[time-nuts] Re: measuring tiny devices

[Brooke Clarke]

Hi Ed:

You might surf the Accessories Catalog for Impedance Measurements.
https://www.keysight.com/us/en/assets/7018-06727/brochures/5965-4792.pdf
They have a number of SMD fixtures for 4-terminal pair LCR meters.
https://prc68.com/I/Z.shtml#KeyDocs

-- 
Have Fun,

Brooke Clarke
https://www.PRC68.com
axioms:
1. The extent to which you can fix or improve something will be limited by how well you understand how it works.
2. Everybody, with no exceptions, holds false beliefs.

-------- Original Message --------
> 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.
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