In article ,
Michael Dunn wrote:
Hello fellow radionuts (radionauts?). For some reason, I've decided
to design and build an FM tuner. Just to see if I can make a good one
I guess. A few questions have presented themselves:

-PLL or quadrature (IC) decoding?

Quadrature. Probably double tuned. (One of the old, now obselete,
demodulator IC's had a good description on double tuned quad coils,
but damned if I can remember which one. I searched for it, as there
was a newsgroup discussion of them going on at the time).

PLLs are feedback loops. Feedback loops are driven by error. In this
case error would show up as distortion.

You could also mix down to a lower IF than 10.7 and use a frequency to
voltage converter. (A precision one shot triggered on each cycle).

-Stereo decoding: single IC or "discrete"?

-Throw in SCA! Why not? PLL?

Narrowband IF ICs like the MC3361 make a good SCA demodulator. Basically
a superhet using the onboard oscillator and mixer that runs a 455 kHz
IF and a quadrature demodulator.

I did one with the MC3359, and I think it's on my ftp pages
at ftp://ftp.eskimo.com/u/m/mzenier .

-Anyone in S Ontario (KW) area?


-I'm still having a hard time figuring out AFC. I can see how I might
do it with a PLL - the DC error voltage being used to pull the LO to
the correct frequency. How was it done in the old days? ;-) If I

Just about all FM detectors (ratio det, discriminator, quad)
output down to DC (from the offset from mistuning) , which can
then be integrated/lowpassed to create the AFC voltage. On
frequency may be at 1/2 the power voltage for a quad detector, not
zero volts.

could design not only AFC, but also automatic tuning of an RF
preselector and perhaps even an antenna tuner, that would be tres cool.

Signal strength is a different signal from AFC. AFC comes after the
signal has been limited, so that's sort of information is lost.

Random comment: Elektor Electronics magazine, the european project
magazine, had a fancy FM tuner project about a half a dozen years ago.

Mark Zenier Washington State resident