User's Manual

FCC ID: MI7-ECSHPA80TX
A7
4. Theory of Operation
4.1 Thermal Equilibration System
“Equilibration” refers to the act of equalizing the operating temperatures of the four
power amplifier modules. By doing so, and by keeping the temperature within reason-
able limits, heat stress on the modules is reduced. This results in longer operating life for
the modules, and increased overall reliability for the HPA8.
Thermal equilibration is accomplished by: 1) attaching the power amplifier modules to a
thick aluminum substrate (heatsink) with heat conducting grease; 2) bonding over-sized
heat radiating fins into the substrate for greater heat transfer; 3) providing a thermo-
statically controlled fan which keeps the heatsink temperature within limits.
4.2 RF Amplifier Section
The HPA8 High Power Amplifier incorporates all circuitry on a single printed circuit
board. Please refer to the schematic diagrams in Section 8 for details while reading about
the circuitry.
Sheet 1 of 6.
Block diagram. This sheet illustrates five functional circuit blocks in the
unit. Sheet 2, Pwr and Gnd, provides regulated power for the logic and fan control
circuitry. Sheet 3 contains transmit keying, over-temperature protection, and over-
voltage protection circuitry. Sheet 4 offers the RF input, driver, and power divider
circuitry. Sheet 5, Pwr Amp Modules, and Sheet 6, Pwr Combiner, are self-explanatory.
Sheet 2 of 6.
Voltage regulators and fan control circuitry. Regulator VR1 provides a 5
volt DC output for all logic circuitry on the board. Regulator VR2 provides an 8 volt DC
output for the 1 watt driver amplifier on sheet 4.
The large fan mounted on the heatsink is controlled by the remaining circuitry. When the
cabinet temperature exceeds approximately 110
o
F, temperature sensor U1 makes the
OUT pin low. This turns on FET Q1, which applies power to the fan via the FP output.
The temperature setpoint is determined by resistor R2.