I guess I just need to think more about the problem. With your explanation I still don't understand why the 1 PPS would be detected differently in the two situations, and I still don't understand why the phase of the 10 MHz reference with respect to the 1 PPS should matter. I can understand some variation, but not 10 clock cycles worth. I know the PPS signal moves around in time, but I know it doesn't move much in the GPS receivers I use, especially over short time scales. As I said, perhaps I just need to think more about what you said. Thanks for the insight.
On Tue, Jun 23, 2020 at 1:27 PM Marcus D. Leech <patchvonbr...@gmail.com> wrote: > On 06/23/2020 03:18 PM, Aaron Smith wrote: > > Marcus, > > They are EndRun Meridian and Meridian II units. > > I am very ignorant on this topic. Is it a standard that the 1 PPS should > coincide with the top of a 10 MHz cycle? I just wouldn't expect the front > end transmit delay, relative to the 1 PPS input, to depend on the 10 MHz > reference phase. I don't understand how the 10 MHz reference and 1 PPS > input are used to synthesize time. Is the 1 PPS detection done at the > master lock rate (200 MHz) or at 10 MHz? > > The 1PPS is used *exactly once*, when you do a "set_time_next_pps", after > which the time-of-day clock on the board is driven by the master > clock which is phase-locked to the 10MHz external reference. So, the > time-of-day clock on the board runs at (in the case of the X310) 200MHz > by default, so each "tick" is 5nsec. The 1PPS signal is probably > "sensed" by logic that's running at the master clock rate. So two X310 > units may > still have a small amount of residual ambiguity about when 1PPS > "happens", by perhaps as much as 5Nsec. But I'm not an FPGA designer, so > this is just an mildly-educated "guess". > > There may be some lose convention about the phase of the 1PPS with respect > to the 10MHz generated reference, but would not expect it > to be an "enforced standard". Different manufacturers will have > different "servo" algorithms for steering the 10MHz clock output (usually, > it's a > voltage-tunable VCTCXO or VOCXO) with respect to the *derived* 1PPS > pulses. The error on the 1PPS signal is typically surprisingly large--it's > 1PPS +/- a few 10s of nanoseconds, and the phasing of that 1PPS with > respect to the 10MHz signal isn't, I think, necessarily a "standard". > > > On Tue, Jun 23, 2020 at 1:06 PM Marcus D. Leech via USRP-users < > usrp-users@lists.ettus.com> wrote: > >> 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. >> >> >> >> _______________________________________________ >> USRP-users mailing list >> USRP-users@lists.ettus.com >> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >> > >
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