Paul Burridge wrote:
On Sat, 03 Dec 2005 21:13:36 -0800, Roy Lewallen
wrote:


As I mentioned in my earlier posting, most people overestimate their
ability to make accurate RF measurements. It's not at all trivial. Be
sure to check your results frequently by measuring known load impedances
close to the values being measured. How do you find the values of those
"known" load impedances? Well, welcome to the world of metrology!


Roy, I've seen your postings hereabouts over the years and you've
always struck me as one of the most knowledgeable posters on this,
*the* most technically-challenging of all hobbies.

Thanks for your vote of confidence. But on the topic of network analyzer
measurements, I gladly defer to Wes Stewart, Tom Bruhns, and other
posters who have spent much more time making real-life measurements with
them than I have. I've used them from time to time, and for some really
challenging measurements, but not by any means as much as those folks have.

I've recently bought a VNA and am going about the laborious process of
setting it up with precisely-cut interconnects to the T/R bridge. Next
thing I need to know is...
Say I have a mica capacitor (for example) that I want to check for its
SRF. How should I mount this component so as to minimize stray L&C
from anything other than the component itself? IOW, what 'platform'
(for want of a better word) do I need to construct to permit accurate
measurements of this cap's RF characteristics in isolation?

In general, you minimize stray inductance by keeping leads short, and
capacitance by keeping conductors apart. The ideal setup is a coaxial
environment right up to the DUT, but even that is subject to coupling
around the DUT, both from one terminal to the other and from each
terminal to ground. If possible, the best plan is to calibrate out the
strays. That's a science and art in itself, and I'll have to yield to
people with more experience than mine for practical information about
how best to do this.

The effect of the strays depends heavily on what you're measuring. For
example, if you're measuring a low impedance, you can get by with more
shunt C than if you're measuring a high impedance. If you're measuring a
high impedance, you can tolerate more series inductance than when
measuring a low impedance. So when you inevitably find that you have to
make tradeoffs in designing a fixture, the trades you make will depend
on what you expect to measure.

Roy Lewallen, W7EL