[time-nuts] Measuring phase shift between 1 Hz DMTD signals by I+Q processing

[Magnus Danielson]

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