Calculating channel B/A in spectrum mode give strange results

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Calculating channel B/A in spectrum mode give strange results

Post by mathiasmagdowski »

Dear all, I would like to measure the transfer function of a simple low-pass filter of 1. order, i.e. an RC circuit or a ohmic-capacitive voltage divider for educational purposes. The transfer function, i.e. the ratio between the output voltage and the input voltage should be displayed in a double logarithmic diagram.

I'm using a PicoScope 2204A with the integrated signal generator to generate a square wave signal with 100 Hz frequency a 50 % duty cycle. The filter consists of a 1 kOhm resistor and 1 µF capacitor giving a corner frequency of around 160 Hz. I'm using channel A to measure the input voltage, channel 2 to measure the output voltage in spectrum mode. With a maximum FFT frequency of 49 kHz, the results looks quite good, see the attachment. Still, interestingly, the transfer function (orange curve) shows maxima at multiples of 200 Hz, not 100 Hz.

For a simple ohmic voltage divider with the same 1 kOhm resistor and a second 2 kOhm resistor, the results looks nonsense, see the secound. There is only a peak at low frequencies. The remaining transfer function shows only noise. Still, the spectra of the input and output signals look useful and correct.

Now the question is: How does the calculation channel B/A works? Is the division done in time domain and the FFT is applied to the results of this OR is the spectrum calculated first and after this the division is applied?

Are there any other suggestions on how to measure the frequency-dependent transfer function of a filter, maybe with some swept excitation using the sweep-mode of the signal generator?

Any help or suggestions on this are appreciated!

Best regards, Mathias Magdowski

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