[time-nuts] SE880 GPSDO

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Apr 28 09:30:15 UTC 2016


Unlikely to be useful.In particular using an off the shelf FSK transmitter is far from optimum.The other issue is the low power and the possibility of interference from others on the same frequency.It would probably be more effective to use GPS and accept a lower time resolution.
Bruce
 

    On Thursday, 28 April 2016 9:00 PM, Ilia Platone <info at iliaplatone.com> wrote:
 

 Please note that not all the frequencies will be utilizable, here only 
433MHz is free-for-all and at low power: only under 50mA transmitting power..

There are some very cheap FSK transmitters that can output at a maximum 
rate of 9600bps: a 1KHz quad signal on these carriers, can drive a GPSDO 
like the 10KHz output of some GPS receivers? The clock being compared to 
this would be 10MHz downscaled by some decade counters.

this would be much simpler to implement.

Ilia.

Il 27/04/2016 23:38, Bruce Griffiths ha scritto:
> On Wednesday, April 27, 2016 09:40:05 PM Attila Kinali wrote:
>> 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
> The solution to this conundrum is to use a high speed serial to parallel
> converter and proces 4/8/16 timeslots in parallel at 1/4, 1/8 or 1/16 the
> serial clock rate as required.
>
> If the high speed serial interface offered by many modern FPGAs could be used
> for this then 100ps or finer timestamp quantisation may be feasible.
>
> Bruce
>
> Bruce
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-- 
Ilia Platone
via Ferrara 54
47841
Cattolica (RN), Italy
Cell +39 349 1075999

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