Graphs showing how a bandpass filter in a radio receiver selects the desired radio signal from all the radio signals received by the antenna.  The center graph shows the voltage from the antenna Vant as a function of frequency. f.  The antenna typically receives many radio signals at different frequencies so the antenna voltage is a composite of different radio signals; here 4 are shown: S1, S2, S3, and S4.  Each radio signal consists of a carrier wave (C) at the center frequency, with the information bearing modulation contained in two sidebands (SB) on either side of the carrier.  The antenna also picks up radio noise (N) from the atmosphere. 
To receive a single radio station, the current from the antenna is passed through a bandpass filter consisting of several tuned circuits. The filter passes the frequency of the desired station, S1, and blocks signals at all other frequencies. The transfer function (TF) of the filter is shown in red in the top graph.  The passband (PB) of the filter is centered on the desired station S1.  The bandwidth (BW) of the passband must be wide enough to pass the sidebands without distortion, but narrow enough to reject interfering radio signals on adjacent frequencies, such as S2.  The output of the filter Vfilter is shown at bottom.  This signal is amplified, then the modulation is extracted from the radio frequency carrier wave by a demodulator circuit, and applied to a speaker.
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