Hi, There should be some delay, but it should be on the order of a few clock cycles (ADC/DAC latency). For the N321 we are observing 100us, corresponding to ~2000 samples. The X310 delay is ~1us, which corresponds to 20 samples. Still a lot higher than I would expect just due to ADC/DAC. The delay changes as a function of the sample rate. If the synchronization is after the DDC (as I think it is), I would expect the delay to be independent of the decimation ratio.
We are doing the calibration and will use that to compensate, but I think this is something that can be mitigated (to <1us) in the FPGA. Eugene. ________________________ Eugene Grayver, Ph.D. Aerospace Corp., Principal Engineer Tel: 310.336.1274 ________________________ ________________________________ From: Rob Kossler <rkoss...@nd.edu> Sent: Monday, January 15, 2024 5:05 PM To: Eugene Grayver <eugene.gray...@aero.org> Cc: usrp-users <usrp-users@lists.ettus.com>; Mark Kubiak <mark.kub...@aero.org>; Jason W Zheng <jason.w.zh...@aero.org> Subject: Re: [USRP-users] Bug/problem aligning PPS to samples Hi Eugene, Are you expecting that the RF output (for Tx case) should be synced to the PPS "at the RF output connector"? It is my understanding that the sync occurs at some place in the FPGA logic for the "radio" block. There will be delay as this goes through D/A and RF chain. Same in reverse for Rx. As long as you get a consistent delay (for a given sample rate), can you calibrate and then choose a playout time that syncs the RF pulse to the PPS pulse? Rob On Fri, Jan 12, 2024 at 4:38 PM Eugene Grayver <eugene.gray...@aero.org<mailto:eugene.gray...@aero.org>> wrote: Hello, There appears to be a bug related to alignment of the PPS to samples. The issue applies to both TX and RX and was replicated on N321 and X310 using UDH 3.15 and 4.6. It therefore appears to be an FPGA issue. TX experiment ---------------------------- * USRP is provided with external PPS and 10 MHz * The PPS input is split and goes to the USRP and a scope * The USRP output goes to a scope * USRP outputs a file * First 1000 samples are 1, remaining are zero * File size = sample rate (i.e. repeats every second) * Setup the experiment using both: * GR file_source + usrp_sink * Sync to unknown PPS * usrp.set_start_time(5) * Standalone C++ application (based on tx_samples_from_file) * Added code to set_time_unknown_pps(0), then set start time using metadata to 5 Results: * The USRP output is delayed relative to the PPS as observed on the scope * The delay is ~1.2 us for X310 and ~100us for N321 * The delay changes slightly (<1us) depending on the sample rate (e.g. 10 Msps vs 20 Msps) RX experiment ---------------------------- * USRP is provided with external PPS and 10 MHz * USRP input is a pulse (generated using technique above) that repeats every second * Pulse is aligned to PPS, verified using a scope * USRP records samples starting on a second boundary (time_t(5)) * GR usrp_source + file_sink * standalone C++ application (based on rx_samples_to_file) * Added code to set_time_unknown_pps(0), then set start time using metadata to 5 * Recorded samples are analyzed to find the first 'large' value Results * Recording appears to start late relative to PPS (only verified on N321, delay is ~100 us, same as for the TX delay) Thoughts * I recall (years ago) there was a fix to a similar problem. The FPGA was modified to trigger ADC/DAC samples after the DDC rather than before. Did it regress at some point? * The delays are very consistent, indicating that the PPS is in fact being used (i.e. it is not random). * We ran some experiments to analyze the stability and accuracy of relative timing between RX and TX (i.e. turn-around) when the start time for TX and RX are specified. The results are excellent – delay is stable and accurate to < 100 ps. This seems like a simple thing to fix in the FPGA – there is no reason for the delay to be > 1 sample clock. Eugene. ________________________ Eugene Grayver, Ph.D. Aerospace Corp., Principal Engineer Tel: 310.336.1274 ________________________ _______________________________________________ USRP-users mailing list -- usrp-users@lists.ettus.com<mailto:usrp-users@lists.ettus.com> To unsubscribe send an email to usrp-users-le...@lists.ettus.com<mailto:usrp-users-le...@lists.ettus.com>
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