[linux-dvb] Re: Spectral Inversion (was Re: Re: patch for scan (-t bitfield select))

["Robert Schlabbach" <robert_s@gmx.net>]

From: "Holger Waechtler" <holger@convergence.de>
> Robert Schlabbach wrote:
> > AFAIK, there is no such thing as "I/Q wires", only a (typically
> > differential) IF signal between the tuner and the demodulator.
>
> I and Q are the names of the two IF wires between tuner and the demod.
> It's the usual notation for complex numbers, the two signals specify
> the modulation point in the complex I/Q plane.

I beg to differ ;) You are correct about I and Q specifying the
constellation point, with I being the In-phase amplitude and Q being the
quadrature amplitude. But you cannot say one of the differential IF lines
carries the I signal and the other carries the Q signal. In fact, you could
only use one IF line to carry the signal, with both I and Q in it...

BTW, I wasn't quite right about the spectral inversion. Here is a good
quick explanation of what it is:

http://www.rfcafe.com/references/electrical/spectral_inv.htm

Basically, it means the signal spectrum is "mirrored" around the center
frequency: What was originally above the center frequency ends up below it,
and vice versa. And this effect happens when the LO (=PLL) frequency in a
downconversion is greater than the target frequency - a scenario you have
with typical tuners, where you add the IF center frequency to the target
frequency.

So to not have a spectral inversion in the tuner, you would have to have a
tuner in which you _subtract_ the IF center frequency from the desired
target frequency to get the PLL frequency. Do such tuner modules really
exist? If not, they *all* spectrally invert the signal...

Regards,
--
Robert Schlabbach
e-mail: robert_s@gmx.net
Berlin, Germany



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