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!
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