This was already SOP for any knowledgeable RF engineer by the
1940's. Somewhere I have a copy of Dishal's paper. I worked
for Zeta Labs in the 1970's and we sold lots of microwave filters
that were tuned this way. We didn't even have VNA's at Zeta.
We just used a directional detector (diode type) and a sweep
oscillator. All the techs could do this in their sleep.
Recently, I needed a 3 pole VHF bandpass filter for a client
requiring only 8% bandwidth. I knew from previous experience
that the Dishal method as the ONLY one that would work. And
sure enough, it worked perfectly.
That's fine, if you want to build a filter, but there are
many other cases where you need to measure immitance unrelated
to a filter.
Rick N6RK
On 5/26/2022 10:27 AM, John Lofgren via time-nuts 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 <[email protected]>
Sent: Thursday, May 26, 2022 10:18 AM
To: Discussion of precise time and frequency measurement
<[email protected]>
Cc: Bob kb8tq <[email protected]>
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 <[email protected]>
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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