I have a low pass RC filter (R=12Ohm, C = 22µF) Which is showing some strange behavior in the phase shift. The cutoff frequency is around 600Hz, as calculated.
But the phase shift goes to -45° only at 890Hz and than it starts ascending back to zero? Could this be due to inductance in the large electrlytic capacitor?
Any ideas? Thanks in advance Tom
Electrolytic capacitors have fairly large ESR values, and they're fairly constant across frequency. As the reactive impedance drops according to frequency, the total impedance becomes dominated by the ESR and you essentially have a resistor. The following figure from Murata shows this:
As you can see, the top curves show the results from a 10uF electrolytic. At 0.1k, the reactance is dominant, about 10X the ESR. By 1k, there's less difference, and by 10k they're essentially the same. Since the reactive component here is almost negligible, you have a voltage divider at this frequency, which doesn't introduce a phase shift.
Your cap goes through the transition earlier in the frequency range because 22uF is less than half the reactance of 10uf.
