On 7/12/22 3:51 PM, ed breya via time-nuts wrote:
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
and a single LC is only a single pole, so the roll off isn't all that
great in a dB/decade sense.
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