电子信息类专业英语Unit_16_Superheterodyne_Radio_Receiver教材教学课件.pptVIP

;Text　　In a broadcasting system, irrespective of whether it is based on amplitude modulation or frequency modulation, the receiver not only has the task of demodulating the incoming modulated signal, but it is also required to perform some other system functions:　　(1) ?Carrier-frequency tuning, the purpose of which is to select the desired signal(i.e., desired radio or TV station).　　(2) ?Filtering, which is required to separate the desired signal from other modulated signals that may be picked up along the way.;　　(3) ?Amplification, which is intended to compensate for the loss of signal power incurred in the course of transmission.　　The superheterodyne receiver, or superhet as it is often referred to, is a special type of receiver that fulfills all three functions, particularly the first two, in an elegant and practical fashion. Specifically, it overcomes the difficulty of having to build a tunable highly selective and variable filter. Indeed, practically all radio and TV receivers now being made are of the superheterodyne type.;　　Basically, the receiver consists of a radio-frequency (RF) section, a mixer and local oscillator, an intermediate-frequency (IF) section, demodulator, and power amplifier. Typical frequency parameters of commercial AM and FM radio receivers are listed in Table16.1. Figure16-1 shows the block diagram of a superheterodyne receiver for amplitude modulation using an envelope detector for demodulation.;;;　　The incoming amplitude-modulated wave is picked up by the receiving antenna and amplified in the RF section that is tuned to the carrier frequency of the incoming wave. The combination of mixer and local oscillator (of adjustable frequency) provides a heterodyning function, whereby the incoming signal is converted to a predetermined fixed intermediate frequency, usually lower than the incoming carrier frequency. This frequency translation is achieved without disturbing the relation of the sidebands to the carrier. The result of the hetero