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