Motorola GM338, Service Manual

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UHF (450-527 MHz) Transmitter Power Amplifier (PA) 25 W 2-5
3.4 Directional Coupler
The directional coupler is a microstrip printed circuit, which couples a small amount of the forward
power delivered by Q5441. The coupled signal is rectified by D5451. The DC voltage is proportional
to the RF output power and feeds the RFIN port of the PCIC (U5501 pin 1). The PCIC controls the
gain of stages U5401 and Q5421 as necessary to hold this voltage constant, thus ensuring the
forward power out of the radio to be held to a constant value.
3.5 Antenna Switch
The antenna switch consists of two PIN diodes, D5471 and D5472. In the receive mode, both diodes
are off. Signals applied at the antenna jack J5401 are routed, via the harmonic filter, through
network L5472, C5474 and C5475, to the receiver input. In the transmit mode, K9V1 turns on
Q5471 which enables current sink Q5472, set to 96 mA by R5473 and VR5471. This completes a
DC path from PASUPVLTG, through L5437, D5471, L5472, D5472, L5471, R5474 and the current
sink, to ground. Both diodes are forward biased into conduction. The transmitter RF from the
directional coupler is routed via D5471 to the harmonic filter and antenna jack. D5472 also
conducts, shunting RF power and preventing it from reaching the receiver port (RXIN). L5472 is
selected to appear as a broadband lambda/4 wave transmission line, making the short circuit
presented by D5472 appear as an open circuit at the junction of D5472 and the receiver path.
3.6 Harmonic Filter
Components L5491-L5493 and L5472, C5491, C5496-98 form a Butterworth low-pass filter to
attenuate harmonic energy of the transmitter to specifications level. R5491 is used to drain
electrostatic charge that might otherwise build up on the antenna. The harmonic filter also prevents
high level RF signals above the receiver passband from reaching the receiver circuits, improving
spurious response rejection.
3.7 Power Control
The transmitter uses the Power Control IC (PCIC, U5501) to control the power output of the radio. A
portion of the forward RF power from the transmitter is sampled by the directional coupler and
rectified, to provide a DC voltage to the RFIN port of the PCIC (pin 1) which is proportional to the
sampled RF power.
The ASFIC (U0221) has internal digital to analog converters (DACs) which provide a reference
voltage of the control loop to the PCIC via R5505. The reference voltage level is programmable
through the SPI line of the PCIC. This reference voltage is proportional to the desired power setting
of the transmitter, and is factory programmed at several points across the frequency range of the
transmitter to offset frequency response variations of the transmitter’s power detector circuit.
The PCIC provides a DC output voltage at pin 4 (INT) which sets the drain current of the first
(U5401) and second (Q5421) transmitter stage via current control op-amps U5402-1 and U5402-2.
This adjusts the transmitter power output to the intended value. Variations in forward transmitter
power cause the DC voltage at pin 1 to change, and the PCIC adjusts the control voltage above or
below its nominal value to raise or lower output power.
Capacitors C5502-4, in conjunction with resistors and integrators within the PCIC, control the
transmitter power-rise (key-up) and power-decay (de-key) characteristic to minimize splatter into
adjacent channels.