[time-nuts] Building a GPSDO & trouble using Jupiter-T
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Mon Jan 30 04:08:29 UTC 2012
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
> <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,
>>>
>> 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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>
>
>
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