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Re: [Discuss-gnuradio] Carrier frequency mismatching?

From: Songsong Gee
Subject: Re: [Discuss-gnuradio] Carrier frequency mismatching?
Date: Fri, 19 Nov 2010 21:19:04 +0900

I understan what you told me...
Then... however, why they have same sign?
One is for up conversion, and the other is for down conversion
they might have opposite signs.
one is MINUS 4 MHz, and the other is PLUS 4 MHz

and one another...
They meet in frequency by DXC,
but, in my opinion, they are basically different in baseband frequency...
I really really cannot understand this magic :(

2010/11/19 Eric Blossom <address@hidden>
On Fri, Nov 19, 2010 at 05:17:53PM +0900, Songsong Gee wrote:
> I use USRP sink and source and set frequency with 2.6 GHz
> When I run a flow graph, I see like below:
> ==============================================
> A: Flex 2400 Tx MIMO B
>     r.baseband_frequency = 2604000000.0
>     r.dxc_frequency = -4000000.0
>     r.residual_frequency = 0.0
>     r.inverted = False
> A: Flex 2400 Rx MIMO B
>     r.baseband_frequency = 2596000000.0
>     r.dxc_frequency = -4000000.0
>     r.residual_frequency = 0.0
>     r.inverted = False
> ===============================================
> Both of baseband frequency are near 2.6 G, but they do not match up
> Can it be problmeatic for communicating each other?

The actual frequencies on the air will be 2.6 G +/- oscillator tolerance.

In the Tx case, the LO is set 4MHz higher than the target, and the
DUC is used to adjust the baseband -4MHz.

In the Rx case the LO is set 4MHz lower than the target, and the DDC
is used to move the digitized IF signal 4MHz lower prior to decimating.

This is easy to confirm with a siggen and a spectrum analyzer.


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