Charles,
On 05/12/2016 10:34 AM, Charles Steinmetz wrote:
Bruce wrote:
For even lower flicker noise using a mixer assembled using
2N2222A's as diodes is even quieter as NIST have shown.
Note that mixers built with 2N2222s only have lower noise than good schottky DBMs below
about 200 Hz -- above that, schottky DBMs are better (see Figures 4 and 5 of the NIST
paper, "Residual PM Noise Evaluation of Radio Frequency Mixers" by Barnes, et
al.). By 10 kHz the MCL ZRPD-1 is about 5 dB better than the 2N2222 mixer. And note
that even if the beat note is 10 Hz, the baseband you analyze is wider than that. (If
the beat note is 100 Hz, there is essentially no advantage to the 2N2222 mixer even at
the beat frequency).
Transistors with diode-connected flicker noise significantly lower than
2N2222s, and other characteristics suitable for this use, are readily available
these days (they were when the NIST work was done, as well, but for whatever
reason they were not used for that research).
Craig just picked up a transistor he knew from ages ago to see where it
took him, as I recall it. He then found multi-transistor chips which is
what he used.
Cheers,
Magnus
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