[time-nuts] GPSDO - probably a stupid question.
kb8tq at n1k.org
Wed Aug 17 20:54:44 EDT 2016
A mixer style phase detector running a GHz range oscillator is one example of a
system that technically updated the EFC several billion times a second. There
does not have to be a DAC involved.
The point is still looking at the noise characteristics of the oscillator and the reference.
It is best done in the frequency domain as phase noise. We substitute ADEV, but that
is not an ideal proxy. Either way you want the loop to cross over from one to the other
somewhere in the vicinity of the “equal noise” point if it exists. If there is no equal noise
point, that makes you wonder a bit about why you are locking one to the other :)
Taking ADEV, since that’s what we have the data on:
GPS starts out somewhere in the 20 ns to 0.2 ns range at 1 second depending on what
you are looking at, which module you are using, and who you trust for your data. That
compares to a good OCXO that should be in the 0.001 ns range at 1 second. There is a
long way to go (larger time span) before the OCXO and GPS are anywhere near
the same noise level. You will need to get to 200 seconds with the best GPS numbers above.
You will need to get to 100X that with the worst ones. Yes, there is a bit of hand waving
in all of that.
> On Aug 17, 2016, at 6:14 PM, Nick Sayer via time-nuts <time-nuts at febo.com> wrote:
> 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:
>> 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.
>> 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;
>>>> and here;
>>>> Even using the 50 bits/sec data rate of the GPS signal would allow updating the
>>>> GPSDO faster than the 1 PPS signal.
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