Reg Edwards wrote:
How can a filter filter correctly when its input is terminated in
an indeterminate impedance?


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No problem! The frequency response of a filter, input to output terminals,
is independent of the source resistance.



That may be true in the Danelaw where you live, Reg, but not in the
rest of the universe. Imagine a circuit consisting of a voltage source
in series with a source resistance followed by a shunt capacitance in
parallel with a load resistor. Write the output voltage equation for this
circuit. Now, replace the source resistance with a short. Write the
output equation for _this_ circuit. Compare the two equations.
Are the frequency responses the same? Now use an indeterminate
impedance for the source resistance. Can you tell what the voltage output
of this circuit is?
73,
Tom Donaly, KA6RUH