VFO Musings

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MARTIN MARRIS:
So, as I get ready to build my first transmitter, I've also been devoting a lot of thought/planning to the future VFO. I've obtained most of the parts for "An Easy-To-Build VFO" by Lew McCoy (QST, February 1962) but I'm hesitating as I start planning the tuned circuit. It's a classic Colpitts oscillator with a buffer/multiplier stage, with a base range of 3.5-4 MHz. Let's bear in mind that I'm exclusively interested in the CW portions of the ham bands and for the time being focusing on 80m, 40m and 20m. Also, please forgive the elementary nature of the questions I am going to pose; I'm still only a novice homebrewer. Several things strike me, and I hope I'm not too off-base here:

(1) Because this is a "multiplying" system rather than a heterodyne system, the band range of this VFO will double each time the frequency doubles. As published, this design has 500KHz bandspread at 80m, which would imply that it tunes across 1MHz at 40m (7 MHz to 8 Mhz) and 2 MHz at 20m (14 MHz to 16 MHz). I am having trouble understanding how this can be made to work. In particular, at 20m the CW portion of the band would be covered by about one-20th of the revolution of the main tuning dial.

(2) It also seems to me that with a multiplying design, frequency stability is much harder to achieve on the higher bands. If there is any instability at all at 80m, then (in absolute frequency shift) that instability is multiplied by four at 20m. Of course there are all sorts of things one can do to build a very stable VFO ... but presumably this is one of the reasons why there was a shift to heterodyning systems.

So I'm thinking of doing a couple of things:

(1) Considerably narrowing the bandspread on 80m, to no more than 100KHz. That in itself will make a big difference to the "resolution" of the dial on the higher bands. I will have to figure out what is the best combination of LC to do this (simply lowering the value of the main tuning cap, on its own, may be sub-optimal).

(2) Adding a bandswitch that would insert some series capacitors so that on the higher bands, the CW sub-band would cover most of the dial. I don't like the idea very much (because the moving parts in a mechanical bandswitch make it harder to achieve good stability) but otherwise I fear the VFO would not be terribly useful on bands above 80m.

Thoughts, anyone?

73 de Martin, KB1WSY

Clark McDonald:
My experience, back in the day, soon taught me that there are no shortcuts, no super-economical or "easy" VFOs worth pursuing.  YMMV


73

Tim Shoppa:
Ever work as a bagger in a grocery store? Remember the old lady who wanted all her groceries in one bag but she didn't want it to be heavy? All designs have some kind of compromise. And the ones with the most "features" or "versatility" could end up being the crappiest in terms of usability and stability.

The design tradeoffs in that QST article are the same ones made in e.g. Heath HG-10B and many other general-purpose VFO's of the era. Most used string-dial drives to get a fast but not ridiculously fast tuning rate, while covering most of 80M+75M, and many reduced the fundamental range for the higher bands just as you suggest.

Today there's no reason to try to "emulate a store bought VFO" including its design tradeoffs. Covering just a sub-section of the CW band is perfectly reasonable.

Doubling in the VFO is in fact a very good idea. If you want an 80M VFO, you might start with 1.75Mc as your base VFO frequency and at least double before going into the transmitter.

Used HG-10B's and similar are out there and you might pick one up and give it a try if you've never used one. Yes, even with string-dial drive, the tuning rate will seem ridiculously fast even on the low bands but you'll get used to the rate and mechanical slop in the tuning mechanism and work it to better than a few hundred Hz. Remember 50 years ago, folks would call CQ and "listen up" or "listen down" many kc for callers, because the modern day expectation of zero-beating to the nearest Hz was simply ridiculous, and even having a VFO was a luxury not everyone (and no novices) had.

Dieter Kuespert:
It is certainly a lot of fun to build those vintage circuits.
Reading through the QST article you'll have seen that the VFO was only designed for 80 and 40 m. I think the author knew very well why.
My approach to higher bands would be a separate VFO. The power supply can be common to both.

Dale Hunt:
I had the VFO assembly out of one of Johnson transmitters (Viking?) and it actually had two
coil assemblies - one running on 160m for that band and 80m, then a second with narrower
tuning range for 40m that was used through 10m.  That also improved the drive on the
higher bands because fewer multiplier stages were used, and it was less likely to tune the
rig up on the wrong harmonic and end up transmitting on 17.5 or 24.5 MHz by mistake.
Switch contacts really don't need to be that bad as long as they make reliable contact. 
(Best if there is some DC flowing through the contacts to keep them clean.)



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