Re: [Discuss-gnuradio] Rx and Tx on same antenna

[Martin Braun]

On Thu, May 20, 2010 at 02:46:38PM +0200, Mirko Heukemes wrote:
> Hello,
> 
> I'm starting to play around with gnuradio and the USRP1. I have the
> RFX400 daughterboard but only one antenna. I used the examples
> usrp_wfm_rcv.py and usrp_nbfm_rcv.py for testing and it worked (I
> only had to change the center frequencies for the use with the
> FRX400). For emitting I used gnuradio-companion and the WBFM
> transmit block that also worked fine. Now, I'm trying to set up a
> transceiver that uses for both the receive and emit path the TX/RX
> antenna of the RFX400. I made a first (simple) flowgraph using
> different classes for the Rx and Tx paths (classes copied from
> working grc flowgraphs). I used the same parameters than for the
> receiver and emitter alone but I set the transmit parameter of the
> USRP sink to Auto T/R. Now, my plan is to remain in the reception
> mode all the time and to switch to transmission mode only if I want
> to speak. In order to do this, I added this code to my  python
> script:
> 
> if __name__ == '__main__':
>    parser = OptionParser(option_class=eng_option, usage="%prog: [options]")
>    (options, args) = parser.parse_args()
>    tr = usrp_wbfm_receive_nogui()
>    tt = usrp_wbfm_emit_rfx400()
>    while 1:
>        tr.start()
>        raw_input('Press Enter to emit: ')
>        tr.stop()
>        tt.start()
>        raw_input('Press Enter to stop emitting ')
>        tt.stop()
> 
> And both the reception and transmission seem to work in first place.
> But if I press Enter the second time, I get this error message:
> 
> RuntimeError: top_block::start: top block already running or wait()
> not called after previous stop()
> 
> Assuming that it's impossible to restart a block after stopping it
> without executing the wait() call, I added tr.wait() and tt.wait()
> after the respective stop calls. But then I get another error
> message:

Hi Mirko,

I haven't read your code (excuse my laziness), but here's some input:
You can't stop and restart a flow graph (just recently, there was a
similar discussion on this topic). Once it's stopped, its state is
undefined. You need two separate flow graphs for Tx and Rx. If you want
them both in one application, you will have to put them both into one
top_block (see the Python-Applications tutorial on the wiki, it also
happens in usrp_nbfm_ptt.py). Also, tunnel.py by default manages to tx
and rx on one antenna by using the auto-tr feature which you've already
discovered.
Hope this is useful,

MB

-- 
Karlsruhe Institute of Technology (KIT)
Communications Engineering Lab (CEL)

Dipl.-Ing. Martin Braun
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