In message , Roy Lewallen
writes
Dave wrote:
Roy Lewallen wrote:
Is this by any chance an exam question?
No, it is not. I was shown it by a lecturer of mine more than 10
years ago. The result is quite interesting.

With the given values, it's a constant-impedance network. I've used one
many times in time domain circuit designs. Its impedance is a constant
real value of 1000 ohms at all frequencies. Since "resonance" implies a
single frequency (at which the reactance is zero), this circuit isn't
resonant at any frequency. The circuit is often used in time domain
applications (e.g., oscilloscopes) where it's sometimes necessary to
provide a constant impedance load but you're stuck with a capacitive
device input impedance. In that situation, the C is the input C of the
device. However, the transfer function isn't flat with frequency-- you
end up with a single pole lowpass rolloff, dictated by the R and C values.

For anyone who cares about such matters, "resonate" is a verb,
"resonant" is the adjective, and "resonance" the noun. A resonant
circuit resonates at resonance.


I think that the principle of this circuit is similar to the
constant-impedance equaliser - such as used to compensate for the loss
of a length of coaxial cable over a wide range of frequencies (very
common in the cable TV world). This is frequency-selective in that it
has essentially zero loss at a pre-determined 'top' frequency (say
870MHz), with progressively increasing loss at lower frequencies (the
inverse of the cable loss). As it has a constant (75 ohm) input/output
impedance, it is therefore resonant at all frequencies from 0 to 870MHz.
--
Ian