Operating instructions

RF Technology PA501 Page 8

2.5 Power Control Circuits (Rev 05/9151)

The forward and reverse voltages from the directional coupler are amplified and

inverted by Q103 and Q101. The amplified voltages are compared to preset reference

levels and the comparison signals are combined in a logical fashion by Q109 and Q108

and used to produce the ALC signal. VR137 sets the maximum permitted reverse

power level.

The ALC signal is filtered and supplied to the rear panel system connector for

connection to the ALC control input of an exciter such as the model T500.

Equipment Type Key Specifications

Power Supply 13.8Vdc, (<25A for 120W)

RF Source 15 Watt source, 450-512MHz, e.g. T500

RF Load / Attenuator

30 dB Attenuator, 50Ω, 150W, SWR<1.2:1

RF Power Meter eg. HP437B or calibrated detector and voltmeter

Table 2: Standard test equipment for the PA501 Power Amplifier

2.6 RF Output Indicator

The forward power voltage is compared with the pre-set dc reference voltage from

VR136 by Q106, Q107. The output of Q113 is used to turn on the RFO LED. The

forward power voltage is buffered to Q116 to turn on the fans via Q6.

VR143 is normally set so that the RFO LED comes ON when output power is 1 - 3db

below the nominal power output level.

2.7 Over Temperature Protection

Thermistor RT144 is mounted to the case of output transistor Q1. If the transistor case

temperature rises above 90 Celsius (about 194

F) the resistance of RT144 increases

and Q117 is turned ON.

This causes the TEMP LED and the fans to come on and also reduces the dc output

voltage of the ALC signal via Q109. The input power will then be reduced by the

transmitter ALC circuits and the output transistor is kept within safe operating limits.