Hi Marcus, Thanks for thoughts on this. We also believe that is has to do with the calibration mechanism of the AD9361, due to the very low sub-sample delay change. Any thoughts from the Ettus team? Is there a way to retrieve calibration information from the RFIC in UHD? This could help to identify if there is a correlation between calibration setting and the group delay change.
Best regards, Emanuel Von: Marcus D. Leech <[email protected]> Gesendet: Montag, 7. Februar 2022 17:48 An: [email protected] Betreff: [USRP-users] Re: B200 group delay variations On 2022-02-07 02:44, [email protected]<mailto:[email protected]> wrote: Hi all, we have a custom Gnuradio implementation with many B200mini USRPs, which enables precise round-trip time (RTT) based ranging among those devices. The MAC is TDMA based with the automatic Tx/Rx switching and to compensate for the USRPs group delay we perform ranging over an RF-splitter which we calibrated beforehand with a vector network analyzer. We observed that the estimated RTT over multiple calibration measurement runs is unstable in the order of up to 1ns. Within one measurement run of at least 5 minutes with 10 RTT estimates per second, it is pretty stable, but when we restart the flowgraph, we observe a bias. Please find at the end of this E-Mail all things we already tried to narrow down the issue (a compressed version). In short: 1. We finally used only two B210 with a single external reference clock, a sampling rate of 25MHz, and a manually set master clock rate of 50MHz. * In this setup the estimated RTT over multiple measurement runs can vary from run to run by <50ps (ok for us), but also by 280ps or 750ps (far too much for us). The standard deviation of RTT estimates within one measurement run is about 70ps, and each run contains 2000 to 3500 RTT estimates. 1. To cross check our implementation, we used the very same flow-graph with two X310 with CBX-120 front-ends, a common external reference clock, and only increased the transmit signal padding by 40us (because the ramp-up time is larger for this front-end). * RTT estimates over multiple runs are picture perfect: the mean RTT varies by less than 5ps over multiple measurement runs. Any idea what could cause those variations for the B200 series? We use UHD 3.15. Best regards, Emanuel Since these variations are very-much sub-single-sample in scale, my guess is that this has to do with the very-complex "calibration" mechanism that the AD9361 undergoes when it is initialized, and when you change frequency. Many of the elements in that sequence are, I think, analog in nature. There may be some "Monte-Carlo" elements in this, either "inherent" (because you're dealing with the vagaries of the analog world), or perhaps explicit in algorithms like I/Q imbalance compensation. In contrast, on the X310 with a CBX, any I/Q balance compensation (which can affect phase), is entirely static, and the signal pathway in the analog domain is very much simpler, with very much less "dynamic tweaking" during initialization and tuning. My understanding is that AD9361 does not allow turning off any of those compensation algorithms. Now, a caveat is that this is an educated hunch, based on the nature of the issue. It could be something else, but the *scale* of the issue suggests something in the analog domain.
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