MICROWAVE RECEIVER

MICROWAVE RECEIVER A typical microwave receiver is shown in figure 2-12. Though not shown, sensing and alarm functions are integral to all microwave communications equipment. During system operation, the signal from the an- tenna passes through a waveguide preselector that eliminates interference from adjacent rf channels. The signal then enters a waveguide filter tuned to its fre- quency, which rejects all other unwanted frequencies. Next, the signal passes through an isolator that minim- izes intermodulation noise and holds the VSWR below 1.2:1. The signal is then mixed with the local oscillator (LO) output to produce the standard 70-MHz inter- mediate frequency (IF). The IF output is amplitude- limited and applied to an automatic frequency control (afc) discriminator, which controls the frequency of the LO. The signal is also applied to an IF discrimina- tor, a de-emphasis circuit, and a squelch circuit that disconnects the baseband amplifier and demultiplex- ing equipment if noise increases above a preset level. After the squelch circuit, the signal passes through a baseband amplifier and then to the demultiplexing equipment, where the original intelligence is retrieved. Microwave communications systems operating in the shf portion of the frequency spectrum use the principle that propagation approaches an optical straight-line path. Propagation takes place in the lower atmosphere and is affected by meteorological factors. Communications in this medium are usually either line-of-sight or tropospheric scatter. LINE-OF-SIGHT (LOS) A line-of sight microwave system consists of one or more point-to-point hops. Each hop is designed to be integrated into a worldwide communications network. Los system characteristics are as follows: Propagation—Free space as affected by the tro- posphere. Communications Capacity/Bandwidth—Up to 600-4kHz voice channels; wideband, can accept TV. Range—Usually 50 to 150 km (31 to 95 statute miles). This depends upon antenna height, earth curva- ture, and intervening terrain. RF Power—Usually less than 10 watts. Antennas—Both transmitting and receiving antennas are horn-driven paraboloids, providing high gain and narrow beam widths. In some applications, plane reflectors are used with the paraboloids. Figure 2-12.—Typical microwave receiver. 2-12