Re: [Discuss-gnuradio] Intermediate frequency question

[Marcus D. Leech]

On 10/05/2011 07:11 PM, Nick Foster wrote:
> By "no IF frequency" he meant the LO frequency is the same as the RF
> frequency. This is called a zero-IF receiver, or direct conversion.
>
> http://en.wikipedia.org/wiki/Direct-conversion_receiver
>
> --n
More precisely, and completely, they use a direct-conversion, quadrature 
(I + Q) signal format.

Without the I+Q bits, then the the two side-bands created from mixing 
the RF with LO==RF would cause
  them to "fold about each other".  In the very, very, early days of 
direct-conversion designs (way back near
  the start of the 20th century), the fact that the two side-bands 
overlapped wasn't an issue, because the modulation
  mode was typically AM, which "doesn't care" about such things.  But 
for anything else, you need to use
  a complex representation, in order to distinguish  (-bandwidth/2-DC) 
from (DC-bandwidth/2).

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