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I want to measure Differential mode HF 9kHz to 100kHz with 3dB accuracy on my 230V 50Hz mains. Is the following concept good?

circuit diagram

Let's imagine a first-oder high pass filter filter with its entry voltage U_e and its exit voltage U_a.

The formula for the voltage relation of a normal first-order high pass is formula, so for 50Hz it is ~ 0.015 so 3V so my oscilloscope is safe.

The assumption that entry impedance is zero is quite satisfied, because it is mains. The assumption that exit impedance is infinity on 50Hz is quite satisfied, because my oscilloscope has 1MOhm >> 10kOhm of the ressistor in parallel in my circuit diagram. The reactance of a 4.7nF capacitor for 50Hz is ~ 677kOhm, so through the 10kOhm resistor only flows 0.34mA so ~ 0.08W. My oscilloscope has a capacitance of 71pF, so for 9kHz the reactance is 250kOhm, for 100kHz it is 22kOhm which is high enough in comparision to 10kOhm, so I expect the measurements to be good enough.

Any flaws in theory or problems with the components metal film resistor/X2 capacitor?

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    \$\begingroup\$ Why not use two channels and subtract them using the normal oscilloscope features \$\endgroup\$
    – Andy aka
    Jul 17, 2023 at 13:28
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    \$\begingroup\$ When working with line sources, it's a good idea to use an isolation transformer on your scope. \$\endgroup\$
    – Aaron
    Jul 17, 2023 at 14:58
  • \$\begingroup\$ Andy aka: I thought a bit about it, but I can't think how to use two channels for my goal \$\endgroup\$ Jul 18, 2023 at 19:13
  • \$\begingroup\$ How to use 2 scope channels to measure differentially: electronics.stackexchange.com/a/76515/166672 \$\endgroup\$
    – Aaron
    Mar 18 at 14:41

1 Answer 1

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Instead of 4.7nF X2 275V I used 3 3kV 330pF ceramic capacitors in series (so 110pF and 9kV voltage spike safe/redudancy in case one capacitors shorts), less capacitance so more secure. I left out the 1MOhm resistor and built the 10kOhm resistance with 2 20kOhm resistance in parallel in case of one failing.

Works well.

Circuit

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