[time-nuts] DMTD was: high rev isolation amps

[Charles Steinmetz]

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).

Best regards,

Charles