[time-nuts] Of rubidium life and piggy-bank anemia....

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Dec 1 04:08:30 EST 2007


Whilst it is difficult to achieve a jitter of 100ps or less when an FPGA
has multiple clocks, achieving a jitter of  less than a few nanosec is
relatively easy.
When time stamping the zero crossings of the beat waveform in a dual (or
multiple ) mixer system with an offset source a resolution of 10ns or so
is adequate as long as the beat frequency is around 10Hz or less. To
achieve the same system resolution with higher beat frequencies such as
1kHz requires timestamp resolutions of a few tens of picosec, whilst
possible, it is complex and expensive to implement. High beat
frequencies like 1kHz also require either a custom low noise offset OCXO
or a complex offset generator. Beat frequency of up to 10Hz or so can be
easily achieved by mechanically tuning a 10811 or similar OCXO.

With a 10MHz mixer input frequency (RF and LO ports), 10Hz beat
frequency and 10ns time stamp resolution a system resolution of around
1E-14/tau is achievable provided the mixer, RF cables and distribution
amplifier temperatures are held constant to 0.01C or better. Otherwise
mixer phase drift (a few ps/C) distribution amplifier phase drift (1
-100ps/C or more depending on the implementation) and RF cable phase
shift tempcos will limit the achievable performance for longer averaging
times.
Using higher ft lower capacitance devices in the distribution amplifiers
will reduce the phase shift tempco as will using higher mixer input
frequencies and tuning the mixer input matching ciruitry to reduce the
mixer input VSWR. Both the mixer RF and LO ports should be driven into
saturation to maximise the system maximum SNR.

NIST produced a dual mixer system in 1976 that had a resolution of about
1E-13/tau for small tau.

Bruce



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