Re: [Discuss-gnuradio] Real-only direct-conversion

On 02/27/2011 06:16 PM, Moeller wrote:
>
> As long as you receive the complete signal bandwidth, you can create the I/Q form later.
> Of course you need the double sample rate, if there's only the real "baseband"
> representation. I call it baseband, but you can also call it IF with the lowest
> possible IF frequency. Strictly speaking it's not a "direct-conversion" receiver,
> since there is a fixed IF in the middle of the baseband spectrum.
> The "data rate" is the same for both, one has double sample rate but only
> half the sample size (real vs. complex numbers).
>
> Complex baseband (I/Q) reconstruction:
> - Hilbert transform eleminates the (symmetric) negative frequencies
> - Frequency shifting the IF frequency to zero by multiplying a complex exp(-j*2*pi*f_IF*t)
>
> This is standard in digital down converters (DDC).
> The TVRX-Board is working this way. According to the schematic,
> only the bipolar A channel is connected to the tuner chip, a real input.
> Other Dboards use both A/B inputs for separate I/Q channels.
>
> I think both variants have their advantages and disadvantages.
>
Right, for a non-zero IF, it's easy to see how to do the Hilbert
transform and convert to I+Q, provided the
sample rates are correct.

But for a zero-IF, direct-conversion, with only a single baseband output
(single mixer), I don't see how you
can make it work.

--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

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