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 _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts