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Need help with Grid Dip Oscillators!

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Rein

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Hello guys!

I lack access to commercial RF measurement gear so I wanted to buid something I could use instead. Started with GDO's. Took one circuit from:

http://w1.859.telia.com/~u85920178/ That's one FET circuit by Harry Lythall. Wanted the circuit mainly for 60MHz - 144MHz air core inductors + antenna EVALUATIONS, meaning, I did not expect to measure anything exactly. Enclosed his design in a metal box as per his recommendations. :idea:

What I found out is in my mind rather annoying: when I tune the oscillator from one end of the band to the other, the RF voltage magnitude generated in the circuit changes by several volt. :( This in turn means that the meter reading also changes from min to max over the said band almost SEVERAL TIMES. So with one hand I tune the cap, with the other I change meter sensitivity. Rather clumsy I gather. :( The change/dip is supposed to happen when another tank is coupled to the GDO. My problem is:
how do I tell the USEFUL change/dip apart from the natural change of RF voltage in the circuit????? :evil:
Then I came accross this place here and I already found tons of useful info.

Anybody here done some experimenting with GDO's??? Is there say, a clever way to somehow balance out the change and thereby increase the sensitivity/usefullness of such a device??? Any good circuits, anyone??? Or are the GDO's purely hobby circuits that are not really expected to perform???

What do you think???

Rein
 

syzzler

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Hi Rein,

setup of a GDO is more matter of mechanics than electronics. Poor circuits lack on sensitivity more than amplitude flatness and your GDO bandwidth it's relatively small. Anyway remeber that you normally meter oscillator current not amplitude. Try to take a look for GDO pictures more than circuits paying special attention to the tank setup.

I hope this can be useful.

Syzzler
 

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