[time-nuts] Building a GPSDO & trouble using Jupiter-T

[Chris Albertson]

On Sat, Jan 28, 2012 at 10:50 PM, Ray Xu <rayxu123 at gmail.com> wrote:

> Also, what is the advantage of using a OCXO instead of a VCXO in terms of
> short-term accuracy?  If the PLL time constant is only a few seconds, then
> a crystal shouldn't deviate in frequency by too much within a few seconds,
> assuming I'm using a crystal bought from a well-known manufacturer...or
> could it? I am inclined towards using oscillators that do not require any
> significant warm up time...

GPS is only a good reference if you average it over a long time
period.  (1000 to 10,000 seconds) There is more short term jitter in
the GPS then in a decent crystal oscillator.   So a very short time
constant does you no good.   Why use an OCXO?  Because of the required
long time constant.  You need to average GPS for such a length of time
(20 minutes to hours) that the ambient temperature will change during
the averaging time.  Of course you could take care that the
temperature does not change but that is what an oven does.    You can
buy a pretty good OCXO for $20 or $25

How long?   That depends on the required accuracy.  You want 1Hz at
10GHz.  That is 1E-10.  Not super hard but no way will you have that
10 minutes after you apply power.

If you need a portable "standard" look at those $40 rubidium nuts that
are on eBay.    It the 1E-10 level, after you calibrate it, if would
stay on-frequency for days and not require much warm up.

Chris Albertson
Redondo Beach, California