This experimental project developed by ChaN. He managed to analyze Radiosignal spectrum and calculate its FFT which is displayed on graphical LCD. When analysing RF frequencies there are some problems with speed. Even when analysing 1MHz radio signal there is over 4000 samples needed for correct ADC.
To solve this problem he managed to shift carrier frequency to lover by using generic method: Frequency conversion, which operates with signals as complex, where operations with negative frequencies are available. So if signal is 100MHz with span ±1MHz, then frequency is shifted to 0MHz±1MHz. This way by Nyquist they there is enough to sample signal at 2MHz. In circuit diagram (PCB (top) and PCB (bottom) )there is seen how this conversion is performed with couple mixer circuits SA612A and reference frequency of 455kHz.