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

[Bruce Griffiths]

M. Warner Losh wrote:
> 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
> ensemble.
>
> Warner
>
>   

Warner

I was describing the approach adopted by NIST in their frequency
measurement systems used to monitor OCXO frequencies at remote sites.
The performance of this system is comparable to what one can achieve
with a TIC having 100ps resolution and a good GPS receiver.


The heterodyne factor is actually 1E6 with a 10Hz offset and a 10MHz
mixer input frequency.
So the resolution is around 3.125E-14/tau for a 32Mhz clock.
JPL use an interpolation technique to reduce the noise contribution of
the offset source when the corresponding mixer beat frequency output
transitions for all channels are widely separated in time.
However they are forced to use a magic 123 Hz beat frequency to get the
best out of the commercial synthesizer they use for an offset source.

A well designed ZCD can achieve a noise floor (due to mixer and ZCD
noise) of less than 10ns even with an a 10Hz beat frequency.
Most of the published ZCDs are far from optimal designs.
The seminal paper on optimum ZCD design was published by Oliver Collins
in an obscure publication (at least for the precision frequency
measurement community).

The principal limitations to the achievable system noise floor for large
tau is the tempco of the mixer and isolation amplifier phase shifts.
Interconnecting RF cable phase shifts may also be significant.

The NIST 1976 dual mixer system used 10MHz inputs and a 10Hz offset
system noise level was about 1E-13/tau, at least for small tau
Currently 100MHz mixer input frequencies are favoured as the mixer phase
shift (measured in ps/C) tempco is significantly lower with higher
frequency inputs.

Minimising the mixer LO and RF port VSWR also lowers the mixer phase
shift tempco.

Mixer IF port output noise is significantly reduced with capacitive
rather than resistive termination.

Bruce