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Old 23rd Mar 2022, 12:29 am   #10
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

Not that much like squelch. Muting is like squelch.... killing the audio if thesignal level is below a set threshold.

Quieting is a smooth change. As an illustration, if you are listening to a good FM receiver with an input signal above the onset of limiting in the IF amplifier, and you smoothly increase the signal level, the level of the wanted audio does not increase, it stays fixed, but the level of the background smoothly falls.

So when listening to an FM signal and trying to estimate the strength of the incoming signal, try to gauge how muchnoise there isn't !

Is it possible to measure....? Possibly. I've never tried it with one of those, but you'll need a decent signal generator to measure gain and 1dB compression point.You'll need a variety of attenuators to play around with to determine if you're measuring your tuner or your test gear. Make that two decent sig gens and a combiner if you want to measure intermod products.For noise figure, you'll need a calibrated noise source. I've got one sitting on the desk here, an HP346B which has an excess noise ratio of 15.65dB at 100MHz. It has a 10dB APC-7 attenuator screwed onto it to reduce the ENR for measuring low noise figures. This noise source covers 10MHz to 18GHz. Easy to say you don't need all that range, difficult to find anything with less that is properly calibrated.

Look up the Y-factor method of noise figure measurement. (HP/Agilent/Keysight AN57-1 and AN57-2 these were updated a few times. The older ones are maybe the best for you, the later ones are more aimed to sell network analysers as the dedicated noise figure boxes went out of production.)

There are other ways: A Russian amateur has a horn antenna kept looking at a cold part of the sky as his reference. It works very well. NPL have resistive loads maintained at liquid nitrogen and melting/boiling water temperatures. These have to be corrected for the temperature gradient along the loss of their connecting cables. Commercial noise sources are traceable back to these standards. NIST in the US had let a building roof fail (saved money on maintenance...) and had lost the parentage of their standards, so we transferred our calibration trail to NPL and got lower quoted uncertainties as a result.

Who'd have thought that all the national standards labs was a competitive industry?

David
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