Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!world!ksr!jfw From: jfw@ksr.com (John F. Woods) Newsgroups: sci.electronics Subject: Re: WANTED: A crystal AM HF recvr, and "one antenna -> 2 recvrs" Message-ID: <3827@ksr.com> Date: 3 Jun 91 14:45:32 GMT References: Sender: news@ksr.com Lines: 44 mrapple@quack.sac.ca.us (Nick Sayer) writes: >I am using a Sony ICF-2010 and a homebrew modem to receive and decode >CHU. It's a bit of a waste to use such a fine radio for something >so simple. So... >1. Is there a cookbook circuit for a crystal controlled single >conversion superhet? It'd be nice if it were frequency switchable >in a simple way. Even better if it were TTL frequency switchable. I don't think I've seen a "cookbook" circuit for this, but just about any simple single-conversion circuit should be easily converted by changing the VFO to a crystal oscillator at the appropriate frequency. Sometime ago (Feb 89? Feb 90? I'll try to look it up) Doug Demaw had an article in QST about how to modify a standard transistor AM radio to be a tunable IF for shortwave receivers; since you're trying to receive an AM signal, you don't even need to modify the AM receiver; tune it to 1000Kc, get an 8.333Mc crystal, (CHU is 7.333Mc, if I remember correctly), build a simple 3N211-based converter (there are abundant "canned" circuits for that) and you're all set (remember to use a good metal case (and probably a good input filter) to prevent stray AM broadcast band signals from getting in; rather than just picking a random IF like 1000Kc, you'd do somewhat better to pick a frequency not in use in your area). The advantage of this approach is the simplicity of wiring (in fact, it occurs to me that you could use an NE-602 IC for plenty of gain instead of a single-MOSFET converter; you don't even have to bias it). To use this strategy, it is best to pick an IF which is on the high end of the range the AM radio will receive, to maximize the rejection by the input tuned circuit; also pick a crystal which is on the high side of the desired signal so the image will be at a higher frequency where the circuit will have (a bit) less gain. Since you're using a modem to decode the signal, you can probably get away with the extra noise from having a not-terribly selective IF. >2. I want to hook one antenna to 2 (or 3, or 4.....) receivers. >And no, a switch won't do. I would like them all to work AT THE SAME >TIME. Is this simple to do? If the application isn't terribly critical, then you ought to be able to get away with just wiring them all in parallel. Sensitivity will suffer a bit, since they're all dividing the input power (this can be ameliorated by a preamp); if the receivers are of different designs and have different input impedances, some receivers will hog the input signal -- to fix that you'll need impedance matching to make all this inputs the same. John Woods WB7EEL