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Old 13th Apr 2020, 6:21 am   #34
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: Top band transmitter

The Vackar is still just an L-C based oscillator. It carefully limits the amount of coupling between the active device and the resonator, so its stability tends towards the stability of the inductor and the capacitors used... provided it is carefully designed. Other oscillators can also be carefully designed.

Oscillators in the better than usual class for stability have measures taken to compensate thermal change. A combination of notmal and NTC capacitors can be designed to produce a compensation effect. It's a matter of curve-fitting, so careful design can get it right at two points on the temperature scale, at least for the individual oscillator the compensation network was designed using.

The next step up in stability needs individual adjustment of temperature compensation. Oxley Developments made the 'Tempatrimmer' and cheaper 'Thermotrimmer' capacitors which gave fixed capacitance and variable temperature coefficient. These were expensive components, and the time taken to vary temperature and to individually adjust them is also going to be very expensive. The Codar stuff was, shall we say, marketed at the time at very attractive prices.

The Vackar has a reputation for stability, but it can be equalled, and it also can be spoiled. But it's entered the folklore as put in a Vackar circuit and you can say it's very stable.

To answer your question, you need someone with an AT5 who's had it in a thermal chamber and made measurements. I've not heard of anyone doing this.

If you're interested in the Vackar and want to know what makes a good one, Peter Martin G3PDM covered it well in the RSGB article for his HF receiver. This can be found in RSGB handbooks from the late sixties and through the seventies. I put a cut-down version in the ARRL handbook from 1995 onwards. It's good advice and still relevant to all VFOs.

I don't think you can still get tempatrimmers and thermatrimmers from Oxley, and they don't seem available on the auction sites. I put a little circuit for a varactor/thermistor variable tempco compensator in the ARRL handbook. It's oddity is that it's designed to be easy to adjust in a single temperature sweep, instead of repeated ones.

The AT5 doesn't have a reputation as a bad drifter, so it should be OK for its task.

There is also a less well known cause of drift. If a VFO is used on the same frequency as the output of a transmitter and a little RF leaks back into the oscillator, this can pull it in mild cases, and induce unstable oscillation in worse cases. People in the day were aware of this and some equipment went to lengths to either multiply up lower frequency VFOs, or use crystal mixer schemes all to avoid this effect.

So have a look at the AT5 circuit. If there isn't individually adjustable temperature compensation, its stability won't be amongst the best, so that shoots down the 'extremely' adjective.

David
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