Frequency Generation Unit - Motorola GTX 2000 Service Manual

Theory of Operation

Frequency Generation Unit

The antenna switch circuit consists of two PIN diodes
(CR161 and CR162), a pi network (C126, L161, and part
of C162), and one current limiting resistor (R163). In
the transmit mode, TX B+ is applied to the circuit to
bias the diodes ÒonÓ. The shunt diode (CR161) shorts
out the receiver port, and the pi network, which oper-
ates as a quarter wave transmission line, transforms
the low impedance of the shunt diode to a high imped-
ance at the input of the harmonic Þlter. In the receive
mode, the diodes are both off, and hence, there exists a
low attenuation path between the antenna and receiver
ports.
The harmonic Þlter consists of part of C121, C122 and
L121, C123, and C124. This Þlter attenuates the har-
monic of the module to a level better than -55 dBc.
To optimize the performance of the transmitter and
receiver into an antenna, a network is used to match
the antennaÕs impedance to the harmonic Þlter. The
network is made up of C125 and L122.
The power control circuit consists of the networks
associated with U151, Q151, Q155, and U111.
U111 is a directional coupler which detects the forward
power of the power module U101. The couple has 21
dB ± 1 dB coupling at port 2. The RF voltage at the cou-
pler port 2 is rectiÞed by CR111, C111, R114, C112, and
is applied to U151A. This signal is ampliÞed by U151A
and applied to summing and integrator ampliÞer
U151B.
POSITIVE MULTIPLIER
16.8 MHz
REF OSC
U210
REFERENCE
DIVIDER
COUNTER FOR
MULTIPLIER
FRACTIONAL
A/D
DIVIDER
U220
FRACTIONAL-N
SYNTHESIZER
2.1 MHz
REFERENCE
MODULATING
CLOCK TO
SIGNAL
ASFIC
2-4
LOOP
FILTER
MOD OUT
PRE-SCALER
DIVIDER
LOW PASS FILTER
FGU CIRCUITS
68P02945C75-A
GTX/GTX LTR/LTS 2000 Portable Radios Service Manual
The voltage at the second input of the summing amp is
supplied by DAC U400-P9. This DAC is controlled by
the microprocessor and provides the reference voltage
for the control loop.
The error voltage at the input of U151B-13 produces a
voltage at its output which is in turn applied to the
drivers Q151, Q152. The voltage from Q151 collector is
applied to the controlled stage of the power module.
Frequency Generation Unit
The Frequency Generation Unit (FGU) consists of the
following major sections: the high stability reference
oscillator (U210), fractional-N synthesizer (U220), VCO
(U260), VCO buffer IC (U270) and Rx injection buffer
(Q280, Q281 and the surrounding components). The
VCO provides the carrier frequency for the transmitter
(U270 Tx output) and provides the local oscillator (LO)
injection signal for the Rx injection buffer (U270 Rx out-
put). The phase-locked loop (PLL) circuit is provided
by the fractional-N synthesizer IC.
The output of the VCO is ampliÞed by the prescaler
buffer, routed through a low-pass Þlter, and applied to
the prescaler dividers of the synthesizer. The divide
ratios are determined from information stored in mem-
ory and bussed to the synthesizer by way of the micro-
controller. The divide ratio determines the VCO
frequency. The resulting VCO buffer signal is applied
to a comparator in the synthesizer. The synthesizer
comparator also receives a reference frequency by way
SWITCHING CCTS
FOR VCO AND BUFFER
RX VCO
RX BUFFER
TX VCO
TX BUFFER
U260
Prescaler
VCO
BUFFER
U270
VCO BUFFER IC
RX OUT
RX INJ.
MIXER
BUFFER
TX OUT
MATCHING
RF PA
CCT
June 1999