M/A-Com 19D902797G10, Руководство по обслуживанию

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CIRCUIT ANALYSIS
3.2 POWER CONTROL
The Power Control circuitry performs three basic functions. It keys and unkeys the PA,
sets the PA output power, and protects the PA against adverse conditions.
3.2.1 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.
3.2.2 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 is done by varying the voltage applied on the Power Set line.
CAUTION
*Misadjustment of R43 may result in PA failure.
3.2.3 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.
3.2.4 Theory of Operation
Power control of the MASTR® III Power Amplifier is accomplished with a feedback
control loop. The three possible 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.
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