[GreenKeys] 170-Hz discriminator?

[AI2Q]

Looking over some old ARRL handbooks and RTTY magazine articles, I see a
number of TUs using the ubiquitous 88-mHy toroid filters that I knew and
loved as a youth. However, most circuits use 2125/2975 "tones," although
there are a few references here and there about putting in an extra
capacitor switch on one filter to permit operation at 170-Hz shift, too.

However, for the life of me, I don't see how that's possible. I've swept
some of the single-toroid/single-cap 88-mH L-C filters that I have here, and
most are quite broad. I find it hard to believe that a simple resonant
filter with a 300-Hz-plus skirt is going to discriminate between a 2125-Hz
tone and a 2295-Hz tone.

I must be missing the point somewhere.

(Not really relevant to this thought is that my application is to pick off
AFSK that my present TU generates and decode it for driving my local loop. I
need to do that because my homebrew TTL logic for driving the selector
magnets on my Model 28 on Rcv is derived from a switched-capacitor filter
bank in an old Kantronics UTU, and the clocking of those filters is turned
off during Xmit. Otherwise, I'd pick off the AFSK from the Rcv string in the
switched-cap circuit when transmitting).

Does anyone have a circuit for a 170-Hz audio filter/detector/discriminator?

Thanks in advance.

Vy 73, AI2Q, Alex in Kennebunk, Maine
http://users.adelphia.net/~alexmm/ai2q.htm

 .-.-.