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:
* 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.
* 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
Here comes the list of setups we tried already with two B210 and common
external reference clock.
Each measurement run comprises 2000 to 3500 RTT estimates, lasts for about 5
minutes, and for each setup we make 5 to 10 measurement runs.
* Sampling rate 25MHz, master clock rate 50MHz
* This gave us the most stable results so far, yet, it is not stable
* Sampling rate 25MHz, master clock rate 25MHz
* Much worse than with master clock rate 50MHz: mean RTT varies with
every measurement run
* Sampling rate 15MHz, master clock rate 60MHz
* We tried this setup to see of the master clock rate to sampling rate
ratio make a difference
* As worse as the setup with 25MHz sampling rate and 25MHz master clock
rate
* Sampling rate 25MHz, master clock rate 50MHz + additional power cycle of
USRP + additional uhd_usrp_probe with manually set master clock rate to 50MHz
* No difference compared to the case without power cycling
* Change of Sink/Source USRP Gnuradio block initialization in top-level
flowgraph python script
* No difference in the RTT variation behavior.
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