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

[Frederik Wing]

> > Hello,
> >
> > I am trying to implement a burst transmitter using GNU Radio Companion
> > and an USRP N210 with WBX board. My problem is when inserting zeros as
> > "gap" between the bursts the USRP is transmitting a non-neglectable
> > carrier signal (due to DC offset I guess?). So I want to try inserting
> > tx_sob and tx_eob stream tags to make the USRP shutting down the TX
> > chain.
> >
> > For that I created a sample flow graph which creates one burst with 6
> > samples. The tags are inserted by a "Message Burst Source". The
> > important part of the generated .py file is as follows:
> >
> > >         ##################################################
> > >         # Variables
> > >         ##################################################
> > >         self.tx_gain = tx_gain = 15
> > >         self.samp_rate = samp_rate = 200000
> > >         self.f_center = f_center = 1.47e9
> > >
> > >         ##################################################
> > >         # Message Queues
> > >         ##################################################
> > >         blocks_message_sink_0_msgq_out =
> > > blocks_message_burst_source_0_msgq_in = gr.msg_queue(2)
> > >
> > >         ##################################################
> > >         # Blocks
> > >         ##################################################
> > >         self.usrp_sink = uhd.usrp_sink(
> > >             device_addr="device",
> > >             stream_args=uhd.stream_args(
> > >                 cpu_format="fc32",
> > >                 channels=range(1),
> > >             ),
> > >         )
> > >         self.usrp_sink.set_samp_rate(samp_rate)
> > >         self.usrp_sink.set_center_freq(f_center, 0)
> > >         self.usrp_sink.set_gain(tx_gain, 0)
> > >         self.usrp_sink.set_antenna("TX/RX", 0)
> > >         self.blocks_vector_source_x_0 =
> > > blocks.vector_source_c((1,1,1,1,1,0), False, 1, [])
> > >         self.blocks_tag_debug_0 =
> > > blocks.tag_debug(gr.sizeof_gr_complex*1, "", "");
> > > self.blocks_tag_debug_0.set_display(True)
> > >         self.blocks_message_sink_0 =
> > > blocks.message_sink(gr.sizeof_gr_complex*1,
> > > blocks_message_sink_0_msgq_out, False)
> > >         self.blocks_message_burst_source_0 =
> > > blocks.message_burst_source(gr.sizeof_gr_complex*1,
> > > blocks_message_burst_source_0_msgq_in)
> > >
> > >         ##################################################
> > >         # Connections
> > >         ##################################################
> > >         self.connect((self.blocks_vector_source_x_0, 0),
> > > (self.blocks_message_sink_0, 0))
> > >         self.connect((self.blocks_message_burst_source_0, 0),
> > > (self.blocks_tag_debug_0, 0))
> > >         self.connect((self.blocks_message_burst_source_0, 0),
> > > (self.usrp_sink, 0))
> > Through the "Tag Debug" block I observe that the tags are set correctly
> > (tx_sob at sample 0 and tx_eob at sample 5). Nevertheless the USRP is
> > still transmitting at the carrier frequency after the actual burst is
> > completed. I observed that with a second USRP as receiver. It seems like
> > the USRP Sink is ignoring the tags.
> >
> > Trying the "tags_demo" application showed the desired behaviour. No
> > carrier between the bursts. I checked the source code and noticed that
> > they are additionally assigning time stamps as tags to the samples. Is
> > this necessary to achieve my goal?
> >
> > I am using GNU Radio 3.7.2.1 and UHD_003.005.005-0.
> >
> > Any comments on this issue are highly appreciated!
> >
> > Frederik.
>
> Good news everyone, I finally found something!
>
> When setting the TX gain in the flow graph above to something less than
> 5.8 (I tried 0 to 5.7) the carrier disappears. If I choose 5.8 or higher
> the carrier is visible. And it's not just that it is attenuated by the
> decreased output gain. There is a difference of 30dB at the receiver
> USRP if I change from 5.7 to 5.8.
>
> I found out by analysing the "tags_demo" application. They are not
> specifying any gain. I added a call to set_gain in the source code and
> observed the same behaviour as in my flow graph. There seems to be a
> magic boarder at 5.8dB.
>
> Can anyone explain this to me? Is it bug in UHD or the USRP firmware?
>
> Frederik

And now the problem is finally solved.

The cause for it was a defective WBX board. I replaced it with a new one 
and now everything works!

Again, thank you all for your productive hints and explanations!

Frederik