Frank said:
Sounds interesting. How do you get -100dB with 12 bits? Would need at
least 17 bits. A moving average filter could work:
http://www.picotech.com/education/oscilloscopes/resolution-enhancement.html
But I guess for the extra 5 bits you would need 32 times the sample
frequency?
In this case it was Doppler processing. The bandwidth of the information
was much lower than in normal aquisition mode (which was also used) and
then you average over many samples plus over time. How much SNR increase
will result depends mostly on the length of the pulsed Doppler receive
window which is called range gate. The actual gating electronics are
often also called range gate.
For this to work the ADCs need to be super-duper quiet, much more so
than they'd have to be for the normal 12-bit work. This also means that
the phase jitter that such an auto-calibration shifter introduces has to
be as much down as your full averaged SNR.
An example how tough this can be: A client had a noise problem in a
similar system. Just a faint line on the FFT. But it annoyed their end
customers. Before they called me they had tried the most expensive
spectrum analyzers on it but could not find anything. So I showed up
with a receiver and head phones. The only way to find it was with the
receiver switched to singel-sideband reception and slowly turning the
dial through a large swath of frequencies. Because in a Doppler system
everything folds into the spectrum at the pulse repetition frequency,
which makes noise hunting tedious.