[time-nuts] High resolution Beat frequency analysis
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Tue Nov 14 00:40:40 UTC 2006
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
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