Figure 1-6-RT-1397/ARC-197 transceiver.In the P-3C aircraft, this system interfaces with four ofthe intercommunication stations in the transmit/receivefunctions. The pilot, copilot, TACCO, and NAV/COMMstations can transmit and receive over this radio. Theother stations in the aircrafl have receive function only.MAJOR COMPONENTSThere are three components to the AN/ARC-197system. These components are the RT-1397/ARC-197transceiver, the C-11067/ARC-197 VHF-AM controlpanel, and the 949880 VHF antenna.RT-1397/ARC-197 TransceiverThe RT-1397/ARC-197 transceiver (fig. 1-6) is asolid-state unit, consisting of a power supply, frequencysynthesizer, receiver modulator, and transmitter. Thereis one indicator, one push button, one microphone jack,and one headphone jack on the unit. The indicator islabeled TRANSMIT POWER, which illuminates whenoutput power is greater than 10 watts. The push buttonis labeled SQUELCH DISABLE, which will disable thesquelch for low signal levels. The microphone andheadphone jacks are used for maintenance andemergency VHF communication in case of ICS failurein-flight.C-11067/ARC-197 VHF-AM Control BoxThe control box (fig. 1-7) controls the operation ofthe system. There are two dual function knobs and adisplay window on the control panel. The displaywindow shows the selected frequency of the system.Figure 1-7.-VHF-AM control box.The outer ring of the dual function knob on the leftapplies system power and selects the test function. Theinner knob changes the frequency of operation in1-MHz steps over the range of control. The outer ringof the dual function switch on the right is labeled VOL,and it is not used in the P-3 aircraft. Volume is controlledby the ICS system. The inner knob of this control is usedto change the frequency of operation in 25-kHz stepsover the range of control.949880 VHF AntennaThe 949880 antenna is located in the tailcap on topof the vertical stabilizer of the P-3 aircraft. This antennaradiates and receives the VHF radio frequency signals.Signals routed to and from the antenna go through aVHF bandpass filter, which reduces the crosstalkbetween the VHF and UHF systems.VHF FUNCTIONAL DESCRIPTIONThere are two modes of operation with theAN/ARC- 197 radio. These two modes are receive andtransmit.Receive ModeIn the receive mode, the received RF signals fromthe antenna are routed through the filter, and applied tothe receiver circuits in the transceiver. The frequencyselected on the control box is applied to the frequencysynthesizer. The synthesizer uses a single phase-lockedloop to generate RF injection frequencies, in 25-kHzsteps, from 116.000 to 155.975 MHz. The RF injectionfrequencies, along with dc tuning voltages,electronically tune the receiver to the selectedfrequency. The AM detected audio is applied to theaudio amplifier circuit. Squelch circuits disable theoutput amplifier if the required signal-to-noise ratio orcarrier level is not present. The output audio is thenapplied to the ICS interconnection box for distributionto the various stations.1-12
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