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I've never attempted to use one at the bottom end of the spec'd frequency range, but based on my (limited) experience with homebrew noise sources, I'd question the stability of the ENR down there. Avalanching gets you a high ENR without operating at incandescent temperatures, but it is flaky. The 1/f corners are high and not particularly stable. I assume that the 346x uses buried devices to moderate the instability, but there's only so much that one can do.

Whether any of that matters depends, of course, on what you need out of the NF measurement. How accurately, and over what frequency range, do you want to measure NF? For terrestrial wireless, for instance, ambient noise typically sdominate over intrinsic thermal noise below about 20-30 MHz. Making accurate NF measurements at 1 MHz, say, isn't particularly important if the idea is to infer the sensitivity of AM radios, for example.

And if you only want to measure NF below tens of MHz, there are several other options for doing so.

-- Cheers
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070