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  3. There are three basic sections which make up a receiver: the tuner or radio signal receiving section, the preamplifier control s

There are three basic sections which make up a receiver: the tuner or radio signal receiving section, the preamplifier control s

admin2010-06-18  69
问题    There are three basic sections which make up a receiver: the tuner or radio signal receiving section, the preamplifier control section, and the power amplifier section. In very elaborate hi fi systems these three sections actually may be purchased as separate units, for increased flexibility and control or as a means of assembling the system in stages using the "building block" approach.
   The Tuner Section
   The tuner section may be equipped to receive only FM signals or both AM and FM signals. Since it is possible to broadcast stereo (two channel) sound via FM, most hi fi receivers have circuitry for decoding the complex composite received signal into separate "left and right" channel signals.
   The front end of the tuner section selects the particular signal desired from the hundreds of radio signals present in the atmosphere. As you tune the dial, resonant or tuned circuits are changed so as to be responsive to a single frequency.  In the case of FM signals, tuning is adjustable from 88 MHz (millions of alternations/second of the radio wave) to 108 MHz. Since each FM station occupies a space of 0.2 MHz, that means a theoretical maximum of 100 stations is possible in one geographical area. Actually fewer stations are assigned in any one area, so that one station’ s signal will not cause audible interference with an adjacent station. In the case of AM, stations broadcast at frequencies ranging from 540 kHz to 1605 kHz (1 kHz = 1000 alternations per second) and selection of desired stations is similar to the technique used in FM.
   The signal is amplified by the front end and mixed with a locally generated signal in the receiver to produce a different frequency known as the IF signal (Intermediate Frequency). In the case of AM, the IF frequency is generally 455 kHz. In the case of FM radio, the IF frequency is 10.7 MHz. The process is known as super heterodyne and since the local signal and received signal are always a fixed amount apart, the succeeding IF amplifiers need not be variably tuned but can be designed simply to amplify 455 kHz (for AM) or 10.7 MHz (for FM).
   Difference Between FM and AM
   In AM radio the amplitude of the radio wave is varied in accordance with the audio information to be broadcast. Hence the name Amplitude Modulation. In FM it is the radio frequency that is varied based on audio signals. It is this feature that makes FM relatively noise free since noise or static is an amplitude phenomenon. If noise accompanies the FM signals the noise can be sliced off by circuits called limiters without impairing the frequency-changing nature of the FM signal.
   While the circuits themselves differ, the amplified IF signals are then applied to a detector circuit which strips off the original audio information.  In FM, the detector is called a radio detector. The output of either the AM detector or the FM detector is an audio signal suitable for application to and further processing by the "preamplifier control" section o{ the receiver.
   In the case o{ stereo FM the recovered audio signal must be further processed or "unscrambled" to recover the separate "left" and "right" audio channels. This is accomplished by the multiplex decoder circuits.
Which feature makes FM radio relatively noise free?
选项 A、Its radio frequency is varied.
B、It is static free.
C、It has "preamplifier control" section.
D、It has a detector circuit.
答案A
解析
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