That's an interesting question, Dan. You seek a low cost oscillator that 
follows GPS without following it too closely.
Do you have a plot of a typical day of GPS atmospheric disturbance?


On Mar 8, 2019 2:55 PM, Dan Werthimer <d...@ssl.berkeley.edu> wrote:

hi robert, randall,  dale and casperites,

thanks for your time/freq standard suggestions.

our problem to get accurate 1 PPS wrt UTC is not limited by internal oscillator 
stability.
the problem is mostly about GPS atmospheric delay corrections.
i don't thinks one needs an ultra stable oscillator in a GPSDO time/freq 
standard if one is only interested in 1 PPS accuracy wrt UTC.
there's no need for rubidium, hydrogen, or microsemi's atomic gizmo for 1 pps 
accuracy
--  an OXCO is fine.

the typical GPS disciplined oscillators (eg: srs and  trimble) output 1 PPS 
that have 100 ns errors from UTC.
the best ones we have found so far are made by endrun technologies (<  10 ns 
error wrt UTC).

endrun claims to have unique algorithms for GPS atmospheric correction.
although one can do next day GPS atmospheric correction for atmospheric delays
(the next day, there are correction tables available for the previous day that 
are location/satellite dependent),
endrun technologies can do some fairly accurate measurements and atmospheric 
delay corrections in near real time.
we'd prefer not to use next day correction tables in our application.

has anybody used GPSDO time/freq standards that have < 10 ns accuracy wrt UTC?
has anybody used the endrun technologies standards ?
has anybody used GPSDO products from microsemi?
https://www.microsemi.com/product-directory/clocks-frequency-references/3826-gps-disciplined-oscillators-gpsdo
the GPSDO's listed at the bottom of that page have 10 ns wrt utc.

best wishes,

dan



On Thu, Mar 7, 2019 at 8:57 AM Robert F. Jarnot 
<robert.f.jar...@jpl.nasa.gov<mailto:robert.f.jar...@jpl.nasa.gov>> wrote:

Dan,

    I looked into GPS disciplined oscillators for a project, and ended up using 
atomic clocks instead, as they are now very small, can be flown, and have 
remarkably low power consumption. We have 2 of them, $7500 each. See 
https://www.microsemi.com/product-directory/clocks-frequency-references/3824-chip-scale-atomic-clock-csac

    I suspect that a good choice of GPS disciplined oscillators would work 
pretty much as well, and be cheaper.

Bob

On 3/7/19 8:51 AM, Dan Werthimer wrote:


in a somewhat related question.

can anybody give us advice about GPS disciplined oscillators time/freq 
standards that are very accurate wrt UTC?
we don't want to buy a hydrogen maser (too pricy).
we have been looking at a company called endrun technologies that sell 
time/freq standards accurate to about +-10 ns wrt UTC.
they might be able to match a pair of them that track each other +- 3ns RMS.
we need a pair of well matched time/freq standards for coincidence time 
stamping/correlation between two observatories for our panoseti experiment.
(the two optical/IR observatories are 500 km apart, and don't have masers).

thanks for any advice on this.

btw, we are using white rabbit for time/frequency distribution over 1 Gbe bidi 
fiber,
and we put the white rabbit hardware (VCO and DAC chips) and software on our 
FPGA boards for this project.
(we made our own FPGA boards with white rabbit and kintex7 because we need a 
few thousand boards)
white rabbit does sub-ns accuracy in timing distribution - some white rabbit 
users have measured 30 ps RMS.

best wishes,

dan


On Thu, Mar 7, 2019 at 12:05 AM Michael Inggs 
<miki...@gmail.com<mailto:miki...@gmail.com>> wrote:
Hi Franco

Simon Lewis in the RRSG at UCT has White Rabbit hardware and expertise (PhD 
incubating). Snag is that it runs on 1GE Fibre. We also have a GPS version. The 
former gives sub ns precision, the latter about 4 ns rms. Send me a message off 
line and I can link you. We also have a scheme of aligning a trigger to both a 
local MHz clock and the 1 pps. This is all open source hardware and software.

Regards

On Thu, 7 Mar 2019 at 08:52, James Smith 
<jsm...@ska.ac.za<mailto:jsm...@ska.ac.za>> wrote:
Hello Franco,

As I understand it, PTP wasn't terribly useful in our application (though I 
wasn't involved with this directly). You can probably sync the little Linux 
instance that runs on the ROACH2, but getting the time information onto your 
FPGA may prove somewhat tricky.

Are you using an ADC card in the ROACH2? Or is the data digitised separately?

What we've done with ROACH and ROACH2 designs in the past is more or less this:

  *   FPGA's clock comes from a timing & frequency reference (TFR).
  *   ROACH2 gets a 1PPS input from the same TFR.
  *   In the FPGA logic there's a counter which is reset as part of the 
initialisation, and some logic that starts the counter going after a set number 
of 1PPS pulses (two to three, I forget exactly now).
  *   The output of this counter is pipelined along with the data and then sent 
out as part of the SPEAD data on the 10GbE network.

The idea here being that you know with a fairly high degree of precision which 
pulse your ROACH was initialised on. The counter that comes through on the 
SPEAD packet counts in FPGA clock cycles (or multiples thereof, perhaps you 
might want to count in spectra), and then you can use the start time to 
calculate the timestamp of each packet (Unix time, MJD, whichever your 
preferred reference is).

Hope that helps.

Regards,
James


On Wed, Mar 6, 2019 at 7:41 PM Franco 
<francocuro...@gmail.com<mailto:francocuro...@gmail.com>> wrote:
Dear Casperiites,

I was given the task of timestamping ROACH2 spectral data in a telescope that 
uses PTP (precision time protocol) as a synchronization protocol. I understand 
that ROACH's BORPH come preloaded with NTP (network time protocol) 
libraries/daemos, but PTP is preferred because is already in use in the 
telescope, and it achieves greater time precision.

Does somebody know if it is feasible to compile/install PTP libraries in BORPH?

Alternatively, we have though of sending the ROACH the current time through a 
GPIO pin using IRIG-B timecode standard. Has anybody done something similar in 
the past?

Thanks,

Franco
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