Hello Martin, > I am a french student with little DSP knowledge working on a school > project and I read the seiries of article from Gerald Youngblood > . > . > and I don't > understand clearly how can quadrature mixers allow to solve the issue of > the image-frequency superposition and to offset the baseband IF to 11kHz > rather than to 0Hz?
The quadrature mixer is just two ordinary mixers. They have a special relation though and that is that RF and/or LO are phase shifted for the output to be phase shifted by 90 degrees between the two mixers. Each mixer will give an output from 0 to some rather high frequency. Gerald has arbitrarily choosen to filter out the signal at 11 kHz. This implicates that the LO has to be 11 kHz above or below the RF frequency. With two mixers there will be two signals at 11 kHz. They are phase shifted by 90 degrees. Let us call the two signals I and Q. The important thing to know is that the phase shift between I and Q changes sign depending on whether the RF signal is above or below the RF frequency. Therefore, by shifting the phase of Q by 90 degrees one will have I and Q in phase on one side of the LO while I and Q will be out of phase on the other side. Just by taking the sum of I and the 90 degree shifted Q one will add two signals with the same amplitude and the same sign for one sideband but one will add the same amplitude with the opposide sign for the other sideband (and not get anything at all because they cancel.) Here is some text on the subject: http://www.sm5bsz.com/pcdsp/cmplmix.htm it is part of an introduction to eadio receivers in general: http://www.sm5bsz.com/pcdsp/genradio.htm Maybe you will find this page and the links it leads to of interest. Regards Leif / SM5BSZ
