Motorola 6864115B62-C, Detailed Service Manual, Page: 277 / 452

UHF (403-470MHz) Transmitter Power Amplifier (PA) 40W 2-11
Customer Programming Software (CPS). Care must be taken not to damage the device by
exceeding the maximum allowed bias voltage. The device’s drain current is drawn directly from the
radio’s DC supply voltage input, A+, via L4421.
5.4 Final Stage
The final stage uses the bipolar device Q4441. The device’s collector current is also drawn from the
radio’s DC supply voltage input. To maintain class C operation, the base is DC-grounded by a series
inductor (L4441) and a bead (L4440). A matching network consisting of C4441-C4444, C4491 and
two striplines transforms the impedance to 50 Ohms and feeds the directional coupler.
5.5 Directional Coupler
The Bi-directional coupler is a microstrip printed circuit, which couples a small amount of the forward
and reverse power of the RF power from Q4441. The coupled signal is rectified to an output power
proportional DC voltage by the diodes D4451 & D4452 and sent to the RFIN of PCIC. The PCIC
controls the gain of stage U4401 as necessary to hold this voltage constant, thus ensuring the
forward power out of the radio to be held to a constant value.
5.6 Antenna Switch
The antenna switch consists of two PIN diodes, D4471 and D4472. In the receive mode, both diodes
are off. Signals applied at the antenna jack J4401 are routed, via the harmonic filter, through
network L4472, C4474 and C4475, to the receiver input. In the transmit mode, K9V1 turns on
Q4471 which enables current sink Q4472, set to 96 mA by R4511 and VR4471. This completes a
DC path from PASUPVLTG, through L4437, D4471, L4472, D4472, L4473, R4496 and the current
sink, to ground. Both diodes are forward biased into conduction. The transmitter RF from the
directional coupler is routed via D4471 to the harmonic filter and antenna jack. D4472 also
conducts, shunting RF power and preventing it from reaching the receiver port (RXIN). L4472 is
selected to appear as a broadband Lambda/4 wave transmission line, making the short circuit
presented by D4472 appear as an open circuit at the junction of D4472 and the receiver path.
5.7 Harmonic Filter
Inductors L4491, L4492, L4493 and capacitors C4448, C4492,C4494, C4496 and C4498 form a
low-pass filter to attenuate harmonic energy of the transmitter to specifications level. R4491 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.
5.8 Power Control
The transmitter uses the Power Control IC (PCIC, U4501) to control the power output of the radio. A
portion of the forward RF power from the transmitter is sampled by the bi-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 PCIC has internal digital to analog converters (DACs) which provide the reference voltage of the
control loop. 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