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There’s an interesting story there. Zenith and GE developed the 6BN6 for the FM job.
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Thanks for the informative history. The attached schematic is the discriminator for Harmon Kardon that uses two 6BN6. It also looks like a wide bandwidth tuned circuit for the first 6BN6 due to the presence of 8.2K resistor across it. The second 6BN6 is connected to a discriminator coils and diodes. Therefore the 6BN6 is not used as a sharp cut-off "chopper" originally designed for a quadrature detector.
The 12AX7 is part of the muting circuit.
The Fisher FM1000 is a very interesting tuner. It uses a total of 6 IF stages; two IF amp and four IF/progressive limiters. One 6BN6 is used as a limiter as shown in the 2nd and 3rd attachments. Interestingly, it opted for ratio detector instead of discriminator. Here ECC88 serves as a muting circuit.
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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.
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Unfortunately my Rigol DG1022 DSS function generator can only go up to 20Mhz. To measure third order intermodulation intercept point, i would need two sine wave generator sources going up to VHF. I have a walkie-talkie FM generator that can go up to 450MHz and a cheapo sweep generator up to 200MHz. But i would need two channels with a combiner for this kind of measurement..
Referring to above video from the designer (Eric) of the TinySA, he used a standard SMA 50 ohm load as a noise source reference ( -174dbm/Hz ) assuming it is standard room temperature. It is not good enough for professional lab but surely it is ok for us hobbyists.
At this moment, there is a listing of a used HP346B noise standard, going for £1500. I read about the y-method in the HP manual previously.
I have many SMA attenuators and a 80db RF step attenuator. Therefore gain and one db compression measurement can be done with my TinySA. I have two these cheap toys, they can be used as VHF signal generators but they only output square waves full of harmonics. The same with NanoVNA, they can be used as sweep generator with appropriate DC isolation capacitors and resistors in the port 1 and port 2. I tried to use NanoVNA to align FM IF transformers and it worked well.
PS: Maybe IEE produced some sort of sky noise map at different frequencies?
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