On 06/23/2020 02:45 PM, Aaron Smith via USRP-users wrote:
Hello,
I am attempting to release a transmission from an X310 every second.
To accomplish this, I must measure, and calibrate the delay in the RF
front end of the radio for my chosen sample rate. I'd like the
transmission to be released within 1 clock cycle of the rising edge of
the PPS.
I am feeding the X310 an external 10 MHz reference and 1 PPS, which
are produced by the same source, and are being supplied to the radio
with matched cable lengths. The source is a GPS receiver and in my lab
I have 2 different generations of the GPS receiver.
While calibrating the front end transmit delay I noticed a discrepancy
in the radio timing between the separate GPS receiver generations. The
1st generation of GPS receiver is 50 ns different than the calibration
for the 2nd generation. When I look at the 1 PPS and 10 MHz output on
a scope, I noticed that in the 1st generation the PPS occurs at the
top of a 10 MHz cycle, and in the 2nd generation it occurs at the
bottom of a 10 MHz cycle. Half a cycle at 10 MHz is 50 ns. I suspect
this is not coincidence because I have now tested 6 different GPS
receivers, 3 of gen 1 and 3 of gen 2, and all 3 gen 1 calibrations are
the same and they are 50 ns different from the gen 2 calibrations.
Is this the expected behavior? Or is there a bug in the X310 code that
handles timing? I have never worked on hardware, but I would not
expect the initial phase of a 10 MHz reference to impact absolute time.
Thanks for your help!
These are external GPS receivers? What kind? Given your scope
measurements, how would this be related to a bug in X310? I'm confused
as to how you're linking the 10MHz/1PPS phasing on your external GPS
receivers to the X310 having bugs.
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