Bruno,

2009/12/3 Bruno L. Albrecht <brunolalb@gmail.com>:
> Yeah, I thought a lot about doing it all in some microcontroller, with FFTs
> and Goertzel et al, but I'm trying to do it all in analog electronics.
>

In that case you're looking at bandpass filtering of some sort then
rectification. Or a number of PLLs as already suggested.
Opamps are probably the way to go  - either as direct filters, or
using them in a gyrator configuration to simulate an inductor and
resonate this with a capacitor for filtering.  Actually using
inductors could work, but the inductor size may be excessive.

There may be a switched capacitor filter configuration that would
work, or look at using the circuit configuration and components used
on graphic equalisers. You may have difficulty getting the bandwidth
narrow enough, but it could be worth a try.

For the PLL implementation the old CD4046 may be an option in place of
the (even older?) LM567 as already suggested.

I did do a FFT program designed to run on an Atmel micro. I used it to
decode DTMF tones among other things. IIRC it had a conversion time of
about 20mS and a max frequency of ~8kHz. Ran on an ATMega32 with a
3.62MHz clock. Half the conversion time was spent taking square roots
to get a voltage result - you may not need to do this for straightout
frequency detection.

RP
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