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From: Cursitor Doom <cd@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: IF transformer VNA Characterisation
Date: Sat, 22 Feb 2025 16:27:47 +0000
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On Fri, 21 Feb 2025 02:48:49 +0530, Pimpom <Pimpom@invalid.invalid>
wrote:

>On 17-02-2025 07:01 pm, Cursitor Doom wrote:
>
>> 
>> 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!
>
>A long time ago, when the Earth was much younger, FM didn't exist where 
>I live but there was an AM station transmitting at 540kHz.
>
>I used to take a 455kHz IF transformer, replaced the built-in 200pF cap 
>with 150pF and used it as the collector load for an RF transistor (an 
>AF117 IIRC). I tuned the IFT directly to the station and the secondary 
>fed a Ge diode (usually OA79) detector. The DC output from the detector 
>served as an AGC bias while the audio signal went to an amplifier. I had 
>a very narrow range of parts to choose from and the amp was usually an 
>AC126 driving an AC187/AC188 complementary pair or a pair of AC128's in 
>parallel push-pull with an output transformer..
>
>The RF input side was a coil of solid copper wire (I didn't have Litz) 
>on a ferrite rod, paralleled by another 150pF cap and tuned by sliding 
>the coil along the ferrite rod.
>
>I must have built at least half a dozen such "radios" with some 
>variations and gave them away to friends. It was a lot of fun.

Sounds like fun, I agree. And only having a limited choice of parts
means you have to be much more resourceful in the design stage if you
are to achieve the best possible results for what you have. Good
training!