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

[Martin Braun]

A couple of comments, maybe this helps you on your tracks:

- You're using an ancient UHD version. Is there a reason you can't use a
newer one?
- In current GNU Radio, you can use 'length tags' to mark bursts, if
that's any help.
- You'll know if your USRP hasn't acknowledged tx_eob if you see 'U's
between bursts.

Cheers,
M

On 10/20/2014 11:43 AM, Frederik Wing wrote:
> 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.
> 
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