[time-nuts] SE880 GPSDO

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Apr 27 05:21:41 EDT 2016


Stabilising the GPS receiver antenna temperature is probably a good idea particularly if it has bandpass filter(s).
Bruce
 

    On Wednesday, 27 April 2016 9:01 PM, Attila Kinali <attila at kinali.ch> wrote:
 

 On Wed, 27 Apr 2016 01:30:49 +0200
Ilia Platone <info at iliaplatone.com> wrote:

> I will use a dedicated FPGA design, and the data will be stored into an 
> SDXC card (UHS), or an IDE drive (maybe not), in RAW mode (no filesystem)..

Please be aware that SDCards performance spec is peak and best case.
Additionally, you need to prepare for writes taking several ms when
the SDCard firmware remaps blocks. Ie you will need to be able to
buffer several MB of data before you write to the SDCards. It would
probably be a good idea to push the data into a CPU (ARM9, or Cortex-A)
and handle the buffering/writing in software instead of an FPGA.

> Each timestamp will be stored into a word of 48bits, the clock will be 
> 400MHz, so the quantization will be 2.5ns, but the maximum error/clock 
> offset should be 1/10 of this, so 500ps , as said by Bruce.

I thought a little bit more about this problem this morning and came up
with something that should be rather simple to implement:

Assuming that you have an amateur radio license, you could use a 
well located central station to transmit a CW signal in the 70cm or
23cm band. There should be some effort put into this station
to make it stable (eg by using a good rubidium as frequency source,
or even an ensemble) and low noise.

Use this CW signal on all the telescope stations to phase lock a local
OCXO. Using a good OCXO, it should be possible to use loop bandwidths
in the 0.1-10Hz range. My guess is, that this frequency transfer system
would yield stabilities in the order of 10^-12 @ 1s (or even better).
For additional performance, one could modulate the CW with a PRN sequence
to get a better SNR and probably get another order of magnitude out of it.
For the simple CW case, the circuitry should be fairly simple and easy
to do. The PRN case would require at least some processing in an FPGA.

Now that all stations have the "same" frequency, one can use the GPS
module to get the time information using long integration times.
Under the assumption that the (sawtooth corrected) PPS is good to +/-10ns
an has a nice, time-invariant distribution, it should be possible to get
below 1ns in precision within 100s. Using common view phase data it
should be possible to get even better than that. 

Of course, the GPS modules would still need to be calibrated against
eachother to get the accuracy below the 10ns level. I am still not
sure how to go below 1ns, though I think this approach should make
that easier. Most likely you will need some temperature stabilization
of the electronics.

It's also quite easy to verify the frequency transfer by letting
each telescope station transmit a CW signal back to the central
station on a different frequency. Then use some SDR system to extract
the relative phase/frequency of each station.

            Attila Kinali

-- 
Reading can seriously damage your ignorance.
        -- unknown
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