Zeta SDR is a simplest SDR using analog mux/demux used as mixer to generate I and Q signals by mixing with oscillator signal of 45 degree phase shift, the IF signal is then passed through low pass filter. The circuit is from the web page http://www.qrz.lt/ly1gp/SDR/.
The basic block diagram is shown below.
The Crystal oscillator frequency should be 4X the required frequency to be received for example 40m band (7-7.2MHz) the crystal between the ranges 28-28.8MHz can be chosen. The 45 degree shift can be achieved digitally by using D-FF as shown.
It requires a sound card of stereo input on mic or line in. Sound card with larger bandwidth can give large spectrum. Usual soundcards have 40 KHz bandwidth. The software i used is SDR# which can be downloaded from here.