User's Manual

FCC ID: MI7-ECSHPA80TX
A8
Sheet 3 of 6.
Transmit keying, over-temperature, and over-voltage protection circuitry.
The transmit keying circuitry is shown at the top of the page. To transmit, the IP8B Base
Station Data Transceiver grounds the KEY input. This input is gated with the normally
high over-temperature and over-voltage inputs to produce a logic low (0V) on the output
of gate U2D. This turns off transistor Q7, applying base current to transistors Q8, Q6,
and Q10. As a result, transistor Q8 turns on FETs Q4 and Q5, applying 13.6 volts DC to
the four power amplifier modules. FET Q4 powers two modules, while FET Q5 powers
the other two modules. At the same time, transistor Q6 turns on FET Q2, applying 5
volts to the 1 watt driver circuit. This makes it active. And finally, transistor Q10 turns
on FET Q9, applying power to adjustable voltage regulator U5. Regulator U5 develops a
low voltage DC source which biases the four power amplifier modules for Class AB
operation.
The over-temperature circuitry consists of temperature sensor U3 and transistor Q3. A
temperature setpoint of approximately 150
o
F is determined by resistor R22. In
operation, the OUT terminal of sensor U3 is normally logic high. This permits normal
operation of the HPA8 via gate U2B. Transistor Q3 is also turned on, producing a low on
the HTA (High Temperature Alarm) output. Should the cabinet temperature exceed the
150
o
F setpoint, the OUT terminal on U3 goes low. This shuts down the HPA8 via gate
U2B and turns off transistor Q3. As a result, the HTA output goes high. If an external
alarm monitoring device is connected to the HTA output, it will warn users there is a
fault with the HPA8. When the cabinet temperature cools slightly below the setpoint, the
OUT terminal goes high, and normal operation resumes.
The over-voltage circuitry consists of reference diode CR1 and op amp U4A. The 16.0
volt threshold is set by resistors R16/R21, and the 2.5 volt output of reference CR1. In
operation, U4A monitors the 2.5 volt reference and the divided-down power input
voltage. When the voltage at the junction of R16 and R21 exceeds 2.5 volts, the U4A
output goes low, turning on OVER VOLTAGE LED D2, and turning off the unit via gate
U2D. This condition occurs when the DC power input is approximately 16.0V. When
the voltage drops to less than 15.75 volts, the U4A output goes high, LED D2 turns off,
and the unit resumes operation. The 15.75 volt drop-out voltage is set by resistor R14.