M/A-Com 19D902797G10, Руководство по обслуживанию

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CIRCUIT ANALYSIS
3.0 CIRCUIT ANALYSIS
3.1
POWER AMPLIFIER
The power amplifier section of the PA Board consists of an Exciter, a Small Signal Gain
Stage, a Low Level Amplifier, a Driver, and the Power Amplifier Finals. All these gain
stages have an input and output impedance of 50 ohms. Figure 1 is a block diagram
showing the signal flow within the Power Amplifier Assembly.
3.1.1
Exciter (U7)
The Exciter stage uses a broadband silicon monolithic microwave integrated circuit
(MMIC) amplifier. The signal from transmitter synthesizer, typically 10 dBm (10 mW),
is input to the Exciter through a 10 dB resistive pad (R1, R2, and R31). The Exciter
amplifies the resulting 0 dBm (1 mW) signal to 12 dBm (16 mW).
The MMIC requires a 5 volt supply source. The 8 volt regulator (U100) provides the 5
volts to the MMIC via a dropping resistor R30.
3.1.2
Small Signal Gain Stage
The Small Signal Gain Stage consists of Q7 and its associated bias and matching
circuitry. Collector voltage is fed through R39, R40, and L23. Resistor R33 sets the
quiescent bias of the part. The transistor input impedance is matched to the 50-ohm
output of the Exciter by C59, C61, C62, and L7. L23 and C49 provide the necessary
output matching. The stage provides 14 dB of gain to amplify the signal from the Exciter
to 26 dBm (400 mW).
3.1.3 Low Level Amplifier (U1)
The Low Level Amplifier (LLA) stage uses a 50 ohm thick film RF Power Module to
amplify and control the output power. Internally, the module is a three stage amplifier.
The power control circuitry controls the gain of the first and second stages by varying the
collector voltage level of Q203. The third stage gain remains constant with A+ providing
the DC supply voltage.
The signal from the Small Signal Gain stage, typically 26 dBm (400 mW), is input into
the LLA. Under typical Power Set conditions, the LLA amplifies the signal to a typical
output level of 40.5 dBm (11.2 W).
3.1.4
Driver (Q1)
The driver is a 6 dB RF amplifier consisting of transistor Q1 and its associated circuitry.
The signal from the LLA, typically 40.5 dBm (11.2 W), is amplified to 46.5 dBm (45.0
W). The transistor input is matched to 50 ohms by C65, C66, C27, C67, and a piece of
printed transmission line. The drive signal is then split with a printed in-phase Wilkenson
splitter, providing equal power to each of the final devices.
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