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

Nick Sayer nsayer at kfu.com
Wed Aug 17 18:14:37 EDT 2016


Well, on a practical level, if you update the EFC that frequently then the DAC change glitches will dominate the actual output even if you’re not actually moving the needle much. 


> On Aug 17, 2016, at 2:53 PM, Bob kb8tq <kb8tq at n1k.org> wrote:
> 
> 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.
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>> 
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