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Old 22nd Jul 2022, 10:16 am   #120
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: 6-gang FM stereo tuner heads

Sig gens are usually somewhat more complicated than Marki suggests, though they're closer in terms of microwave band generators where most of their mixers are aimed. For the 100MHz band you're most likely to have an output level control loop with an RF detector on the output of the amp of the sig gen, certainly any generator with an output level controllable enough not to spoil any accuracy of these tests.

Cross-coupled signal from one generator to another will go through whatever value of output attenuator the generator has engaged and will beat with the generator's own signal in the levelling loop detector. This will AM modulate the generator's output forming just the third-order products you're trying to measure.

To get reasonably useable results coutermeasures like disabling levelling, or adding power amps and external attenuators are often needed.

I did a lot of measuring aviation COM and NAV radio designs through the last decade and they were not quite in the class of a basic packaged diode ring mixer (mini TFM-2 for example) and quite a bit of elaboration was needed to get specified TOI results. Quick tests before elaborations were introduced could easily be several dB out, and this was with low power transistor tree mixers (Called Gilbert cells by everyone except Barrie Gilbert himself and those who know the real Gilbert Cell analogue computing element)

The choice of spacing for a TOI test is also important in receiver tests. It sets how far down the structure of the receiver both tones will penetrate and how much removal of one or both happens due to the selectivity of different stages. Tone spacing sets which aread of the set get their linearity measured. For narrow spacing on an FM set, you'll eventually start measuring the linearity of the limiter stages! and they are intentionally not at all linear. Wide spacing can give too encouraging results because RF selectivity enters the scene and controls what reaches the mixer.

If you were measuring just a diode ring double balanced mixer alone, there is no selectivity at play, and so tone spacing ceases to be so important.

Just being a few dB off in TOI has a larger effecto on your SFDR calculations.

David
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