|Subject:||Re: [Discuss-gnuradio] Odd use of LO phase lock feature on USRP for RFID application|
|Date:||Tue, 19 Apr 2011 16:31:55 -0700 (PDT)|
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
--- On Tue, 4/19/11, Colby Boyer <address@hidden> wrote:
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