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Re: [Discuss-gnuradio] XCVR 2450 spurs

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] XCVR 2450 spurs
Date: Tue, 08 Dec 2009 18:49:09 -0800
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20090922 Fedora/3.0-3.9.b4.fc12 Thunderbird/3.0b4

On 12/08/2009 02:47 AM, hanwen wrote:
Hi *,

I'd like to measure the out-band spurious generated by XCVR2450.
There is a spreadsheet which can be used for calculating the location of
mixer harmonics.
And what I need is the IF and lo frequency.
I guess XCVR2450 use a zero-IF scheme, but why there is a small DUC
frequency about 254Hz?
I'm totally a layman of RF and transceiver design.
Thanks in advance!

The XCVR2450 uses direct conversion. The LO is generated by a fractional-N frequency synthesizer which has very fine resolution. However, it does not have infinite resolution, so it may only be able to get within a couple of kilohertz, and thus there would be some frequency offset. The DDC or DUC is used to correct that offset, and that is why you are seeing 254 Hz. If you choose a different frequency, you will see a slightly different offset.

In most cases you don't care about that residual, so you could change the software to assume the LO is exact, and that would give you a 0 Hz DUC frequency.


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