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

[Colby Boyer]

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 <address@hidden> 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 <address@hidden> wrote:

From: Colby Boyer <address@hidden>
Subject: Re: [Discuss-gnuradio] Odd use of LO phase lock feature on USRP for RFID application
To: address@hidden
Cc: "Matt Ettus" <address@hidden>, "GNU Radio Discussion" <address@hidden>
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, <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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True, for a SISO system with TDD(not FDD) theres no problem for your
kind of application.
Regards
Agile Solutions

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