Re: IF transformer VNA Characterisation

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!