Re: [Discuss-gnuradio] USRP Synchronization

[Eric Blossom]

On Sat, May 06, 2006 at 01:26:15PM -0700, Ges wrote:
> Hi,
>  

>  I am trying to do to and fro transmission using 2 USRP boards. I
>  have connected the tx and rx daughter cards together using SMA Tees
>  (3 SMA Tees connected together giving me 4 ports for connecting the
>  2 tx and 2 rx). I am trying to do a TDMA type of transmisson. the
>  TX on one usrp transmits and then after x secs the tx on the other
>  transmits.

What daugherboard(s) are you using?

What you're describing is currently tough with the basic tx since it's
effectively "always on" once you fire it up.  If you underrun, the
FPGA will continue transmitting the same value to the DAC (which has
the upconverter in it), thus even if you're "not transmitting", it's
highly likely that you really *are* still transmitting.

There are a couple of ways to solve/work-around this behavior:

 * use a RFX-<foo> board with auto transmit switching enabled.
 * modify the FPGA code such that when the TX fifo is empty, you
   ramp the value fed to the DAC down to zero over say 8 clocks.

If this doesn't make sense to you, please attach an oscilloscope to
the DAC output and watch what you're currently getting.

> The way I am controlling the trasnmission is by not
> writing to the buffer on the host side. But even if I am giving a
> large time gap, the data I get at the receiver seems to be
> garbled. Only the very first guy that transmits gives me perfect
> reception. How can I ensure that the USRP sends out data only when
> I want it to and not otherwise, such that there is no overlap
> between the txs of the two USRPs?
> 
> Thanks
> Gesly

Eric