MAJOR COMPONENTS

Figure 1-8.-C-11950/ARC-187 control box. Transmit Mode In the transmit mode, the VHF XMTR control signal from any one of the four ICS master control panels applies a ground to the transceiver as the VHF key signal. This VHF key signal provides the push-to-talk command to the transceiver to switch it from the receive to the transmit mode of operation. The synthesizer generates transmitter drive frequencies from 116.000 to 155.975 MHz in 25 kHz steps. Audio from any of the four ICS master control boxes are applied to the modulator circuit. The modulator circuit provides 90-percent amplitude modulation. The compressor and limiter circuits are included to prevent over-modulation. The transmitter uses five stages of amplifiers to raise the output to 20 watts. The RF output is routed to the antenna, through the filter, for radiation. The transceiver also produces a sidetone output, which is provided to the ICS system in the same manner as the receiver audio. UHF COMMUNICATIONS Learning Objective: Recognize components and operating principles of the UHF radio system. The AN/ARC-187 UHF radio system consists of the UHF-1 radio set, UHF-2 radio set, and the interfacing equipment. The UHF communication system provides amplitude modulation, frequency modulation, and frequency shift key (FSK) operations. The frequency range of this radio system is 225.000 to 399.975 MHz. MAJOR COMPONENTS Each radio set in the system consists of an RT-1571/ARC-187 receiver transmitter, a C-11950(V)3/ ARC-187 radio control, an AT-879/ARC antenna, and a 740645 antenna switching relay. The UHF-2 radio set also contains a 974924 antenna select panel. The antenna and the antenna switching relay will not be discussed in detail in this manual. RT-1571/ARC-187 Receiver Transmitter The RT-1571/ARC-187 receiver transmitter contains all the circuitry for the normal operation of the UHF radio set. C-11950/ARC-187 Radio Control Box The C-11950/ARC-187 control box (fig. 1-8) provides all the controls for normal operation of the radio set. We will be discussing each of the controls and 1-13