Bruce Griffiths wrote:
If one uses a mixer output frequency of several kHz then one can avoid the flicker noise region if one uses a high pass filter between the ADCs and the mixer preamps.
Traditionally the beat frequency balance against the flicker noise in which a lower beat frequency increases the "gain" where as eventually flicker frequency comes and haunt us.
An alternative approach is to choose a higher initial beat frequency around say 1 kHz, sample that using traditional audio samplers and then digitally further mix down the signals before detection. That way you can get say 1 Hz beat frequency with the noise performance dominated by the 1 kHz noise. However, the processing could be performed on the original 1 kHz waveform directly or for that matter a direct sampled variant of the original waveforms.
A second mixdown requires that the first and second LOs is locked to each other for best performance.
Does such a system have a performance advantage over direct RF sampling? Perhaps it does if and only if the phase noise floor of the lower bandwidth ADCs that are used is lower than the noise floor of the ADCs that would be required to sample the RF signals directly? The noise floor of state of the art ADCs suitable for direct RF sampling is around -150dBFS/Hz. The noise floor of "typical" high resolution ADC(AD7762, AD7641) capable of sampling at around 1MSPS or so appear to be similar.
Direct sampling is ofcourse another method to consider, but put higher demands on up-front processing. It has however become fairly cheap.
Cheers, Magnus _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
