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Re: [Discuss-gnuradio] Odd use of LO phase lock feature on USRP for RFID

From: Colby Boyer
Subject: Re: [Discuss-gnuradio] Odd use of LO phase lock feature on USRP for RFID application
Date: Tue, 19 Apr 2011 14:00:01 -0700

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.



On Tue, Apr 19, 2011 at 12:44 PM, <address@hidden> wrote:
On Tue, 19 Apr 2011 11:41:46 -0700, Matt Ettus <address@hidden> 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
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> Discuss-gnuradio mailing list
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True, for a SISO system with TDD(not FDD) theres no problem for your
kind of application.
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