On Sun, 01 Jun 2008 07:10:56 -0000 "cocokcocok" <[EMAIL PROTECTED]> wrote:
> What would be the advantage of having baseband complex I/Q signal feed > to sound card instead of having direct 12KHz IF signals which can be > digitally splitted to I/Q signals for further processing. > TenTec and Winradio with their old models using the later approach as > far as I know. The problem is the mirror image. One way is to use a filter that passes a bandwidth of something like 10 kHz at i.e. 455 kHz. That would mean that 455 is converted to 8 kHz while the entire passband would be from 3 to 13 kHz. The mirror frequency that creates a signal at the end point of the passband would be 6 kHz away and a reasonable filter could attenuate it by 80 dB. The filter would guarantee freedom from spurs caused by the mirror image. With direct conversion there would be no high Q filter at RF frequencies. One would feed two wideband audio signals into the soundcard and this would increase the useful bandwidth quite a lot. There would be a false signal at the image frequency. With 1% components one can expect the image level somewhere around -30 to -40 dB. Careful balancing with trimmers or by special software procedures is required to bring the image spurs to a reasonable level. The advantage with "direct conversion" giving I and Q is that the useful bandwidth becomes at least two times larger. Typically more depending on the shape factor of the filter in use. Here is a description of a system with the filter method that uses an extreme filter to get nearly the maximum possible bandwidth: http://www.sm5bsz.com/pcdsp/hware.htm and particularly the filter here: http://www.sm5bsz.com/pcdsp/pcif.htm As you can see it takes many crystals to get a usable bandwidth of 20 kHz when sampling at 44.1 kHz - but it can be done. The links show my first two-channel receiver in a PC computer. Nowadays I use I and Q with a four-channel soundcard. 73 Leif / SM5BSZ
