[time-nuts] SMD TADD-1 distribution amplifier

Charles Steinmetz csteinmetz at yandex.com
Mon Dec 21 16:52:08 EST 2015


Anders wrote:

>How is that calculated? I only get this far:
>9.6nV/sqrt(Hz) into a 50R load is 1.8e-18 W/Hz or -147.3 dBm/Hz
>what then?

As I said on Dec. 18 in response to the original post, the in-band 
(10MHz) noise is NOT the main problem with respect to AM and PM 
noise.  The main problem is the BASEBAND noise (near DC, say, 
0.1Hz--10Hz).  And the AD8055, as well as the MAX477 originally used 
in the TADD-1, are absolutely horrible in this regard.  Both of them 
have noise densities > 1,000nV/sqrtHz at 10Hz (and even worse below 10Hz).

See my earlier post for a brief explanation of why this is so, and 
how baseband noise is converted to AM and PM noise in the RF signal 
band.  There are also more in-depth explanations in the 
archives.  One search term you can use is "PM conversion."

For low AM and PM noise, you want an opamp with a noise density of 
10nV/sqrtHz, or lower, *at 10 Hz*.  The ADA4899, LME49713, and AD8010 
are three possible choices (these days, there are many others, as well).

Best regards,

Charles





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