[time-nuts] Building a GPSDO & trouble using Jupiter-T
lists at rtty.us
Mon Jan 30 13:16:07 UTC 2012
The GPS ops signal (or 10 KHz signal) you will be trying to track will be moving by 10's or 100's of ns per second. 10 ns per second is 10 ppb. 10 ppb at 10 GHz is 100 Hz. You need to smooth this out or your LO will be moving all over the place.
On Jan 29, 2012, at 10:20 PM, 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
> <albertson.chris at gmail.com>wrote:
>> 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,
>>> a crystal shouldn't deviate in frequency by too much within a few
>>> 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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> 73, Ray Xu
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