Motorola ASTRO Digital Saber, Service Manual

Page: 42 / 128

6
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5
Transmitter
The 800MHz RF power amplifier (PA) is a 5-stage amplifier (U502). The
RF power amplifier has a nominal input and output impedance of 50
ohms.
An RF input drive level of approxi3dBm, supplied from the
VCO buffer IC (U303), is applied to pin 1 of U502. The dc bias for the
internal stages of U502 is applied to pins 2, 5, and 6 of the module.
Pins 2 and 5 being switched through Q502 and pin 6 being
unswitched B+ to the final amplifier stage. Power control is achieved
through the varying of the dc bias to pins 3 and 4, the third and fourth
amplifier stages of the module. The amplified RF signal leaves the PA
module via pin 7 and is applied to the directional coupler (U501).
The purpose of U501 is to sample both the forward power and the
reverse power. The reverse power will be present when there is other
than a 50-ohm load at the antenna port. The sampling will be
achieved by coupling some of the reflected power, forward and/or
reverse, to a coupled leg on the coupler. The sampled RF signals are
applied to diode CR501 for rectification and summing. The resultant
dc signal is applied to the ALC IC (U504 pin 2) as RFDET to be used as
an strength indicator of the RF signal being passed through the
directional coupler (U501).
The transmit ALC IC (U504) is the heart of the power control loop. The
REF V line (U504 pin 7), a dc signal supplied from the D/A IC (U503),
and the RF DET signal described earlier, are compared internally in the
ALC IC to determine the amount of C BIAS, pin 4, to be applied to the
base of transistor Q501. Transistor Q501 responds to the base drive
level by varying the dc control voltages applied to pin 3 and 4 of the
RF PA, controlling the RF power level of module, U502. The ALC IC
also controls the base switching to transistor Q502 via pin 12, BIAS.
The D/A IC (U503) controls the dc switching of the transceiver board.
Its outputs, SC1 and SC3, pins 12 and 14 respectively, control
transistor Q503, which then supplies TX 5V and RX 5V to the
transceiver board. The D/A also supplies the dc bias to the detector
diode (CR501) via pin 7, and the REF V signal to the ALC IC (U504).