800/900Mhz Transceiver Board; Frequency Synthesis - Motorola Handie-Talkie MTX Series Theory/Troubleshooting Manual

3. ALC Circuits
Coupler module U104 samples the forward power
and the reverse power of the PA output voltage.
Reverse power is present when there is other than
50 ohms impedance at the antenna port. Sampling
is achieved by coupling some of the forward and/or
reverse power, and apply it to CR102(VHF) or
CR101(UHF) and CR103 for rectification and sum-
ming. The resultant dc signal is then applied to the
TX ALC IC (U101 pin 2) as RFDET to be used as
an RF strength indicator.
The transmit ALC circuit, built around U101, is the
heart of the power control loop. Circuits in the TX
ALC module compare the signals at U101 pins 2
and 7. The resultant signal, C BIAS, at U101 pin 4 is
applied to the base of transistor Q110. In response
to the base drive, transistor Q110 varies the dc con-
trol voltages applied to the RF PA at U105 pin 3,
thus controlling the RF power of module, U105.
Thermistor RT101 senses the temperature of the
TX ALC IC. If an abnormal operating condition
exists, which causes the PA slab temperature to
rise to an unacceptable level, the thermistor forces
the ALC to reduce the set power.
IV. 800/900MHz TRANSCEIVER BOARD
A. Frequency Synthesis
The complete synthesizer subsystem consists of the
reference oscillator (U203), the voltage controlled
oscillator (VCO), U205, a buffer IC (U201), and the
synthesizer U204).
The reference oscillator contains a temperature-
compensated 16.8 MHz crystal. This oscillator is digitally
tuned and contains a temperature-referenced 5-bit ana-
log-to-digital (A/D) converter. The output of the oscillator
(pin 10 on U203) is applied to pin 14 (XTAL1) on U204
via capacitor C284 and resistor R222.
Module U205 is the voltage controlled oscillator,
which is varactor tuned; that is, as the voltage (2-11V)
being applied to pins 1 and 7 of the VCO varies, so
does the varactor's capacitance, thereby changing the
VCO's output frequency. The 800MHz VCO is a dual-
range oscillator that covers the 806-825MHz and the
851-870MHz frequency bands. The low-band VCO
(777-825MHz) provides the first LO injection frequen-
cies (777-797MHz) that will be 73.35MHz below the
carrier frequency. In addition, when the radio is operated
through a repeater, the low band VCO will generate the
transmit frequencies (806-825MHz) that will be 45MHz
below the receiver frequencies. The low-band VCO is
selected by pulling pin 3 high and pin 8 low on U205.
When radio-to-radio or talk-around operation is neces-
sary, the high band VCO (851-870MHz) is selected.
This is accomplished by pulling pin 3 low and pin 8 high
on U205.
The 900MHz VCO is also a dual-range oscillator
that covers the 896-902MHz and the 935-941MHz fre-
quency bands. The low-band VCO (861-902MHz)
provides the first LO injection frequencies (861-
867MHz) that will be 73.35MHz below the carrier
frequency. In addition, when the radio is operated
through a repeater, the low-band VCO will generate the
transmit frequencies (896-902MHz) that will be 39MHz
below the receiver frequencies. When talk-around oper-
ation is necessary the high-band VCO (935-941MHz) is
selected.
The buffer IC, U201, includes a TX, RX, and
prescaler buffer whose main purpose is to individually
maintain a constant output and provide isolation. The
TX buffer is chosen by setting pin 7 of U201 high; the
RX buffer is chosen by setting pin 7 of U201 low. The
prescaler buffer will always be on. In order to select the
proper combination of VCO and buffer, the following
conditions must be true at pin 6 of U201 (or pin 38 of
U204) and pin 7 of U201 (or pin 39 of U204). For the
first LO injection frequencies 861-867MHz (A), pins 6
and 7 must both be low; for the TX repeater frequencies
896-902 MHz (B) pins 6 and 7 must both be high, and
for talk-around TX frequencies 935-941 MHz (C) pin 6
must be low while pin 7 must be high.
(A) = 777-797MHz for 800MHz; 861-867MHz
for 900MHz
(B) = 804-825MHz for 800MHz; 896-902MHz
for 900MHz
(C) = 851-870MHz for 800MHz;935-941MHz
for 900MHz
The synthesizer IC, U201 consists of a prescaler, a
programmable loop divider, a divider control logic, a
phase detector, a charge pump, an A/D converter for
low frequency digital modulation, a balance attenuator
to balance the high frequency analog modulation to the
low frequency digital modulation, a 13V positive-voltage
multiplier, a serial interface for control, and finally a filter
for the regulated five volts. This filtered five volts is pre-
sent at pin 19 of U204, pin 9 of U205, and pins 2,3,4,
and 15 of U201. It is also applied directly to resistors
R214, R215, and R220. Additionally, the 13V, being
generated by the positive voltage multiplier circuitry,
should be present at pin 35 of U204. The serial interface
(SRL) is connected to the microprocessor via the data
line (pin 2 of U204), clock line (pin 3 of U204), and chip
enable line (pin 4 of U204).
The complete synthesizer subsystem works as fol-
lows. The output of the VCO, pin 4 on U205, is fed into
the RF input port (pin 9) of U201. In the TX mode, the
RF signal will be present at pin 4 of U201. On the other
hand, in the RX mode, the RF signal will be present at
pin 3 of U201. The output of the prescaler buffer, pin 15
on U201, is applied to the PREIN port (pin 21) of U204.
The prescaler in U204 is a dual-modulus type with
selectable divider ratios. This divider ratio is controlled
by the loop divider, which in turn receives its inputs via
15