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Monday, June 20, 2011

The Superheterodyne AM Receiver

A block diagram of a superheterodyne AM receiver is shown in figure 5.12. The receiver shown consists of an antenna, an RF (radio frequency) amplifier, a mixer, a local oscillator (LO), an IF (intermediate frequency) amplifier, a detector, an audio amplifier, a power amplifier, and a speaker.

A block diagram of a superheterodyne AM receiver


    The antenna picks up all radiated signals and feeds them into the RF amplifier. These signals are very small (usually only a few microvolts).

    RF Amplifier                                                   
     This circuit can be adjusted (tuned) to select and amplify any carrier frequency within the AM broadcast band. Only the selected frequency and its two side bands pass through the amplifier. (Some AM receivers don't have a separate RF amplifier stage.)

    Local Oscillator                                              
   This circuit generates a steady sine wave at a frequency 455 KHz above the selected RF frequency.

    This circuit accepts two inputs, the amplitude modulated RF signal from the output of the RF amplifier (or the antenna when there is no RF amplifier) and the sinusoidal output of the local oscillator (LO). These two signals are then "mixed" by a nonlinear process called heterodyning to produce sum and difference frequencies. For example, if the RF carrier has a frequency of 1000 KHz, the LO frequency is 1455 KHz and the sum and difference frequencies out of the mixer are 2455 KHz and 455 KHz, respectively. The difference frequency is always 455 KHz no matter what the RF carrier frequency.

    IF Amplifier                                                     
    The input to the If amplifier is the 455 KHz AM signal, a replica of the original AM carrier signal except that the frequency has been lowered to 455 KHz, The IF amplifier significantly increases the level of this signal.

   This circuit recovers the modulating signal (audio signal) from the 455 KHz IF. At this point the IF is no longer needed, so the output of the detector consists of only the audio signal.

    Audio and Power Amplifiers                         
   This circuit amplifies the detected audio signal and drives the speaker to produce sound.

Signal flow through an AM receiver

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