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Re: choice of IF frequencies



I may be partially wrong. The direct conversion (zero-IF) design eliminates
the DC component of the baseband signals because the DC offsets in the phase
detectors are much larger than the received signal. I assumed that the DC
component of the signal needs to be preserved to demodulate FM.

The easy method to demodulate FM is:

         I * dQ/dt + Q * dI/dt
        ----------------------------
              I * I + Q * Q

This works whether or not the passband is centered on the carrier frequency
and the DC component can be used for AFC.

If there is always modulation present, I suppose that FM could be treated as
a form of PM and Costas loop used to phase lock the local oscillator to the
FM signal. d(arctan(I/Q))/dt then gives the rate of change of the phase of
the signal which would be the frequency offset.

This wouldn't work very well for FM voice as the loop would unlock as soon
as the speaker pauses. However, it may work fine for data as long as the
baud rate is high enough. At low rates, 1/f noise will compete with the
baseband signal.

73,

John
KD6OZH

----- Original Message -----
From: "laura halliday" <marsgal42@hotmail.com>
To: <amsat-bb@AMSAT.Org>
Sent: Monday, 11 December 2000 17:23 UTC
Subject: Re: [amsat-bb] choice of IF frequencies


> John Stephensen, KD6OZH <kd6ozh@gte.net> wrote:
>
> >Direct conversion radios work nicely for SSB, AM and PM.
> >The only problem for me is that they can't support FM
> >reception and almost all AMSAT digital satellites have
> >a FSK downlink.
>
> Hence the reference to *I/Q* baseband processing. The
> information to demodulate FM is there. You just need to
> use it, rather than throw it away.
>
> I'll worry about 30 dB carrier suppression when I start
> receiving satellite signals that are loud enough to hear
> a carrier that is 30 dB down. A little bit of carrier
> to lock a PLL on to is handy anyway. If it's good enough
> for NASA... :-)
>
> Sideband suppression is a non-issue with DSP. It is, in
> any case, also a non-issue with zero-IF/Weaver/3rd Method
> radios. Though for different reasons.
>
> Laura Halliday VE7LDH     "Que les nuages soient notre
> Grid: CN89mg               pied a terre..."
>                                    - Hospital/Shafte
>
>
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