[time-nuts] Frequency Ensemble

[Tom Van Baak]

> Hi Everyone,I like to know if it possible to run let say 10 GPSDO, 16 Rb clock
> together and take the average to control 1 "master clock" and have better stability ?
> like what BIPM or NIST doing.
> I have search about ensemble system but I have no idea how much advantage
> I get from some clock that I already have.Thank You Anton 

Anton,

The rule-of-thumb is that, *under the right conditions*, N clocks will perform sqrt(N) better than 1 clock.

So yes, NIST, USNO, PTB, BIPM -- all the big boys -- use ensemble techniques. But the key is that they mostly use cesium clocks, not OCXO or Rb clocks from eBay. Laboratory cesium standards don't suffer from frequency drift. The other key is that the clocks are independent. Under these conditions one can obtain sqrt(N) advantage.

The problem with using cheap OCXO or Rb clocks is that they drift, and this drift may depend on make / model / environment; all of which are possibly common mode for you. This means the full sqrt(N) assumption is likely not valid.

The problem with using GPSDO is that they are not independent clocks. In fact, they aren't clocks at all: they are just noisy radio receivers, implementing "time transfer" from the USNO GPS master clock, which is related to but not equal to UTC(USNO) which is related to but not equal to UTC itself. There's a lot of common mode error amongst a set of GPSDO. This means the full sqrt(N) assumption is likely not valid.

Those who use GPS for highest accuracy tend not to use GPSDO. Instead they just collect raw timing information and post-process it some hours to weeks later. That is, they want to know
    what time-it-was-precisely
rather than
    what time-it-is-approximately.
A GPSDO only does the latter.

/tvb