The sidebands produced are the images or aliases. In the i rst case we have fs 1 fm,
or 50 1 70 5 120 MHz and fs 2 fm, or 50 2 70 5 220 MHz. The difference is a negative
frequency, which as it turns out is still a valid signal of 20 MHz. It is this difference
signal that is the alias we are interested in. Using this process with the upper- and lowerband
signals of 60 and 80 MHz produces a spectrum that extends from 10 MHz to
30 MHz, still a bandwidth of 20 MHz. What we have done is translated the 70 MHz
signal down to 20 MHz while retaining all the sidebands and information within the
20 MHz bandwidth. A low-pass i lter is used to remove all of the higher-frequency harmonic
signals and their aliases.