RADIO FREQUENCY AMPLIFIER

3-6 The transmitter unit provides an upper sideband (usb), lower sideband (lsb), independent sideband (isb), cw, fsk, or compatible AM signal. The output of the transmitter has enough power to drive the radio frequency amplifier. Depending on the model, the transmitter tunes across the frequency range in 100- or 500-hertz increments. Digital circuitry is used to accomplish this process. Transmitter outputs are also applied to the rf amplifier to automatically tune it to the correct frequency. We will go through a detailed breakdown of the transmitter unit later in this chapter. RADIO FREQUENCY AMPLIFIER.—The rf amplifier unit is a two-stage linear power amplifier that produces an output of 1,000 watts with a nominal input of 100 milliwatts. Nineteen frequency bands are used to cover the operating frequency range. The operating band is automatically selected by digital coding generated by the transmitter. The code controls two motor-driven band switch assemblies. Automatic control circuits protect the unit against overload and compensate for variations in system gain, mode of operation, and loading. All low voltages required for operation (except two of the relay control voltages) are internally produced. The high voltages required in the amplifier stages are produced by the associated power supply (when using 60 hertz primary power) or the optional internally mounted power supply (when using 400 hertz primary power). Let's take a look at figure 3-6 to see all the operating controls and indicators located on the front panel. Some controls are used only for initial setup and are protected by a hinged access cover. All connections are made at the rear of the case. The amplifiers and the associated interstage broadband transformer assemblies are cooled by forced ventilation. Cooling air is drawn through a filter on the front panel and exhausted through a port on the rear of the case. You should always take particular care to clean or replace any filter in electronic equipment as a regular part of your preventive maintenance program.