Don't confuse the PLL loop "time constant " with the time constant of
the analog or digital filter.
They are not the same.
Bruce
Ray Xu wrote:
Hi Chris
Thanks for your helpful input.
What do you mean by "average"? Do you mean that the GPS and PLL must be
kept on for "20 minutes to hours", or did you mean that the PLL loop filter
must have a time constant of 20 minutes to several hours? To me, the
latter seems really unpractical for analog filters...Yet I have seen many
of them built using analog filters. Especially JAmes Miller's
http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htm and his FAQ
says that the time to wait is perhaps 15 minutes or so to be usable. The
previous GPSDO that James built has its schematic; the filter he used
doesn't look like they're anywhere close to a time constant of 20 minutes.
I may consider the Rb standard, but I'm more inclined on using GPS since I
actually get to build some stuff on my own :-)
Thanks again
Ray Xu
On Sun, Jan 29, 2012 at 6:52 PM, Chris Albertson
<[email protected]>wrote:
On Sat, Jan 28, 2012 at 10:50 PM, Ray Xu<[email protected]> 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
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