>
> Hello Harold,
>
> thanks for the info.
>
> Would you happen to have a datasheet? I don't know what a "ceramic
> discriminator" is. What of the other 3 filters? They can't all be used
> for the same thing?
>
> Thanks, TTYL
>

I don't have datasheets on those specific parts, and they may be OEM part
numbers. The ceramic filters I used were from Murata. You might check them
out.

A "ceramic discriminator" is just a ceramic resonator that is used in a
discriminator (FM demodulator). It replaces a parallel LC filter used in a
quadrature detector FM demodulator. As I recall, the IF is applied to the
filter (whether ceramic or LC). As the frequency varies (as in FM), the
phase between the voltage and current varies. This phase difference is
used to determine the instantaneous frequency and demodulate it.

There are lots of different methods of demodulating FM. Precision monitors
in radio stations have used "pulse counting" demodulators where the FM is
heterodyned down to a 200kHz IF. The 200kHz signal then drives a
monostable multivibrator with a constant output pulse width. As the
frequency varies, the duty cycle varies. This variable duty cycle signal
is passed through a low pass filter to recover the audio. This yields a
very low noise and very low distortion demodulation of the FM.

I suspect most recent receivers are demodulating the FM using DSP. For
example, Silicon Labs has several FM receiver chips that are DSP based.

So, good luck with your filters! The band pass and notch filters probably
have an input, an output, and a ground terminal. They need to be driven
and loaded with a specified resistance for them to work properly. The
discriminator is probably a two terminal device that is designed to work
with a specific chip (like the SA639 I mentioned before).

Harold




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