Re: [Discuss-gnuradio] Welcome and brief update

[John Turner]

The sampling rate of a system is determined by the information bandwidth not
necessarily the carrier frequency.

In the case of an FM modulated carrier, at 107.5MHz, all the information is
contained within a 300kHz bandwidth around the carrier, i.e. between
107.35MHz and 107.65MHz. You could sample the signal at >215MSPS and all the
information would be maintained.

However,

Since a "Mixer" allows two frequencies to be added or subtracted from one
another. If a signal of 107.349999 MHz is mixed with the modulated carrier
and then low pass filtered the resulting spectrum will contain all the
information of the original carrier and would reside between 0Hz and 300kHz.
The sampling rate necessary to process this bandwidth, conceptually, is
600ksps.

If the signal is converted to 10.7MHz, a 30MSPS converter again could be
used to sample the signal, however it is also possible to use a technique
known as IF sampling to extract the information at a much lower sampling
rate. Again, Analog devices has many application notes that describe the
process.

JT

----- Original Message -----
From: Chris Albertson <address@hidden>
To: Marshall White <address@hidden>; <address@hidden>
Sent: Saturday, January 11, 2003 9:32 PM
Subject: Re: [Discuss-gnuradio] Welcome and brief update


>
> 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
>   KG6OMK
>
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