Thanks Leif, for the explanation. It does make sense. Is it fair to say that 12KHz IF version will not suffer any I/Q imbalance (phase, amplitude and DC offset)as it is created and manipulated in digital domain while the I/Q complex base band produced by analog circuit (component tolerances.
It is still difficult for me to visualise why "direct conversion" would have twice the bandwidth. Regards, --- In [email protected], Leif Asbrink <[EMAIL PROTECTED]> wrote: > > 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 >
