Graphs showing how a superheterodyne| radio receiver works.  All the graphs horizontal axis is frequency f.  The blue graphs represent signal strength (voltage) at various point in the circuit.  The red graphs are the transfer functions of the filters; the width of the red band represents the fraction of the signal from the graph above it that passes through the filter at various frequencies.  The top graph shows the voltage from the antenna applied to the receiver.  It is a composite of signals (blue shapes) from several radio transmitters at different frequencies. Signal S1 is the signal received by the radio.  Each signal consists of the carrier frequency (dark blue vertical line) with sidebands on either side containing the modulation (light blue bands).  The signal first passes through the RF filter.  Its purpose is to remove any signal such as S2 at the image frequency, fLO−fIF{\displaystyle f_{\text{LO}}-f_{\text{IF}}}, which would otherwise interfere with the received signal.  The 3rd graph shows the signals applied to the mixer, consisting of the local oscillator signal fLO{\displaystyle f_{\text{LO}}\,} and the four radio signals.  In the mixer the LO signal beats with the four signals, creating heterodynes at the difference between the signal frequency and the LO frequency. The 4th graph shows these heterodynes.  The IF filter (5th graph) selects the frequency of the desired signal, S1 and filters out the others, along with unwanted modulation products
Own work
Chetvorno