[time-nuts] measuring tiny devices

[Lux, Jim]

On 5/25/22 3:16 PM, ed breya via time-nuts wrote:
> 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. 


You might check out the NanoVNA - people have made a variety of novel 
fixtures for measuring small parts (i.e. 0604 SMTs)

It certainly has the measurement frequency range you need. The trick is 
figuring out whether you want to do a series or shunt measurement, and 
that sort of depends on the reactance of your device at the frequency of 
interest.