[time-nuts] GPSDO - probably a stupid question.

[KA2WEU at aol.com]

There are no (rarely maybe ) stupid questions, mostly silly answers 
 
 
In a message dated 8/17/2016 5:03:07 P.M. Eastern Daylight Time,  
elfchief-timenuts at lupine.org writes:

Wouldn't  you also not be able to actually sync to the individual chips, 
since you  can't really see the start of any given chip so much as you 
just see the  correlation over larger sections of the stream? Plus you'd 
have to track  only one SV at a time (right? Since I doubt the edges of 
every chip are  perfectly aligned across all SVs even under the best 
conditions), so  things like brief multipath excursions or even 
atmospheric/ionospheric  fluctuations would push you off by a bit as well...

(which is why, of  course, you have the long control loop that GPSDOs  use)

-j


On 2016-08-17 11:41 , Didier Juges wrote:
> In  fact, you do not want to "update the crystal one million  
times/second".
> The whole point of a GPSDO is to combine the excellent  short term 
stability
> of the crystal with the excellent long term  stability of the GPS signal. 
If
> you update the crystal in real time  from the GPS data, you do not need 
the
> crystal...
> The control  loop of GPSDOs usually have an effective bandwidth measured 
in
> minutes  or even hours in the case of rubidium oscillators.
>
>
> On  Wed, Aug 17, 2016 at 11:57 AM, Peter Reilley  
<preilley_454 at comcast.net>
> wrote:
>
>> You can  get crystal oscillators that have a frequency control signal and
>>  are more
>> stable than the run of the mill oscillators.    Changing the GPS 
oscillator
>> would
>> require modifying a  very tightly populated circuit board.   Perhaps not
>>  possible.
>>
>> What about some of the SDR (software defined  radio) projects that aim to
>> implement GPS  functionality?   If you used the GPS chipping rate  (1.023
>> MHz)
>> to dicipline the 10 MHz oscillator then  you are less sensitive to 
crystal
>> instabilities.
>> You  are updating the crystal one million times a second rather than  
once
>> per second.
>> This is assuming that the chipping  rate of the transmitter is just as 
good
>> as the
>> 1 PPS  signal.   This info from here;
>>  https://www.e-education.psu.edu/geog862/node/1753
>> and  here;
>>  https://en.wikipedia.org/wiki/GPS_signals
>>
>> Even using  the 50 bits/sec data rate of the GPS signal would allow
>> updating  the
>> GPSDO faster than the 1 PPS signal.
>>
>>  Pete.
>>
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