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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 17th Jan 2020, 3:31 am   #21
Terry_VK5TM
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Default Re: Kenwood DM-81 GDO; Anything Special?

The only FET based GDO schematics I can find also measure either the RF coming off the source of the FET or the gate voltage.
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Old 17th Jan 2020, 5:46 am   #22
Radio Wrangler
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Default Re: Kenwood DM-81 GDO; Anything Special?

The gate DC voltage isn't simply a measurement of oscillation amplitude, it is also the command parameter controlling the amplifier gain, so it is a combination of both methods and it works quite well.

Oscillators have to have some form of amplitude stabilising mechanism which amounts to feedback. If an oscillator design has strong control of its amplitude, then the amplitude varies little if a nearby resonator sucks a little power from it. In this case sensing the oscillation amplitude becomes a poor dip indication, you have to monitor the amplitude control mechanism.

Simple oscillator circuits vary in how tightly their amplitudes are controlled, but most are not very tight, so you do see a dip in amplitude and you get variation from circuit to circuit of how good a dip you get. The less well controlled oscillators tend to have trouble starting, or dropping out over part of a tuning band. The oscillators which seem to survive everything tend to give poor dip indication.

For simple circuitry, the triode is a remarkably difficult GDO to beat. A 50uA meter in a grid leak with the RF AC coupled to the grid shows both the amplitude and how hard the oscillator is working. Heaters and HT are inconvenient. For a battery powered all-in-one-box job, the JFET seems to be the closest alternative - the gate current will be rather low so an amplified meter helps.

Bipolars have a sharper non-linearity which reduces the obviousness of the dip in terms of tank amplitude.

David
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Old 17th Jan 2020, 9:24 am   #23
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Default Re: Kenwood DM-81 GDO; Anything Special?

Interesting thread. Last year I came across a couple of G3WPO units at a rally. Definitely inferior to even the heathkit GD-1 believe it or not so that may indeed back up the assertion of valve superiority. I may look out for a Kenwood unit as that’s a step above all the FET units I’ve seen.

Not sure why I seem to collect the things as I never actually use them!
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