[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.