[time-nuts] SE880 GPSDO

[Attila Kinali]

On Wed, 27 Apr 2016 20:18:10 +0200
Mike Cook <michael.cook at sfr.fr> wrote:

> > 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.
> 
> It might be possible to clock the FPGA directly from a suitably massaged CW. 
> Do any clock at 1GHz+???
> It would be also possible to do away with LO’s in this case. 

That would make the system more complicated than simpler, because
you need to extract a signal from a noisy environment, pass it through
narrow band filter so you get something that resembles a sinus in order
to use it in the electronics. This kind of works when the reference
signal comes in via cable. With over the air transmission, this wont work.

Using an OCXO together with a PLL basically forms a very narrow band
filter that has a very small tempco, adjustable (and adaptive)
frequency and allows to change the filter coefficiencts quickly
to acquire the signal at start-up.

Very few chips (of any kind, not just FPGA) allow input clocks higher
than a couple 100MHz. Single ended CMOS inputs usually go only up
to 200MHz, often much lower than that. Differential (LVDS and PECL)
ends usually in the 500MHz range.

Also. Running an FPGA at 1GHz is not trivial at all. Most designs
don't do more than 500MHz even on the fastest FPGAs out there.
100-300MHz are common values. And unlike with CPUs, there is often
no need to run an FPGA faster than the data arrives or leaves, as
the functions can be run in parallel and use a pipelined architecture.

> The CW could also carry time, and if it was feasible, local GPS would be
> unnecessary.

If the CW would carry time, it wouldn't be CW anymore ;-)

Yes, that's the idea with the PRN modulation i mentioned in the other mail.
But it wont lift the necesity of GPS. There is an unknown delay from the
sender to the receiver, through multiple filter and frequency conversion
stages. Some of them can be measured, some of them come from the ambiguity
of the phase in the system. Others, like the path delay, cannot be measured.

One solution would be, to use 2 transmitters with known positions and
known phase relations, then it would be possible to extract time,
given one knows the positions of the receivers exactly. 
To get around that requirement, one would need at least 4 transmitters...
Ie. one would be recreating the GPS system... at which point it becomes
simpler to just use GPS and live with the degraded accuracy.

Also keep in mind, that with GPS it is well known where the errors
come from and how big they are. Also lots of techniques are implemented
to counter those. With a DIY-GPS system, one would need to implement
those and measure their performance again, which would be a whole lot of work. 

			Attila Kinali
-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson