec 4 '18 at 7:00
  • \$\begingroup\$ The flickering wave is probably due to triggering on noise spikes, which appears to be significant in your picture. \$\endgroup\$ – Sven B Dec 4 '18 at 8:07
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Treat the capacitor as a frequency dependent resistor and you end up with a voltage divider. With low resistance to the GND side your signal will be near 0V.

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