Re: [Discuss-gnuradio] Implementing timed transmission: UHD API or Stream Tags?

[Martin Braun]

Yes, you can time down to the clock cycle. The time it takes for a
signal to propagate to the antenna after it is released depends on the
interpolation rate and the hardware, but it doesn't vary a lot (assuming
everything else constant, such as temperature). You can calibrate that
out quite well -- a few years back I did some radar measurements this
way. I had to calibrate an initial offset, but once I knew that there
was no tracking etc. required.

M

On 07/13/2016 09:24 AM, Lakshay Narula wrote:
> Hi Martin,
> 
> Appreciate your reply to my questions. I agree that I cannot do better
> than a clock cycle, so that is settled. Let us say that I use GPSDO for
> my reference and I wish to transmit at GPS time t. Is it possible to
> control the transmit time to within (t plus/minus 1 clock cycle)? I have
> read other threads on this topic and it looks like the "tx_time" tag
> controls the time at which the packet is released to the DSP on-board
> USRP. So it would take some additional time for the packet to go through
> the DSP, DAC, and the antenna.
> 
> Now I would like to perform the experiment myself and see if this delay
> is something that I can calibrate out, but unfortunately we do not have
> the hardware on hand (I currently have DBSRX2 which cannot transmit). So
> I was wondering if you have some intuition about the kind of
> consistent/variable delays I would be seeing, and how bad you would
> expect the variation to be (~10 ns, ~100 ns, ~1 ms)?
> 
> Thanks,
> Lakshay.
> 
>     > Hello,
>     >
>     > I am a new GNU Radio user. I am looking to build a system that can
>     > transmit a packet at a pre-defined time with very high accuracy
>     (about 1
>     > nanosecond). Having gone through the mailing list I am aware that
>     timed
>     > transmission is a common task and many people have asked similar
>     > questions. However, I am still a bit confused.
>     >
>     > 1. I see that there is an example tx_timed_samples that comes with the
>     > UHD source code. This is in C++ and uses the UHD API. Am I right in
>     > thinking that when implemented this way, it has nothing to do with GNU
>     > Radio at all? Is there any "reported accuracy" of this method in terms
>     > of difference between actual and required time of transmission?
> 
>     Yes, that's accurate. This is more of a UHD/USRP issue. Note that you
>     can time to a clock cycle, which is longer than a nanosecond. Whether or
>     not your sample is lined up with a time reference of your choice to
>     sufficient accuracy also depends on the reference signal you're
>     providing.
> 
>     > 2. I also see that it is possible to achieve similar objectives using
>     > tx_time stream tags in GNU Radio. My question is if this method is
>     > equivalent to method 1 in terms of what goes on "under the hood"? If
>     > not, how do these differ, and which method would provide better
>     accuracy
>     > in terms of agreement between actual and required time of
>     transmission.
>     > Does GNU Radio use the UHD API "under the hood"?
> 
>     Yes, it does, and tags are no better or worse than the API calls. They
>     may be more convenient, depending on your software. Internally, the UHD
>     send() call (which is how samples get to devices) is populated with a
>     timestamp in both cases.
> 
>     > Please feel free to point me to resources I can read to get a better
>     > understanding of this architecture and relationship between UHD, GNU
>     > Radio, and USRP.
> 
>     There's also the usrp-users mailing list, and UHD has a bunch of
>     examples. gr-uhd code is also a useful reference.
> 
>     Cheers,
>     M
>