 The heterodyne technique is used for ( 1 ) shifting an input signal into a new frequency range as well as ( 2 ) amplifying a weak input signal ( assuming use of an active mixer ) . A weak input signal of frequency f1 is mixed with a strong reference frequency f2 from a local oscillator ( LO ) . The nonlinear combination of the input signals creates two new signals , one at the sum f1 + f2 of the two frequencies , and the other at the difference f1 − f2 . These new frequencies are called heterodynes . Typically only one of the new frequencies is desired , and the other signal is filtered out of the output of the mixer . The output signal will have an intensity proportional to the product of the amplitudes of the input signals . 
