[time-nuts] Re: Noise down-converter project

[ed breya]

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