Motorola ASTRO XTS 3000 R Detailed Service Manual page 51

The second LO/VCO is a Colpitts oscillator built around transistor Q1.
The VCO has a varactor diode (VR401), which is used to adjust the
VCO frequency. The control signal for the varactor is derived from a
loop filter consisting of C426, C428, and R413.
Transmitter
The 800MHz RF power amplifier (PA) is a five-stage amplifier (U502).
The RF power amplifier has a nominal input and output impedance of
50 ohms.
An RF input drive level of approximately +3dBm, 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 3 and 4 of the module; pin 3
is switched through Q502 and pin 4 is unswitched B+ to the final
amplifier stage. Power control is achieved through the varying of the
dc bias to pin 2, the third and fourth amplifier stages of the module.
The amplified RF signal leaves the PA module at pin 5 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 indicator of the strength 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 2 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).
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