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.
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?
Thanks,
Paul
--

"What is now proved was once only imagin'd" - William Blake

On Sun, 04 Dec 2005 15:25:51 +0100, Paul Burridge
k wrote:

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?

Hi Paul,

Accuracy and precision is no good unless you can duplicate the test
rig to the eventual environment of use. That said, precision
capacitors and inductors are three leaded devices. The third lead
goes to the shield around them. Obviously for either, a shield
changes what would have been the nominal value for the component.
However, that change also swamps all the variables that could disturb
the accuracy. In other words, the shield enforces a fixed environment
that reduces all other stray influences to a minimum.

In so doing, I've been able to measure standard capacitors and
inductors out to 9 places. Without those third lead configurations,
the same components would easily lose 3, 4, or 5 of those digits.

So one way to mount a mica cap would be over and close to a ground
plane that extends beyond its foot print by a significant distance.
This proximity would swamp the effects of other components nearby
causing a shift in the resonance (if and when they were added, or
removed). Building a cage around the capacitor would reduce these
effects even further. Of course, all such measures would shift the
native resonance, but you are never going to achieve that frequency
anyway.

You can, of course, elect to go the other way with a minimal ground
proximity. In that case you would use microstrip techniques to build
the test rig, making the strip with equal to the width of the
component (presumably being surface mount). However, SRF becomes
rather meaningless except as a general indicator. This is because
changing the board material from alumina to epoxy; or changing from a
series to shunt application can shift this frequency by 20% to 40%.

Another issue is with the leads themselves. ESR for caps can easily
tally up to a tenth of an Ohm and you have to select your caps on this
basis as much as for their inductance. In this regard, you measure
the D of the cap (dissipation factor) not Q (although each is the
inverse of the other, there are D instruments specifically for this).
This tenth Ohm is NOT necessarily in the wire lead (a common
misconception) but rather in all the parallel (or worse, series of the
wrapped cap) plate connections. For surface mount caps, you may want
to mount them 90° (up on edge rather than flat on face) to the board
to double the first PRF resonance and reduce the insertion losses
there and above.

The short answer to your question is how stable, and how accurate do
you want to reproduce the measurement to your application?

73's
Richard Clark, KB7QHC

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