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Re: [Discuss-gnuradio] Welcome and brief update

From: Chris Albertson
Subject: Re: [Discuss-gnuradio] Welcome and brief update
Date: Sat, 11 Jan 2003 21:32:25 -0800 (PST)

OK, basic radio theory.

The "tunner" is really a kind of filter that letts the 107.5Mhz
signal pass.  It don't move the signal.

THe next stage is a "mixer".  Here the 107.5Mhz signal
get mixed with, lets say a 50Mhz sin wave signal.  THe result
is a signal with frequency compnents at both 107.5+50 and
at 107.5-50  Mix that signal in another mixer with (say)
40Mhz and then apply filters and you have a 10.7Mhz signal
that you can sample at 30Msps.  In a real system the numbers
would be all diferent but the result the same, you "mix and
filter" the desired radio freq. down to a standardized "IF"
freq. and then sample it.  
It works because the content being carried by the 107.5 carier is
maybe just voice or NTSC video. 

--- Marshall White <address@hidden> wrote:
> --- Daniel Rogers wrote:
> > actually, it seems that I choose 40Msps from a lack of
> understanding of 
> > the amount of samples/Hz of radio frequency needed for correct
> digitization.
> > 
> > That said, how is it that a 400 kHz sample rate is sufficient for a
> > 107.5 MHz FM modulated signal?  How is it that you can extract
> useful 
> > information when you have much muhc less that one sample per cycle.
> > 
> > And given all this information, what is the max frequency you could
> > demodulate at 40Msps?
> > 
> > --
> > Dan
> > 
> Dan,
> The answer is that you must have a "tuner" that takes the signal at
> 107.5 MHz and "moves" it down
> to a much lower frequency (called an IF, or intermediate frequency).
> If you have a 40 MHz sampling rate, any IF less than 20 MHz should
> more or less be okay.
> If you had a 400 kHz sampling rate, then your IF would have to be
> CENTERED at 100 kHz.
> Marshall
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> http://mail.gnu.org/mailman/listinfo/discuss-gnuradio

Chris Albertson
  Home:   310-376-1029  address@hidden
  Cell:   310-990-7550
  Office: 310-336-5189  address@hidden

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