Products > Test Equipment
ZYT GPSDO - Trimble guts
DJ2XH:
Hi Folks,
I am just wondering whether the ZYT-GPSDO-2 LCD SYMMETRICOM 10MHz 1PPS OCXO GPS Diszipliniert Oszillator offered in AliExpress provides more phase stability in comparison to the BG7TBL GPSDO GNSSDO GNSS Disciplined Oscillator Disciplined Clock with 10MHz Output, also offered in AliExpress.
The BG7TBL GPSDOs we tested two of those each connected with a separate GPS antenna to synchronize two RF-synthezisers at 110 MHz.
We compared the phase drift of the two 110 MHz signals with a amplitude- and phase comparison meter.
We measured a phase drift permanetely variating between of more than 180 deg within of app. 60 seconds.
The RF-synthezisers keep their phase within 1 deg when connected with the same 10 MHz reference signal -> so, they are ok, as well as the phase comparison meter.
My question would be if with the ZYT-GPSDO-2 LCD SYMMETRICOM 10MHz 1PPS OCXO GPSDOr better results can be expected in terms of phase stability?
Thanks DJ2XH
nctnico:
GPS receivers will drift about (ballpark) +/-50ns in respect to eachother over a 24 hour period. So it is not a surprise that you are seeing large phase shifts on a signal that has a period time which is a magnitude smaller compared to the GPS drift. You'll be better of using a lower frequency that is derived from the GPS receiver. Like 1MHz or so.
Comparing GPS receivers without a good reference (like a Cesium clock) is not easy but doable. You'll need to use 3 GPS receivers and run a (at least) 48 hour measurement and compare the phase offset between the receivers. The offset between each combination of GPS receivers should be within the same extremes and have similar stability (Allan deviation).
Also make sure the GPS receivers have enough time to establish their fixed position and are running in fixed position mode. This means letting them run for 24 hours (rule of thumb; check the manual for exact specification) before doing any measurement.
DJ2XH:
Hi nctnico,
Thank you for your quick and helpful reply.
Actually the 110 MHz is a measurement frequency which can not be changed for the planned application.
Further is was planned to use the 2. GPSDO with the RF-syntheziser set on board of a hovering drone.
So there is no fixed position at least for one set during the measurements.
We actually do not need a reference with global accuracy.
For the application we only need a relative phase stability or coherence of about 10 deg between the two sets.
Best Regards
DJ2XH
nctnico:
So you are looking for ballpark 250ps time accuracy. It is not impossible but you won't be able to do this using a GPSDO. How big is your budget? There are companies that offer wireless frequency / time transfer solutions that might be able to meet your requirements but I don't think it will be cheap.
The basic idea would be to use the carrier frequency of a transmitter to convey the frequency to the drone so the drone uses the same frequency reference as the ground station but you likely need to adjust for atmospheric conditions.
DJ2XH:
Your suggestion sounds interesting.
Actually the distance between the static and the hovering set is not more than 100m.
I would have hoped that both sets see nearly the same atmospheric conditions.
Do you think that GPSDOs also have a drift of +/-50ns?
Even the solution you mentioned might be pretty expensive, can you give me a hint where to look in the internet for those companys?
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