Re: 6-gang FM stereo tuner heads
Plenty of devices don't give a nice, straight, 3:1 slope for the third order intercept level.
Quite often the10, 20, I've seen 40! dB can be erratic and sometimes not even re-traceable.
So you say to yourself that you're using TOI to determine SFDR and you slap a ruler along the linear section and use that to determine the intercept point. This is where I expect a one-shot TOI measurement is likely to go astray.
To do a real TOI test, the levels of the two tone generators need to be varied over a decent span and the resulting plot needs careful interpretation. A bit of software in an analyser might give you a number, but you need to know how much you can trust it.
Once distortion effects in quartz crystals started to be known about, there was a trend away from using quartz filters in critical receivers. The HP3745A and 3747A were hex-superhets for example. Feedback RF amplifiers and also mixers can give 'interesting' TOI plots.
David
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
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