[USRP-users] B210 -- various questions on sampling/clock rates

Rob Heig robhhh6 at gmail.com
Thu Oct 26 01:35:58 EDT 2017


Thanks a lot for your answer! The link you gave me is also full of very
very interesting material :)
Have a nice day!

On 25 October 2017 at 22:16, Anon Lister <listeranon at gmail.com> wrote:

> Try specrec to record data.
> https://github.com/garverp/gr-analysis
> As to why it works better see this presentation:
> http://www.trondeau.com/grcon15-presentations#
> wednesday_Lincoln_Synchronized
> (The link is down at the time of writing, perhaps it will be up again soon)
> With it I am able to do 50Msample w/o overruns.
> Search for master_clock_rate in the uhd documentation. Setting that will
> allow you to manually set the master clock rate for the b2xx and e3xx
> devices. As to why I believe oversampling has some benefits a more dsp
> focused individual will be able to elaborate on. I believe the logic for
> automatic choice is something like the highest multiple of the desired
> sample rate that is less than the maximum clock rate.
> -Anon
> On Oct 25, 2017 12:12 PM, "Rob Heig via USRP-users" <
> usrp-users at lists.ettus.com> wrote:
> Hi,
> I am experimenting a bit with the B210 board and I have a couple of
> questions concerning sampling/clock rates:
> - is it normal that, on a decent modern PC with plenty of cores, memory,
> and working on a SSD, a simple program that dumps I/Q data on file gives
> more often than not overflows ("OOOO" messages from UHD) starting at around
> 40Msps (sometimes, with a very lightweight charge of the CPU, even at
> 10-20)? I have been monitoring USB traffic using vUSBAnalyzer to see if
> anything's wrong, and it seems that from time to time the transmission
> simply stops and gets reset after a few seconds (meaning that the board's
> buffers overflowed, I guess), but I couldn't find a reason why this
> happens. I have asked a colleague to increase the size of the buffers on
> the FPGA, but that improved the situation only slightly... Is there any
> architectural documentation explaining the behavior of UHD with a USB
> device to see where the bottleneck could be (without having to delve into
> the UHD code)?
> - playing with filters I saw that, when the sampling rate is set below
> 16Msps, the clock rate is set at a multiple of the desired sampling rate
> (for instance, 32MHz for 2Msps, 40MHz for 5Msps, ...). Actually, I realized
> it only after having spent a whole morning wondering why a custom FIR
> filter that was supposed to work nicely at 8Msps was not filtering at all
> (I guess the messages printed on the console are there for a reason....
> ;)). What is the reason behind this choice? Is it imposed by the RFIC (I
> couldn't find anything on the reference manual, but I might be
> mistaken....) or by the board design? What is the rule that governs the
> choice of the clock rate? Is there any documentation about it?
> Thanks a lot in advance and have a nice day!
> Rob
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