Directional Coupler; Antenna Switch; Pa Stages - Motorola MCX600E Service Manual

9.2

PA Stages

The bipolar transistor Q8520 is driven by Q8510. To reduce the collector-emitter voltage and in turn
the power dissipation of Q8510 its collector current is drawn from the antenna switch circuit.
In transmit mode the base of Q8520 is slightly positive biased by a divided K9V1 signal. This bias
along with the rf signal from Q8510 allows a collector current to be drawn from the antenna switch
circuit and in turn switches the antenna switch to transmit, while in receive mode the low K9V1
signal with no rf signal present cuts off the collector current and in turn switches the antenna switch
to receive.
The following stage uses two enhancement mode N-Channel MOS FET devices (Q8530, Q8531)
and requires for each device a positive gate bias and a quiescent current flow for proper operation.
The voltages of the lines BIAS VLTG and BIAS VLTG 2 are set in transmit mode by two Digital to
Analog (D/A) converters (U0731-4, U0731-11) and fed to the gates of Q8531 and Q8530 via two
resistive dividers. The bias voltages are tuned in the factory. If one or both transistor are replaced,
the bias voltages must be tuned with the Dealer Programming Software (DPS). Care must be taken,
not to damage any device by exceeding the maximum allowed bias voltage. The collector currents
are drawn from the supply voltage A+ via L8531 and L8532.
The final stage uses the bipolar device Q8540 and operates off the A+ supply voltage. For class C
operation the base is DC grounded by two series inductors (L8533, L8534). A matching network
consisting of C8541-C8544 and two striplines transform the impedance to 50 Ohms and feed the

directional coupler.

9.3 Directional Coupler
The directional coupler is a microstrip printed circuit which couples a small amount of the forward
power off the rf power from Q8541. The coupled signal is rectified to an output power proportional
negative DC voltage by the diode D8553 and sent to the power control circuit in the controller
section via the line PWR DETECT for output power control. The power control circuit holds this
voltage constant, thus ensuring the forward power out of the radio to be held to a constant value.
9.4

Antenna Switch

The antenna switch is switched synchronously with the K9V1 voltage and feeds either the antenna
signal coming through the harmonic filter to the receiver or the transmitter signal coming from the PA
to the antenna via the harmonic filter.
In transmit mode, this K9V1 voltage is high and biases Q8520 and, along with the rf signal from
Q8510, allows a collector current to be drawn. The collector current of Q8520 drawn from A+ flows
via L8542, L8541, directional coupler, D8551, L8551, D8631, L8631, R8616, R8617 and L8611 and
switches the PIN diodes D8551 and D8631 to the low impedance state. D8551 leads the rf signal
from the directional coupler to the harmonic filter. The low impedance of D8631 is transformed to a
high impedance at the input of the harmonic filter by the resonant circuit formed by L8551, C8633
and the input capacitance of the harmonic filter.
In receive mode the low K9V1 and no rf signal present from Q8510 turn off the collector current of
Q8520. With no current drawn by Q8520 and resistor R8615 pulling the voltage at PIN diode D8631
to A+ both PIN diodes are switched to the high impedance state. The antenna signal, coming
through the harmonic filter, is channelled to the receiver via L8551, C8634 and line PA RX.
Theory of Operation
Transmitter Power Amplifier (PA) 5-25W
4-19