[USRP-users] packet transmission question

bob wole bnwole at gmail.com
Wed Sep 25 23:28:52 EDT 2013


Hi Marcus,

Thank you for your response. I'll search the gnu radio mailing more,
although I am reading it already. If you could please point out the bounds
for the delay, maximum and minimum?

Bob


> Bob,
> this topic has been heavily discussed the last few weeks.
> Matt himself has made some statements on this, please use a search engine
> on the gnu radio mailing list archive to find out.
> "Sample on air" is not really what's happening. You've got to be aware
> that there is, as you've already read in these replies, a variable digital
> delay. After that, you're still mixing to RF! That also introduces group
> delay. So, basically, even though there are some bounds for the delay, your
> actual delay depends on multiple factors and if you really care about that,
> you will need to figure out a way to measure it that fits your need
> yourself.
>
> Hope that helped a little bit,
> Marcus
>
> On 09/25/2013 09:36 AM, bob wole wrote:
> > Anybody please,
> >
> > --Bob
> >
> >
> > On Fri, Sep 20, 2013 at 10:28 PM, bob wole <bnwole at gmail.com <mailto:
> bnwole at gmail.com>> wrote:
> >
> >
> >     Hi Ian
> >
> >     Lets say I enter 500e3 as sample rate in USRP sink (N210). And on a
> sample I tag tx_time=5.65, then what delay will sample encounter from FPGA
> to going over the air ? Or by what time the sample would be on air ? On
> what thing this delay depends on? Your comment would be appreciated.
> >
> >     Bob
> >
> >         On 19.09.2013 17:52, Ian Buckley wrote:
> >         > Francois,
> >         > The logic that counts time and controls the presentation of TX
> data
> >         > to the DAC is located at the start of the USRP DSP chain after
> all the
> >         > transport, protocol and (de)packetizatiion logic. Between
> there and
> >         > the DAC are the CIC filter, both Halfband filters, and the
> CORDIC, all
> >         > of which contain some pipelining in the logic. For any given
> >         > interpolation ration from your GR sample rate to the DAC
> 100(or 64)MHz
> >         > sample rate, this logic should present a constant group delay.
> It
> >         > should change somewhat for different interpolation ratios due
> to
> >         > changes in the CIC configuration.
> >         > -Ian
> >         >
> >         > On Sep 19, 2013, at 1:53 AM, Francois Quitin <
> fquitin at ulb.ac.be <mailto:fquitin at ulb.ac.be>>
> >         > wrote:
> >         >
> >         >> Hi all,
> >         >>
> >         >> We have a little question for you: we use a USRP+WBX with a
> code
> >         >> derived from tx_timed_burst.cpp to create our custom
> transmitter, and
> >         >> we use another USRP that records the received data to a file.
> >         >>
> >         >> When we look at the received signal, we observe a slight small
> >         >> staircase (around 30 samples long at 10MHz = 3us), as can be
> seen in
> >         >> the attached figure. It seems that our actual transmitted
> packet
> >         >> starts after this ?stair?. When we decrease the sample rate
> to 5MHz,
> >         >> the ?stair? is still 30 samples long (6us this time).
> >         >>
> >         >> As we are working in a case where timing is very critical,
> this 3us
> >         >> delay is a bit of an issue (unless it?s constant in which
> case we can
> >         >> compensate for it). Any idea where this is coming from ?
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.ettus.com/pipermail/usrp-users_lists.ettus.com/attachments/20130926/975b489f/attachment-0002.html>


More information about the USRP-users mailing list