It may not be the most eye-catching of names, but the superheterodyne receiver vastly improved radio technology in the early twentieth century by filtering out unwanted radio signals.
The "superhet," as it is commonly called, is the invention of Edwin Howard Armstrong (1890-1954). While working on ways to amplify radio signals during World War I, he hit upon a cunning idea. An incoming radio signal is mixed with an internal signal generated by the receiver (known as a local oscillator). The two signals are multiplied together. This creates two new signals: One is the sum of the two originals, and one is the difference. So, if you have an incoming radio frequency at 5 MHz and a local oscillator at 4 MHz, signals will be seen at 9 MHz and 1 MHz, This is known as heterodyning. A filter can be used to remove any signal that does not fit a narrow frequency range around the difference signal (here 1 MHz). The beauty of the system is that precise tuning can be achieved by varying the frequency of the local oscillator. For example, if you have a filter fixed at 1 MHz, and radio stations broadcasting at 5, 6, and 7 MHz, the three stations can be "tuned in" by turning the dial—and therefore the local oscillator—to 4, 5, and 6 MHz respectively.
Armstrong patented the technique in 1918, and it was quickly adopted by the military. The superhet principle was later used to tune TV and radio broadcasts and is still widely used today. Armstrong went on to invent FM radio, but he also spent many years fighting patent lawsuits and eventually committed suicide.
