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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