[time-nuts] Re: measuring tiny devices

[Bob kb8tq]

Hi

That’s one of the methods. There are others for various filter topologies. Some are
more practical than others …

Bob

> On May 26, 2022, at 9:27 AM, John Lofgren <John.Lofgren at lairdconnect.com> wrote:
> 
> Bob,
> 
> You may be thinking of Dishal's method.
> < https://www.johansontechnology.com/dishal-bandpass-filter-tuning-using-lasertrim-chip-caps>
> 
> -John
> 
> -----Original Message-----
> From: Bob kb8tq via time-nuts <time-nuts at lists.febo.com>
> Sent: Thursday, May 26, 2022 10:18 AM
> To: Discussion of precise time and frequency measurement <time-nuts at lists.febo.com>
> Cc: Bob kb8tq <kb8tq at n1k.org>
> Subject: [time-nuts] Re: measuring tiny devices
> 
> EXTERNAL EMAIL: Be careful with attachments and links.
> 
> Hi
> 
> The real answer to the problem is to dig into the bowels of 1940’s electronic craft.
> There are various methods for setting up an L/C filter. You short this / open that sweep to find a dip or a peak. You move it to the “right” place. Just what you do depends very much on the filter design. Many L/C’s got done this way or that way simply because they would fit a known alignment method.
> 
> While it all sounds very cumbersome and obscure it actually isn’t. Long ago I stumbled upon a gal setting up very complex L/C IF filters this way. The display gyrated this way and that way as she did this or that. I don’t think it took her more than a minute to get the whole thing set up….. to this day, I’m amazed by how fast she was.
> 
> Do I have any useful links to actually read up on  this magic? … sorry about that.
> 
> Bob
> 
>> On May 26, 2022, at 4:58 AM, Lux, Jim via time-nuts <time-nuts at lists.febo.com> wrote:
>> 
>> 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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