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Old 16th Oct 2022, 8:05 pm   #1
Join Date: Aug 2007
Location: Walsall Wood, Aldridge, Walsall, UK.
Posts: 2,760
Default Fun With Tubes and Fun with Transistors

I came across this site written by Max Robinson whilst looking for theory on Intermodulation Distortion for Part 7, Chapter 55 of my T & M Theory and Repair Book !

I can whole–heartedly recommend Max's Website, there is an enormous amount of very useful information and constructional ideas, including notes on how to make replacement valve–type i.f. transformers, amongst other things !

He has also written a complementary site, "Fun with Transistors".

Enjoy, there's tons and tons to read !

Chris Williams
It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!
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Old 17th Oct 2022, 8:52 am   #2
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,248
Default Re: Fun With Tubes and Fun with Transistors

You can derive intermodulation distortion behaviour from just a model of the curvature of any non-linear device. Feed it a single signal and you get harmonic creation.

Feed it two signals ands you get harmonics of each, you also get mixing products between the fundamentals, and you get mixing products between all the combinations of those harmonics - these are what get called intermodulation products. It happens that the odd-order term products come out close to and surrounding the two fundamentals, with spacings equal to the spacing of the fundamentals.

With so many harmonics and combinations, it's like unravelling a pullover... where do you stop.

For normal series representation of most (not all) intermod and harmonic terms, if you reduce the fundamental(s) by 1 dB, the products will all reduce by X dB where X is their order number. This is a good check to try to work out what the recipe is behind an unwanted sproggy. This rule of thumb is good for lowish levels but goes bad as you approach the TOI level. Feedback amplifiers can break the 'rule'. Crystal filters can really go wild at higher levels.

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