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

[Bob kb8tq]

Hi

You can update the EFC a billion times a second.  Update rate and bandwidth are not the same thing. If you want good ADEV, the loop better not have a bandwidth greater than 0.01 Hz. GPS ADEV is pretty awful at 1 and 10 seconds. It is starts to be good past a few thousand seconds. Yes, older modules are a bit worse than newer ones. Also sawtooth correction can make things a bit better.

Bob

Sent from my iPad

> On Aug 17, 2016, at 2:51 PM, Nick Sayer via time-nuts <time-nuts at febo.com> wrote:
> 
> Updating the EFC more quickly reduces the ADEV, though. I find that the fiddly part of tuning a GPSDO design is balancing the ADEV against phase control. If you want keep an iron fist on the phase, you can only do so by constantly swatting around the frequency.
> 
> I won't say that getting more frequent phase feedback is a bad thing, but if you're trying to get the PLL time constant to be longer rather than shorter that it won't help a lot. 
> 
> Sent from my iPhone
> 
>> On Aug 17, 2016, at 9:57 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.
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
> 
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.