USRP mux configuration bug

I have been experiencing a weird bug when transmitting packets using the
USRP. I am part of the Hydra group at UT ( We have
been developing a multiple antenna system using the USRP. It seems to
quite well except for the following bug:

Since we’re using two transmit chains, I set the mux value in
usrp.sink_c to
0xBA98. I have noticed however, that at random the transmitter will flip
IQ channels of the transmitter. I have verified that this occurs by
analyzing the trace of the received signal. I noticed that this same bug
seems to occur when using a single antenna system as well (there I set
mux value to 0x0098). In the single antenna case I can just reset the
value to 0x0098 everytime I send a packet in order to alleviate the
This same approach does not work in the two-antenna case.

Any thoughts on why the mux value in the transmitter might be changing
why the IQ channels might get flipped? Please note that I have already
verified that the EXACT same waveform is being sent to the usrp
everytime I
transmit a packet; hence, I am certain that the bug is not being caused
I am currently using a multiple antenna system, so I set the usrp.sink_c
value to 0xBA98. This should enable the USRP to support to transmit RF
chains, right?

On Fri, Jul 20, 2007 at 05:02:55PM -0500, Ketan M. wrote:

mux value to 0x0098). In the single antenna case I can just reset the mux
chains, right?

Ketan M.

Are you running one of our standard fpga images, or are you using one
that you have modified? That is, are you writing verilog code?

If you are compiling your own verilog, are all timing constraints
being met? Did you start with our project file?

It’s seriously unlikely that the contents of the mux register is
getting randomly disturbed.