superheterodyne receiver
The common type of AM, FM and TV receiver, which uses intermediate frequency (IF) stages. Rather than demodulating the actual carrier frequency of the transmitting station, which was the approach taken in the early days of radio, "superhet" receivers shift the desired frequency to a single frequency that the receiver can handle very efficiently. The intermediate frequency is then demodulated using direct conversion or homodyne techniques. See IF stage, direct conversion receiver and homodyne receiver.
An AM Radio Example
The carrier frequencies for AM operate from 530 kHz to 1610 kHz. Many superheterodyne AM radios use a demodulation circuit designed for 455 kHz. When the listener tunes in a station, an oscillator generates a signal 455 kHz less than the frequency of the desired station. For example, tuning in an 800 kHz station would generate a 345 kHz signal (800-455=345) that would be subtracted from the incoming signal (800-345=455). A bandpass filter then allows only the 455 kHz signal to go to the demodulator circuit to recover the audio. The following FM radio example illustrates this technique.
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| Bandshifting to 10.7 MHz IF |
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| In this FM example, the radio is tuned to 101.5 MHz (FM is 88-108 MHz). The intermediate frequency (IF) is 10.7 MHz, and the incoming signals are bandshifted into that frequency, plus and minus 50 kHz for the audio signal and subcarriers. The 1 MHz crystal frequency is an arbitrary reference signal for ensuring the accuracy of the local oscillator. See PLL. |