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Re: [Discuss-gnuradio] Transmitting bursts with GRC by inserting SOB and

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] Transmitting bursts with GRC by inserting SOB and EOB
Date: Tue, 21 Oct 2014 14:40:13 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.7.0

On 21.10.2014 13:47, Frederik Wing wrote:
> Hello Marcus,
> thank you very much for your hints!
>>>> - You're using an ancient UHD version. Is there a reason you can't
>>>> use a
>>>> newer one?
>>> I am using Ubuntu 14.04 LTS and just pulled GNU Radio and UHD from the
>>> packet sources. There is no newer version available. And I don't
>>> like to
>>> compile myself unless it is absolutely necessary.
>> Consider it outdated. Ubuntu's packet sources are most of the time
>> horribly out of pace compared to GNU Radio's development process.
>> For UHD it's the same. Note that Ettus ships its own Ubuntu packages
>> for UHD *and* GNU Radio, so you can get a non-ancient version of both
>> without even compiling it.
>> By the way: GNU Radio is a software radio developer's framework. You
>> can do great things with it without touching a compiler, but at many
>> points it becomes helpful when one can write his own C++ block, so
>> you definitely need to lose your fear of compilers if you want to use
>> GNU Radio to its fullest potential :)
> After some hours of compiling time I could finally update GNU Radio to
> version 3.7.6 and UHD to 003.008.
>>> I don't think it's because of an outdated version of UHD as the
>>> "tags_demo" application is working perfectly with my current version.
>> ... Well, my old tube radio also still works ;)
> Unfortunately, even with the newest version the USRP is still
> transmitting its carrier over the air. I tried both with the Message
> Burst Source as well as with the Stream to Tagged Stream Block
> combined with setting a Length Tag name in the USRP Sink. With the Tag
> Debug Block I see tx_sob+tx_eob and the Length Tag, respectively. They
> all seem to be at the right place and have the right value.
:( ok, so we see something that looks like significant LO leakage.

You can use offset tuning, by specifying a tune request instead of
simply a target frequency f_t. By doing so, you instruct the USRP to
tune the oscillator of the daughterboard to some frequency f_LO, and let
the FPGA shift the resulting signal to baseband by a second frequency
f_FPGA, so that f_LO+f_FPGA = f_t.

You thereby would move the LO leakage out of your desired signal. Would
that help you?


> The Length Tag should arrive properly at the Sink. I checked by
> changing the tag's name at the Stream to Tagged Stream Block to
> something stupid and finally got an "tG" printed out.
> Another interesting observation:
> I don't get the "U" underrun output if I use a larger number of
> samples for my burst (in Vector Source). Even with only Vector Source
> and USRP Sink (no blocks in between) I don't get any "U" printed. I
> tried that with 500 samples.
> Has anyone ever succeeded in transmitting proper bursts with a GRC
> flow graph? Or at least on Python level? I really feel desperate right
> now...
>>>> - In current GNU Radio, you can use 'length tags' to mark bursts, if
>>>> that's any help.
>>> I tried using length tags by inserting an "Stream to tagged Stream"
>>> block and adding the length_tag_name property to the USRP Sink. Here is
>>> my sample code, now with a burst of 5 samples:
>> The GNU Radio doxygen contains documentation for the current GNU
>> Radio build, containing the newly available message passing
>> functionality.
>> Maybe that's of interest to you!
> That's definitely an interesting feature. But first of all I'd like to
> make sure that transmitting bursts works as expected before making
> other improvements in my transmitter chain.
> Frederik
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