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

M. Warner Losh imp at bsdimp.com
Sat Dec 1 19:17:33 EST 2007

In message: <4751F17C.9030707 at xtra.co.nz>
            Bruce Griffiths <bruce.griffiths at xtra.co.nz> writes:
: The conventional approach adopted by NIST is to divide each frequency to
: be measured down to 1 PPS, then timestamp the PPS transitions for each
: channel as well as the PPS transitions from a GPS timing receiver. By
: using the same setup at NIST with their frequency standards most of the
: noise due to ionospheric delay variations is a common to both systems
: and is eliminated on subtraction.

NIST has gone to an approach where the signal under test is
heterodyned with a signal that's ~10Hz low.  Since one cycle in the
test frequency is one cycle in the heterdyned frequency, you can get
measurements of signal down to the noise floor of your hardware (since
even a simple 32MHz counter gives one several order of magnitude
better than the noise in the zcds).

At 10MHz, the heterodyne factor is 1e9.  This means you can measure
the phase difference of the signal to 31.5e-18 (assuming a 32MHz
clock).  Since the noise in the 32MHz oscillator is at 1e-13, we can
measure the 10MHz down to a few parts 1e-13 (since the noise dominates
over the resolution of the measurement).  With a better oscillator,
one can hit the noise floor of the ZCDs that we used in the project
with a more stable 32MHz oscillator (down to 1e-14 or 5e-15 or so in
some of the tests I ran in the lab).  The 32MHz oscillator is a
relatively cheap OCXO.  The design of the system is such that almost
all of the 32MHz noise subtracts out...  The noise is such that
switching to 100Hz gives a factor of 3 better noise floor.  But any
faster than that doesn't help much.  A 2Hz signal is 5x worse noise
floor because the 32MHz noise starts to dominate.

With a good quality TIC, one is doing good to get PPS measurements in
the picosecond level (1e-12).  But that's a lot more expensive than
the above device.

At least that's what NIST is using to measure the cesiums in its clock


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