How to measure Q of a varactor at 50MHz?

Any ideas? Bias it, resonate it with a coil at 50MHz, sweep thru it
with a network analyzer and figure Q from Fo and BW?

 

[night dalits]

<sdeyoreo@hotmail.com> wrote in message
news:eaj5f15vab7cdn77svf3ejkn1ik1pjjfee@4ax.com...

Any ideas? Bias it, resonate it with a coil at 50MHz, sweep thru it
with a network analyzer and figure Q from Fo and BW?

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.

 

[Tim Wescott]

sdeyoreo@hotmail.com wrote:

Any ideas? Bias it, resonate it with a coil at 50MHz, sweep thru it
with a network analyzer and figure Q from Fo and BW?

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Wes Stewart]

On Thu, 04 Aug 2005 22:24:35 -0400, sdeyoreo@hotmail.com wrote:

Any ideas? Bias it, resonate it with a coil at 50MHz, sweep thru it
with a network analyzer and figure Q from Fo and BW?

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.

 

[John Larkin]

On Thu, 04 Aug 2005 22:24:35 -0400, sdeyoreo@hotmail.com wrote:

Any ideas? Bias it, resonate it with a coil at 50MHz, sweep thru it
with a network analyzer and figure Q from Fo and BW?

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

John

 

[sdy]

That's what I'm doing, no?

 

[sdy]

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

 

[John Larkin]

On 5 Aug 2005 10:06:17 -0700, "sdy" <sdeyoreo@hotmail.com> wrote:

That's what I'm doing, no?

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.