[Discuss-gnuradio] USRP behavior during underflow

[Francois Quitin]

Dear all,

 

We’re working on a project where a receiver tracks the LO phase and frequency offset of a transmitter by using periodic messages sent by the transmitter. We have a small problem we need some help with:

 

The transmitters creates a packet (with the packet encoder), modulates it with a GMSK modulator and sends it to the USRP. Then it stop sending samples to the USRP, so an underflow occurs. After a certain time interval (+/-50 ms), a new packet is transmitted in the same way. At the receiver, the packets are received and decoded correctly. However, when we try to measure the absolute LO phase offset, we get inconsistent results.

 

Our guess is that when we stop sending samples to the transmitting USRP (when the underflow occurs), the transmitter (or parts thereof) is turned off. When the next packet arrives, the transmitter is turned on again, but the absolute phase of the carrier is lost.

 

Is this something to be expected? Is there a way to keep the transmitter on during an underflow?

 

Thanks for your help!

 

François Quitin, Ph.D.

 

BAEF Post-doctoral research fellow

Electrical & Computer Engineering Dept.

University of California

Santa Barbara, CA 93106-9560

Phone: +1-805-883-8599

Email: address@hidden

Home: www.ece.ucsb.edu/~fquitin