Testing a varactor for capacitance while under use?

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How about putting it in the tuned circuit and hitting it with a pulse... see at what frequency it rings at... you have a 'scope?

George H.

Do you mean a C-meter? Ohm it out to see if it has a DC path. If it does, just add a healthy series cap to block DC.

John

"Dave"

Few capacitance meters work when there are other components wired across the cap under test.

Eg, the cap meter function on my DMM will not read the capacitance of my scope probe when the scope is connected.

... Phil

How about putting it in the tuned circuit and hitting it with a pulse... see at what frequency it rings at... you have a 'scope?

George H.

Hey George,

Thanks for the reply. I do have a scope, and I am already hitting it with a pulse and tuning the circuit to increase the received pulse's amplitude, but the signal disappears into the noise as I go up the frequency range and I am wondering how close to my actual desired frequency the circuit is tuning to before the signal disappears. Problem is I don't know what the capacitance of the varactor is at any given point, and I am operating solely with the inductance of the whip antenna and connecting wire due to the constraints of the frequency I am trying to pick up (10KHz). My calculations tell me I only need a few picohenries to make a resonant circuit at the required frequency, and I believe the whip and connecting wire more than provide for that.

Open to ideas, and appreciate the response.

Dave

PS: am now thinking that it is 60Hz noise that I am fighting, and am thinking that if I ground the antenna somehow I can eliminate this. Will work on that...

Hey John,

Yes, either my LCR meter or my DMM, which has a capacitance test feature. And I already am blocking DC at this point, so that I am only working with the signal (and the noise that surrounds us all.)

Thanks,

Dave

Hmmm. My Mastech LCR meter has properly read the capacitance of caps still in the circuit, and I believe my (can't remember the nameof the brand) DMM has also. Assuming (hate that word) this is the case, is there any way to measure the capacitance of a varactor while it is in use?

Thanks,

Dave

DMM cap ranges are usually pretty bad.

This is excellent:

John

Picohenries? 10KHz? Fishy

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"Dave"

Assuming (hate that word)..

Cos it is all you ever do.

... Phil

"Jitt"

We have ourselves a genuine live one here - folks.

Needs to be kept on the hook long as possible.

.... Phil

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Hi Dave, I don't know much about antenna's and such. I don't think you can ground the antenna though. If there is low frequency crap getting in you can try a high pass filter.

The varactor spec sheet should give you some idea of the C as a function of V. I was suggesting that if you wanted to measure C(V) that you could let it ring with a known L and figure out C.. or look at some RC time with a known R.

You might try some other variable caps see if you can get that to work and then go back to the varactors once you have a better idea of the C value you need.

George H.

Characterize the varactor's C-V curve out of circuit, then monitor the (average, DC bias) voltage on the varactor (which can be done without disturbing the AC operating conditions).

Some varactor uses (frequency doubling) with high AC excitation can't be usefully probed this way. That's because the 'capacitance' is a linear characteristic, and varactors are intrinsically not linear devices.