Re: [Discuss-gnuradio] A question on wfm_rcv_gui.py

[Sachi]

Thanks Eric, so the usrp accepts negative
IF¡¡frequency?


Sachi

--- Eric Blossom <address@hidden> wrote:
> On Sun, Mar 20, 2005 at 04:47:11PM -0800, Sachi
> wrote:
> > I still can't figure it out. Can anyone of you
> guys
> > help me with it?
> > 
> > > Hi guys
> > > 
> > > I got a bit confused by the code for FM
> demodulator.
> > > In function parseargs(), why we return freq1 -
> 128e6
> > > instead of freq1 itself? What does 128M stand
> for?
> > > 
> > > Thanks
> > > 
> > > Sachi
> 
> Let me take a shot at it.  Matt wrote it, but I
> think he's off-line
> for the weekend.
> 
> wfm_rcv_gui was written assuming that there was no
> RF front end doing
> any downconversion.
> 
> The A/D sample rate is 64M.
> 
> The Nyquist zones are therefore:
> 
> [0, 32M]    normal
> [32M, 64M]  inverted
> [64M, 96M]  normal
> [96M, 128M] inverted
> 
> etc.
> 
> The problem is that 96M, one of the folding points,
> occurs right in
> the middle of the broadcast band.
> 
> This means that 95.0 and 97.0 both alias down to the
> same frequency
> and can't be distinguished from each other.
> 
> freq1 - 128M will give a valid frequency if freq1 is
> >= 96M.
> 
> Bottom line: it's a kludge that sort of works.
> 
> Eric
> 


                
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