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EDIT: I found the problem: with these values cut-off freq is 33MHz and the signal is only 1MHz, duh.

Please have a look at my very simple schematic

enter image description here

And here is the scope dump enter image description here BLUE is output from square gen. RED is X3 output, and 0.4V/div 5uS/div

The square is rock steady, but after C1 all the fluctuation begins, but I'm expecting a stable amplitude. I even switched to battery power to eliminate any potential noise from the power supply, but the problem persists. I'm not using a breadboard, and wires and component legs are max 20mm (3/4 inch).

It's probably a normal occurrence with hi-pass, but I have no experience with it, any ideas or suggestions?

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  • \$\begingroup\$ Is that a simulation or a real setup? In both cases, what are the exact components involved, and what exactly do you want to tell with that schematic like drawing? \$\endgroup\$ – PlasmaHH Nov 21 '17 at 16:44
  • \$\begingroup\$ @PlasmaHH I have amended the schematic. There are only two components, one resistor and one capacitor. And it's a real setup, no sim. \$\endgroup\$ – r.reindeer Nov 21 '17 at 18:06
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You are feeding a high pass filter a signal with a frequency well below the filter cutoff. In that case, the filter acts as a differentiator. Thus the output is the derivative of the input. For a square wave, the output will be a positive maximum at the positive going transition of the square wave and a negative maximum at the negative going transition of the square wave. That explains your output waveform. Another way of looking at it is that most of the input voltage appears across the capacitor. Since the current through a capacitor is the derivative of the voltage, the output voltage is this current multiplied by the output resistance.

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  • \$\begingroup\$ This doesn't explain why the output varies so much. The morphology is a differentiator, but the spikes should all be about the same size \$\endgroup\$ – Scott Seidman Nov 22 '17 at 0:27
  • \$\begingroup\$ Scope sampling rate issue? \$\endgroup\$ – Scott Seidman Nov 22 '17 at 0:28
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This is a huge nonlinearity, and not normal behavior for a high-pass system. You should be looking toward intermittent connections, faulty scope probes, and all your scope settings.

Further, I don't understand what your 5V supply is doing, or how your function generator is hooked up. A passive circuit does not need power. Please update your question to better describe all your connections.

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  • \$\begingroup\$ You are right, I have amended the circuit diagram, the previous was ripped out of a greater schematic context. \$\endgroup\$ – r.reindeer Nov 21 '17 at 18:04

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