Basically, if possible use the same LO, or lock the two LO's together. Going back to the original question: is locking the LOs for an RX card and a TX card on a USRP1 feasible?
Thanks for your comments Vijay, you helped to add focus to my question. --Colby On Tue, Apr 19, 2011 at 4:31 PM, Vijay Pillai <[email protected]>wrote: > Hi Colby, > > Even if the two boards have slightly different frequencies, this should not > impact decoding of the receive signal as the received signal jumps between > the I and Q channels (depending of course on the packet lengths and assuming > that you have a packet decoder on I and Q separately). > > More seriously using separate uncorrelated LO signals for transmit and > receive significantly degrades receiver sensivity. The transmit signal is > typically 30dBm+ and the same antenna or nearby antenna is used to get the > receive signal - the received signal has this huge transmit signal along > with a -60dBm backscattered signal from the tag that is 50 to 200kHz away > from carrier. If the same LO is used for transmit and receive, then at the > dowconversion mixer, there is a high degree of correlation between signals > at the LO port and the RF port (esp. the transmitter leakage), and much of > the transmitter noise shows up as a DC offset. If separate LO's then the > noise at the two ports are large uncorrelated contributing to the baseband > noise. You can expect 5db to 20db difference in SNR between using same LO's > vs separate LO's > > Best regards, > -Vijay > > --- On *Tue, 4/19/11, Colby Boyer <[email protected]>* wrote: > > > From: Colby Boyer <[email protected]> > Subject: Re: [Discuss-gnuradio] Odd use of LO phase lock feature on USRP > for RFID application > To: [email protected] > Cc: "Matt Ettus" <[email protected]>, "GNU Radio Discussion" < > [email protected]> > Date: Tuesday, April 19, 2011, 5:00 PM > > > The two boards should have different clocks, so there should be some > frequency offset. Even in typical SISO systems, you use a PLL block to deal > with this since you can't access the other LO because its physically > somewhere else. > > While receiving, the transmitter is still running at full power to run the > RFID tag. The transmitters carrier is down converted by the receiver board. > Unless I have a misunderstanding, and the two daughter boards share the same > clock there should be some frequency offset. > > ? > > Thanks, > Colby > > On Tue, Apr 19, 2011 at 12:44 PM, > <[email protected]<http://mc/[email protected]> > > wrote: > > On Tue, 19 Apr 2011 11:41:46 -0700, Matt Ettus > <[email protected]<http://mc/[email protected]>> > wrote: > > On 04/19/2011 11:38 AM, Colby Boyer wrote: > >> Hi All, > >> > >> In RFID applications, a reader receives (backscatter from RFID tag) and > >> transmits (constant tone) at the same frequency. With commercial > >> readers, a single LO will be shared by the RX and TX chain. However, in > >> the USRP case, two separate daughter boards are used so different LOs > >> are in use for the RX and TX chain. So you should end up with some > >> frequency offset in RX chain due to mismatched clocks. > >> > >> Is it possible to lock the LOs of a TX daughter board and a RX daughter > >> board, as you would for a traditional MIMO 2 TX or 2 RX setup? There > >> appears to numerous discussions and examples of the latter. I'm thinking > >> it would be possible. But I'm more of a systems guy and less of a RF > >> hardware guy, so any comments would be appreciated. > >> > >> Thanks, > >> Colby > > > > > > As long as you set them to the same frequency, they're already locked. > > No need to do anything different. > > > > Matt > > > > _______________________________________________ > > Discuss-gnuradio mailing list > > [email protected] <http://mc/[email protected]> > > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > True, for a SISO system with TDD(not FDD) theres no problem for your > kind of application. > Regards > Agile Solutions > > > > -----Inline Attachment Follows----- > > > _______________________________________________ > Discuss-gnuradio mailing list > [email protected] <http://mc/[email protected]> > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > >
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