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

[Magnus Danielson]

Hi,

I agree.

There is however a subtle detail, how they leak out over time.

At one time we had to lock an 155,52 MHz oscillator up to 8 kHz, this 
for a 2,48832 Gb/s link, which needs to pass the SDH STM-16 jitter and 
wander specifications. The first attempt at that PLL was using a 4046, 
and the charge-pump was being used. The charge-pump has dead-time, and 
well, they thought it was good to only push the EFC here and there. What 
this meant was that they created a triangle-waved frequency modulation 
of low rate, which then created phase modulations as it went through the 
integration of the oscillator. The scale-up factor made this quite 
noticeable at the actual bit-rate. It made the point that you need to 
update often to keep deviations limited, and when doing it at a higher 
frequency, they are easier to filter out.

In essence, you need to think what each comparison or update creates as 
a step response and how it is averaged out over time.

In this regard a PWM is a really bad signal, as it can push the 
strongest amplitude at the lowest frequency, which becomes hardest to 
filter. For one design I needed to increase the resolution, so I made an 
interpolation but with inversed spectral density to that of PWM, to push 
the highest amplitude to the highest frequency so that filtering becomes 
easier. Turned out to be quite easy and work well.

High update rates can be very useful even if the bandwidth of the loop 
is low. The bandwidth only limits how low the updaterate can be, but the 
phase-noise purity makes update rates and smoothing mechanisms interesting.

Cheers,
Magnus

On 08/17/2016 11:53 PM, Bob kb8tq 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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