[time-nuts] Re: DIY Low offset Phase Noise Analyzer (Erik, Kaashoek)

[ed breya]

I forgot to mention that you should also consider possible effects from 
the RF present, on the LNA. This can be more significant than SMPS 
frequencies getting where they don't belong, especially since the RF is 
intentionally right at the opamp's input. Your LPF only reduces, and 
does not eliminate, the 2F and harmonics, so there can be significant RF 
present on the LNA circuit.

A simplistic view is that the RF is far beyond the opamp's GBW or closed 
loop gain and should have no response, but it's not at all beyond 
upsetting or altering the operation. This can result in extra DC offsets 
and noise due to RF rectification in the input circuits, which only 
remain "linear" at frequencies where the output and feedback can keep up 
with the input.

This can be fixed if necessary, by adding extra RF filtering, 
particularly some built to low-pass at a higher cutoff frequency well 
above the analysis frequency, and well below the expected f and 2f.

For instance, in your circuit it looks like L1 is 1 mH, with 100 nF 
caps, which ideally cuts off quite low. However, 1 mH is a pretty big 
choke, and will tend to have a lot of inter-winding capacitance (and 
high resistance - don't forget to include it in noise), making it less 
effective at the higher frequencies. Adding an LC section in front of 
it, but set up for something in the MHz region, will give much greater 
rejection of the f and 2f, due to having more appropriate smaller L and C.

Anyway, if it works fine as is, then no problem, but it's something to 
be aware of if you get strange effects down the road.

Ed