Quote:
Originally Posted by G6Tanuki
Pragmatically, a sensibly-designed free-running VFO shouldn't suffer enough drift to be a problem when used with 1.8/3.5MHz AM.
Keep the heaters on all the time, keep the VFO running all the time, and use a diode-switched capacitor to shove the VFO up a few tens/hundreds of KHz when on receive.
FET and transistor VFOs - in the absence of the always-on-running mitigations - have low thermal-inertia and so just swap slow frequency-drift for fast-drift.
Klaas Spaargaren PA0KSB's "Huff and Puff" stabiliser from the 1970s is an interesting fix for the problem; it will happily lock to a VFO-tuned frequency and hold it to within 50Hz.
http://www.hanssummers.com/huffpuff.html |
Last point first, I was talking to someone a while ago who absolutely did not want to add a FET to his vintage receiver even though it could have been
the way to have given it a feature he wanted. If someone is keen to stay true to a
vintage chassis or a design and won't use a single FET, the idea that Huff and Puff will be acceptable seems unlikley.
As to the other comments about making a good valve VFO; those all seem very sensible, but it may well be possible that the use of a battery valve could just make it that bit better. Seems unlikely that it would make it worse, but I've never seen it tried.