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From: Cursitor Doom <cd@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: IF transformer VNA Characterisation
Date: Mon, 17 Feb 2025 22:20:01 +0000
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On Mon, 17 Feb 2025 07:54:58 -0800, John Larkin
<jjlarkin@highlandtechnology.com> wrote:

>On Mon, 17 Feb 2025 13:31:28 +0000, Cursitor Doom <cd@notformail.com>
>wrote:
>
>>On Mon, 17 Feb 2025 10:21:25 +0100, Jeroen Belleman
>><jeroen@nospam.please> wrote:
>>
>>>On 2/16/25 21:19, Cursitor Doom wrote:
>>>> When I lived in Germany, I joined DARC (as you do) and showed my new
>>>> sausage-noshing friends some examples of my construction handiwork. As
>>>> a result of that, they gave it a specific German portmanteau term  to
>>>> describe it: Scheissebau. I haven't looked up the translation but I'm
>>>> guessing it means 'ingeniously-resourceful.' ;-)
>>>> Anyway, here's a prime example. I have several hundred broadcast radio
>>>> intermediate frequency transformers manufactured in the early 1970s.
>>>> 
>>>> https://disk.yandex.com/i/Ym1YrWS2YGTnxw
>>>> 
>>>> I was curious as to what IF they were made for. Each of them is
>>>> color-coded to indicate this, but I have no chart to de-code this and
>>>> online sources conflict in many respects. The obvious answer was to
>>>> test them all and create a chart from those findings. This
>>>> necessitated the building of a test fixture to accommodate the
>>>> transformers, which can be plugged into it and swapped around for
>>>> purposes of comparison. Having built this, I then needed to make up a
>>>> calibration kit to establish a reference plane to subtract the effects
>>>> of the hook-up cabling and connections. Fortunately, de-embedding and
>>>> whatnot is no big deal as these IFs are low, so the parasitics (which
>>>> I'm not proud of) in this construction shouldn't materially affect the
>>>> measurements.
>>>> 
>>>> Here's the fixture:
>>>> https://disk.yandex.com/i/NE8B4i5Yh0jWYA
>>>> 
>>>> A specimen IF for testing:
>>>> https://disk.yandex.com/i/BUpamDpN8us8pQ
>>>> 
>>>> The ad-hoc calibration kit:
>>>> https://disk.yandex.com/i/CaV7QGfA-KtP_w
>>>> 
>>>> 
>>>> 
>>>
>>>I don't think it's meaningful to use them in a 50 Ohm
>>>environment. They were used as collector loads of common
>>>emitter stages, so were driven by a high impedance, say,
>>>20 kOhm or so. The target load impedance on the secondary
>>>varied according to the specific purpose of the stage.
>>>(I think the ones with the yellow screws were optimized
>>>to drive diode detectors in AM radios, and red ones were
>>>AM band LO oscillator coils.)
>>>
>>>Jeroen Belleman
>>
>>Well, all modern test equipment is normalized for 50 or (more rarely)
>>70 ohms. That is the so-called 'system impendance' and there's not
>>much one can do about it. However, I really just want to see where the
>>resonant point is for each of these devices. I only had the time last
>>night to do one (a green one) which turned out to be 10.7Mhz, so I can
>>now sell them off as such. I'll do the other colors when I have time.
>>It'll be interesting to see the difference in construction between a
>>10.7Mhz and a 455khz one. Interesting for *me* at any rate, although I
>>accept that others might fail to see the point of this. Each to his
>>own!
>
>Rip some open and see what's inside.
>
>I agree that you might not learn much in a 50 ohm environment. I'd
>play with a signal generator and an oscilloscope and a few resistors
>and caps to get into the right frequency and impedance ballpark.
>
>Just an ohmmeter will figure out a lot.

That would certainly have been a damn sight simpler! It took me
*hours* and 3 attempts just to come up with that crummy jig.
Fortuntely it works - enough to see where the resonances are at any
rate. And that's all I really need to know.
Come to think of it, I do have a vintage 'wobbulator' in my test gear
collection. That would have been perfect for this job D'oh!