M/A-Com 19D902797G5 Maintenance Manual - page 3
PA Output Power Set
PA output power is set according to the level of the Power
Set line. Four (4) volts on this line will produce minimum
power. As the voltage increases toward eight (8) volts, the
power will increase to the maximum rated output. The PA
output power is initially set for an output of 110 watts at J104.
This is done by adjusting R204 while injecting a 10 mW
signal at J1 and applying 8 volts to J201-3. After setting the
maximum power level, changing the output power is done by
varying the voltage applied on the Power Set line.
PA Protection
The power control also protects the PA against over tem-
perature and high VSWR conditions.
An over temperature condition exists when the flange
temperature of the final output transistor reaches 80
°
C. At
this point the output power will drop below its set level. The
output power will continue to drop such that when the flange
temperature reaches 125
°
C the PA output drops at least 10 dB
below its set level.
Reflected power is limited to 25% of the set power. If the
output VSWR degrades to worse than 3:1 the forward power
will be reduced to limit the reflected power to 25% of the set
power. The Power Sensor line indicates when the PA is
operating in a cutback condition. If the PA is keyed and the
power control is cutting back, the Power Sensor line will drop
to zero (0) volts and the PA alarm light on the station will turn
on.
Power Monitor
A DC voltage proportional to forward power, provided by
the detection circuit of the Power Sense stage, is buffered and
delivered to the PWR Monitor line of the interface connector.
Theory Of Operation
Power control of the MASTR III Power Amplifier is
accomplished with a feedback control loop. The three possi-
ble feedback signals are: representation of forward power,
temperature sensitive scaled representation of forward power,
or representation of reflected power. The three signals are
input to a diode summing junction which selects the largest
of the three for use as the feedback.
The microstrip directional coupler samples the output
power and produces a voltage, Vf, proportional to the forward
output power. The power control compares the forward volt-
age, Vf, to a reference voltage at U3. The output of U3 adjusts
the control voltage at pin 5 of the MMIC of the Small Signal
Gain Stage. This varies the gain through the stage, and
controls the power output level of the Power Amplifier As-
sembly.
During over temperature operation, a scaled repre-
sentation of the forward power is maintained constant by
varying the control voltage line. Thermal resistor RT1 sens-
ing an increase in temperature causes the output of U1.1 to
increase. If the output of U1.1 becomes larger than the other
feedback lines, the output of U3.2 will begin to decrease.
This causes the gain of U101 to decrease. Since the scaling
is a function of temperature the power is reduced as the
temperature increases.
Under VSWR cutback operation the reverse voltage, Vr,
representative of the reflected output power is held below a
threshold by reducing the control voltage as necessary. If Vr
increases at U1.2 beyond the preset threshold an increase at
U3.2 will result. This causes a subsequent reduction in the
control voltage to U1. Thus the power control circuit reduces
the output power in order to limit the reflected power to 25%
of the set power.
Signal Interface
The signal interface to the MASTR III Power Amplifier
is supported by a six position feedthrough connector, J201,
with the following pinout:
1 - PWR Sensor
2 -
PA Key
3 -
PA PWR Set
4 -
PWR Monitor
5 - Ground
6 -
Fil A+
Pwr Sensor
This line indicates when the PA is experiencing adverse
conditions. Under normal operation, while the PA is keyed,
this line will be proportional to forward power. Minimum
power (zero watts) corresponds to 2.5 volts while maximum
power corresponds to 4.5 volts. This voltage is not tempera-
ture compensated and no effort is made to calibrate this signal
to an absolute power level. It is intended to provide a relative
indication of forward power and to discriminate between
normal and cutback operation.
Zero volts on this line, when the PA is keyed, indicates the
forward power is cutback. This power cutback may be due
to high reflected power or may be due to high PA tempera-
tures. This fault condition may indicate a problem with the PA
or may indicate a system problem external to the Power Ampli-
fier. High VSWR may be due to a poor antenna and high
temperature may be due to a blocked cabinet vent. Zero volts
on this line, when the PA is keyed, does not indicate zero
forward power. Zero volts indicates the PA is protecting itself
due to adverse conditions. If the adverse condition, either high
VSWR or high temperature is eliminated, the power will return
to normal and the PWR SENSOR voltage will rise above 2.5
volts.
PA Key (Interface Connector Pin 2)
This line is used to key and unkey the PA, UNKEY=0 volt
and KEY=5 volts. The driver of this line must be capable of
supplying 5 volts at 1.0 mA. The appropriate key sequence
requires RF from the transmit synthesizer be input to the PA
before the KEY line is energized.
PA PWR Set (Interface Connector Pin 3)
This line is used to set the RF Power Output of the PA.
Minimum power output equals 4 volts and maximum power
output equals 8 volts. The driver of this line must be capable of
supplying 8 volts at 1.0 mA.
Fil A+ (Interfaces Connector Pin 6)
This line provides the filtered supply voltage for the Power
Control. The driver of this line must be capable of supplying
13.4 volts
±
20% at 100 mA.
PWR Monitor
This line provides a DC voltage proportional to forward
power.
LBI-39127H
2