How to measure Q of a varactor at 50MHz?

I would bias it, resonate it with a series coil at the frequency of interest, then find the effective resistance of the coil/diode pair. Then I'd subtract out the series resistance of the coil to get the series resistance of the diode, figure out the capacitive reactance of the diode (assuming I know the coil's inductance) and calculate Q.

But then, I don't have a network analyzer.

could be swamped from loaded Q of the circuit (loading from the input and output) or from coil Q, but you could back out some of that, by math. I would check out similar specs on similar device to get an idea, too.

Yes.

Your problem is then determining the parameters (primarily Q) of the inductor so that its effects can be factored out.

I would suggest that to do this you substitute a capacitor of known, or at least assumed very high Q, such as a ceramic piston/tubular trimmer. These should have Q in the thousands so for this purpose can be considered infinite.

If you've got a network analyzer, why not just use that?

John

That's what I'm doing, no?

The required Q is 200, so I'll use some piston caps I have.

But can't you just measure the vector impedance and infer Q? That should work unless the Q is extreme, or the na is crap.

If you resonate with an inductor, you may well be measuring the inductor's Q.

John

Yikes, the Q is >200... I think Tim wins. Make a T with a couple of R's. the coil & diode to ground, Sweep thru it, from the depth of the notch, get the series R. Short the diode, from the level get and subtract the coil's R. I know the diode's capacitance from an auto tester, so I can figure Xc, then get Q.