Block diagram of a double-conversion superheterodyne receiver a more sophisticated version of the superheterodyne receiver invented in 1918 by Edwin Armstrong and used in many modern radio receivers.  In order to achieve both good adjacent channel selectivity and image rejection, the double-conversion receiver uses two intermediate frequencies (IFs).  The incoming radio frequency (RF) signal from the antenna is first mixed with a sinusoidal signal from the 1st local oscillator (LO) to give a high 1st IF frequency.  Since the separation between the RF and the image frequency is twice the 1st IF, this high IF results in wide separation between the RF and the image, allowing the image filter in the RF amplifier to reject the image.  Then this 1st IF is mixed with a 2nd local oscillator frequency to give a lower 2nd IF.  This lower IF allows the 2nd IF filter to have a narrow bandwidth, giving the receiver good adjacent channel selectivity.  Then the 2nd IF is demodulated to extract the modulation from the radio frequency signal, and the audio is amplified and applied to the speaker
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