usrp reverse signal

[jtudor]

Hi!!

I'm trying to built a little radar with the USRP 2920  ..

but I have a problem.

I connect the two port of tghe USRP (TX1 RX2) with a low loss cable of 50 cm..

after transmitting a chirp signal, sometimes  i found my signal on rx port shifted by 180 degrees .. totally reversed

it's possible that is a problem of the LNA on board of the USRP ?.

Francesco

Ps

I Had this problem also with the USRP 1 of Ettus

[Anthony_F]

Hello Francesco,

Can you post any pictures of what you are receiving and what you are transmitting?  When does this shift happen and when does it not happen?

Since you also had the same problem with the USRP1 I would suspect the setup you have, maybe you are seeing a physical reflection or multipath interference.

I would try a loop back test using the 30db attenuator that comes with the USRP and see if it the shifting still persists.

[Yupeng_Jia]

the loop back test configuration can be found in the link Out-of-the-Box Video With NI USRP

[jtudor]

I connect the two port (TX1RX1 - RX2) with the SMA SMA cable without the attenuator ...

I send you two Images of the rx signal and tx signal ..

I found the shifted signal only sometimes not always..

[Anthony_F]

Hello Francesco,

Are you continuously sending the chirp signal or are you sending it only a few times (finite)?

It looks like you are only using one USRP to both send and receive, is this correct?

[jtudor]

It's a trasmission of a chirp signal for one second...

I use it for both sending and receiving..

[ErikL · Accepted solution]

The RX and TX share a 10 MHZ reference, but not their respecive derived Local Oscillators. This means that the derived LO clocks can lock at different phase shifts which will change each time the application is run, but will stay at a constant offset (coherent) while running.

[jtudor]

thank's a lot Erik