Re: [Discuss-gnuradio] Carrier frequency mismatching?

[Eric Blossom]

On Fri, Nov 19, 2010 at 09:19:04PM +0900, Songsong Gee wrote:
> 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 :(

Try drawing a picture...

Eric


> 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.
> >
> > Eric