Re: [Discuss-gnuradio] High LO leakage in xcvr 2450 with new UHD?

[Josh Blum]

On 07/14/2012 12:23 PM, Apurv Bhartia wrote:
> Hi All -
> 
> I've been using the new UHD driver (003.004.002) stable release w/ XCVR
> boards on the USRP N200.
> 
> I'm using the xcvr as a transceiver - it receives a packet, then waits a
> while before sending a packet. However, I can see some spikes in the
> preamble detection stage which seemingly correspond to the time around
> which the packet is transmitted. I know XCVR is not full duplex, so I'm
> *guessing* it's some kind of LO leakage, perhaps?
> 

Its just the the LO is directly in the center of the passband. I
recommend tuning with an LO offset:
http://files.ettus.com/uhd_docs/manual/html/general.html#tuning-notes

> On replacing the UHD driver with an old one (003.004.000), the issue seems
> to have disappeared.
> 
> Any ideas? Attached is a snapshot which shows the smaller spikes
> (corresponding to tx) next to the bigger ones (corresponds to the rx).
> 

This LO offset behavior was automatic for 003.004.002, however that was
causing more trouble than it was worth. For applications that tuned both
RX and TX. Because the setting of one side would stop on the other (due
to shared LO), and you would end up with incorrectly tuned CORDICs in
the DSP.

-josh

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