Ericsson 19D902783G1, Руководство по обслуживанию

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POWER CONTROL
The Power Control circuitry performs three basic func-
tions. It keys and unkeys the PA, sets the PA output power,
and protects the PA against adverse conditions.
Keying and Unkeying the PA
To key the PA, the digital controller places 5 volts on the
PA key line, J201-2. Zero volts on the PA key line causes the
PA to unkey. If the control cable (W13) is disconnected, with
nothing actively driving the PA key line, the PA will remain
unkeyed.
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 its maximum rated output.
The PA output power is initially set at the factory. This is
done by adjusting R43 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 in done by varying
the voltage applied on the Power Set line.
PA Protection
The power control also protects the PA against over
temperature 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.
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. These three
signals are input to a diode summing junction which selects
the largest of the three for use as the feedback.
The stripline directional coupler samples the output
power and produces a voltage, Vf, proportional to the for-
ward output power. The power control compares the forward
voltage, Vf, to a reference voltage at U3. The output of U3
controls the current flow thru Q5 and the output of Q203.
The collector output of Q203 adjusts the control voltage,
Vct1 and Vct2. This control voltage is capable of adjusting
the total PA output power since it provides the first two stages
DC supply to the Low Level Amplifier, U1.
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 U3.1 to
increase. If the output of U3.1 becomes larger than the other
feedback lines, the output of U3.4 will begin to decrease.
This in turn will cause the output of Q203 to decrease
reducing the supply voltage to U1. Since the scaling is a
function of temperature the power is reduced as the tempera-
ture 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 U3.1 beyond the preset threshold an increase at
U3.4 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 – NC
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 temperatures.
This fault condition may indicate a problem with the PA or may
indicate a system problem external to the Power Amplifier.
High VSWR may be due to a poor antenna and high tempera-
ture 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+ (Interface 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.
SYMPTOM AREAS TO CHECK INDICATIONS
1. No Power or low Power at 1. Measure the transmitter output The presence of power at this port
Antenna Port. power before the duplexer or is an indication of a defective
antenna switch (for simplex mode). duplexer, switch, or cables.
2. Measure the transmitter output The presence of power at this port
power before the low pass filter. is an indication of a defective filter
or cables.
3. Measure the transmitter output The presene of power at this port
power before the optional isolator is an indication of a defective isolator
at the PA output port. or cables.
2. No power at PA output port 1. Station is in receive mode.
and PA ALARM is OFF.
3. No power at PA output port 1. No RF input to PA. Check TX Synthesizer should deliver
and PA ALARM is ON. connection between PA and TX a minimum of 10 mW
Synthesizer. (10 dBm) to the PA.
2. Check the logic or DC inputs to
the PA from the Interface Board
through J201.
a. J201-2 PA KEY 5volts during transmit
b. J201-3 POWER SET 4 volts to 8 volts (4 volts
represents zero RF power)
c. J201-6 13.8 VF 13.8 Vdc
±
20%
3. Defective PA Replace PA
TROUBLESHOOTING GUIDE
LBI-38674F
2