Thanks Mike, for info on LCR alternatives. It's good to know of others
out there, if needed. I have an HP4276A and HP4271A. The 4276A is the
main workhorse for all part checking, since it has a wide range of LCZ,
although limited frequency coverage (100 Hz - 20 kHz). The 4271A is 1
MHz only, and good for smaller and RF parts, but very limited upper LCR
ranges. I think it works, so I can use it if needed, but would have to
check it out and build an official lead set for it. I recall working on
it a few years ago to fix some flakiness in the controls, so not 100%
sure of its present condition.
The main difficulty I've found in measuring small chokes is more of
probing/connection problem rather than instrument limitation. For most
things, I use a ground reference converter that I built for the 4276A
many years ago. It allows ground-referenced measurements, so the DUT
doesn't have to float inside the measuring bridge. The four-wire
arrangement is extended (in modified form) all the way to a small
alligator clip ground, and a probe tip, for DUT connection, so there is
some residual L in the clip and the probe tip, which causes some
variable error, especially in attaching to very small parts and leads.
When you add in the variable contact resistance too, it gets worse.
Imagine holding a small RF can (about a 1/2 inch cube) between your
fingers, with a little clip sort of hanging from one lead, and pressing
the end of the probe tip against the other lead. All the while, there's
the variable contact forces, and effects from the relative positions of
all the pieces and fingers, and the stray C from the coil to the can to
the fingers. I have pretty good dexterity, and have managed to make
these measurements holding all this stuff in one hand, while tweaking
the tuning slug with the other.
I had planned on making other accessories like another clip lead to go
in place of the probe tip, but not yet built. I also have the official
Kelvin-style lead set that came with the unit, so that's an option that
would provide much better accuracy and consistency, but the clips are
fairly large and hard to fit in tight situations, and the DUT must
float. Anyway, I can make all sorts of improvements in holding parts and
hookup, but usually I just clip and poke and try to get close enough -
especially when I have to check a lot of parts, quickly.
The other problem is that the 4276A is near its limit for getting
measurements below 1 uH, with only two digits left for nH. The 4271A
would be much better for this, with 1 nH vs 10 nH resolution.
If I get in a situation where I need to do a lot of this (if I should
get filter madness, for instance), then I'll have to improve the tools
and methods, but I'm OK for now, having slogged through it this time.
Ed
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