Q3: MDS measurement
The signal disappears into the noise floor with the attenuator at 66
dB or
10 mV/2000 = 5 uV input signal.
I believe the USRP ADCs are 12 bits with VFS = 2V, so the LSB is 2V/
4096 or
about 488 uV. How is the
system resolving signals that are 20-40 dB below the LSB size?
I think I figured this out over night. It's been quite a while since
I looked at
A/D converter response to very small signals.
If an ideal ADC is somehow biased at a code step threshold point, any
size signal will cause toggling
between two ADC codes, and a subsequent fft will readily pick out the
frequency components related to this toggling.
Signal size (on the low end) is ultimately limited only by the
resolving capability of the comparator
element(s) in the converter.
If the ADC input sees wide-band noise that is 1 LSB or more in
amplitude, the noise acts as a dither signal, and the input
no longer needs to be biased at a code step threshold. Adding a very
small sinusoid to the noise will alter the ADC code pattern
in a detectable way. Again, the signal size should be limited by the
resolution of the comparator.
Dick...
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