[time-nuts] High resolution Beat frequency analysis

[Dr Bruce Griffiths]

A resolution of 1E-15/T has been achieved (at JPL) by measuring the 
period of a 1Hz beat signal produced by mixing a pair of  100MHz  
sources, the zero crossing detector used requires heroic measures to 
combat the high phase shift temperature coefficient (~60us/K with 
polypropylene capacitors, NP0/C0G capacitors are impractical because of 
large value required) of the 1Hz RC filter employed to define the noise 
bandwidth.

It would appear that this difficulty may be circumvented by employing a 
high resolution wide dynamic range sigma delta ADC in conjunction with a 
somewhat higher cutoff frequency antialiasing filter. The internal 
digital filters can have very stable delays determined by a crystal 
oscillator and the antialiasing filter may have a phase shift tempco 
below 1us/K so that temperature control to within 0.1K or so, should 
suffice to achieve a phase stability of better than 100ns.

ADC integral nonlinearity will generate harmonic distortion in the 
sampled signal, however this isnt critical when measuring the relative 
stabilty of the 2 sources. The mixer non linearities will generate more 
distortion than the ADC.

Suitable sigma delta ADCs with 130 dB dynamic range are available.

Has anyone considered implementing anything like this?

Bruce