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Frequency Modulated Receiver
SINGLE-SIDEBAND

Neets Module 17-Radio-Frequency Communications Principles
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2-13 combines the incoming rf with the local oscillator signal to produce the intermediate frequency, which is then amplified by one or more IF amplifier stages. You should note that the fm receiver has a wide-band IF amplifier. The bandwidth for any type of modulation must be wide enough to receive and pass all the side-frequency components of the modulated signal without distortion. The IF amplifier in an fm receiver must have a broader bandpass than an AM receiver. Figure 2-10.—Block diagram of an fm receiver and waveforms. Sidebands created by fm differ from the AM system. You should recall that the AM system consists of a single set of side frequencies for each radio-frequency signal modulated. An fm signal inherently occupies a wider bandwidth than AM because the number of extra sidebands that occur in an fm transmission is directly related to the amplitude and frequency of the audio signal. You should observe that only two fundamental sections of the fm receiver are electrically different from the AM receiver. These are the discriminator (detector) and the limiter. Beyond the IF stage, the two receivers have a marked difference. AM demodulation involves the detection of variations in the amplitude of the signal; fm demodulation is the process of detecting variations in the frequency of the signal. In fm receivers a DISCRIMINATOR is a circuit designed to respond to frequency shift variations. A discriminator is preceded by a LIMITER circuit, which limits all signals to the same amplitude level to minimize noise interference. The audio frequency component is then extracted by the discriminator, amplified in the af amplifier, and used to drive the speaker. ADVANTAGES.—In normal reception, fm signals are almost totally absent of static while AM signals are subject to cracking noises and whistles. Fm followed AM in development and has the advantage of operating at a higher frequency where a greater amount of frequencies are available. Fm signals provide much more realistic sound reproduction because of an increase in the number of sidebands. This increase in the number of sidebands allows more of the original audio signal to be transmitted and, therefore, a greater range of frequencies for you to hear. As you can see, fm requires a wide bandpass to transmit signals. Each transmitting station must be assigned a wide band in the fm frequency spectrum. During fm transmissions, the number of significant






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