Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Dec 21 18:43:21 EST 2015

Such blanket statements aren't a particularly useful guide unless calibrated by measurements. For example the OPA653 has a measured PN floor of around -163dBc/Hz for a +13dBm input and the measured PN @1Hz offset is -150dBc/Hz (comparable with the NIST isolation amps).However the voltage noise is (estimated) to be 300nV/rtHz @1Hz and about 8nVrtHz @ 10kHz. Whilst the measured PN floor agrees closely with the measured value. The input voltage noise @1Hz can't be used directly to estimate the PN noise at 1Hz offset.

Bruce

On Tuesday, 22 December 2015 12:02 PM, Charles Steinmetz <csteinmetz at yandex.com> wrote:

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
are three possible choices (these days, there are many others, as well).

Best regards,

Charles

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