Post date: Oct 26, 2014 3:28:31 PM
Looking through my past project list, I found one of the most interesting project that I've ever done: building a double conversion, superheterodyne RF receiver. The fancy words captured my attention, but at first, I was neither able to recall the detailed operations of the receiver, nor make sense of its Hellenic name as I used to. So this blog is a review of the subject.Â
Firstly, the Greek name, superheterodyne, can be broken down into two parts, according to [1]: super, which is from supersonic, referring to RF signals, and heterodyne, which is (hetero)different-(dyne)power. The later is kind of a misnomer, since it actually refers to a modulation/demodulation technique commonly used in telecommunication with local oscillators/mixers. The technique itself is based on a simple, yet beautiful mathematical truth: the Euler formula of cosine. The implication of the formula is the following simple engineering rule
, if (low side injection)
, if (high side injection)
where is the intermediate frequency, which is usually lower than the RF frequency , to make the final demodulation easy, and is the oscillator frequency. Another implication is a natural explanation of image frequencies, which is the other RF frequency (think "The Other Woman" comedy :) ) which can interfere at IF frequency given a mixing/oscilation frequency
, as follows.
, if (low side injection)
, if (high side injection)