I agree that there are lots of contributors to overall uncertainty. However, I don't think a bias inductor is used to feed the diode with the correct current.?
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The HPJournal from April 1983 gives the internal circuit for the 346B and this shows how the diode is matched using some resistors and a stub. I assume the 346A is the same but with a larger value attenuator at the output.
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Looking at my old notes, I worked out that there is something like a 16dB attenuator at the tip of the 346A noise source and the blocking capacitor must be something like 1200pF.? I worked this out from a 1 port measurement of the 346A and I compared this to a model based on what is inside the 346 noise sources. Based on my findings, I think the coupling cap is something like 1200pF in my 346A but this may vary across device to device. I also worked out that the equivalent resistance of the diode and the matching resistors was about 44 ohms for my 346A.
