[time-nuts] Beginner's time reference

John Miles jmiles at pop.net
Fri Dec 11 01:47:43 UTC 2009


> As a
> newbie to the field of timekeeping, I'm a bit uncertain what would make a
> good first system. The price tag of the high end masers unfortunately puts
> them outside my range, based on what I could see on eBay. I'll be happy to
> pay for good equipment, but once it gets to 4 digits it starts
> putting some
> serious dents in my bank account. I'm hoping to build my own
> circuit around
> the device, which might cut down the costs somewhat. Some rubidium sources
> on eBay go for less than $100, but I'm not sure what quality to
> expect. Any
> advice or suggestions are appreciated!

It's interesting to consider how well a GPS clock can perform relative to a
local cesium.  I took a plot (attached) of a Thunderbolt with a 10811-60109
(t=850s) and one of the sub-$100 LPRO-101 rubidiums, then drew some lines on
it to indicate the spec limits of one of the better cesium standards (the HP
5071A).  I also tried to copy the traces from Tom's Z3801a at
http://www.leapsecond.com/pages/gpsdo/ onto the plot.

It seems that the *unlocked* OCXO in Tom's Z3801A is turning in a
performance better than the specs of a 5071A with the high-performance tube
option, all the way out to tau ~= 400 seconds.  It's better than a standard
5071A until about 2000 seconds.

If the Z3801A had a much longer lock bandwidth -- say if there were a
rubidium standard between the GPS receiver and OCXO that could be locked at
tc >> 2000s -- then the GPS receiver might be pretty competitive with the
standard 5071A model, at 1/20 the price.

-- john, KE5FX
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