Motorola ASTRO MT 1500, Service Manual

Page: 86 / 817

August 16, 2007 6816985H01-B
4.2-2 UHF1 Detailed Theories of Operation: Antenna Switch
• A filter for the regulated 5 and 3 volts
The 13V, being generated by the positive voltage multiplier circuitry, should be present at pin 47 of
U202. The serial interface (SRL) is connected to the microprocessor via the data line (pin 7 of U202),
the clock line (pin 8 of U202), and the chip-enable line (pin 9 of U202).
The complete synthesizer subsystem works as follows:
1. The buffered output of the VCO will be present at R267 in receive mode and R289 during
transmit mode.
2. The output of the Prescaler filter is present at C291 and applied to pin 32 of U202. The
prescaler in U202 is a dual-modulus type with selectable divider ratios. This divider ratio is
controlled by the loop divider, which in turn receives its input from the SRL. The loop divider
adds or subtracts phase to the prescaler divider by changing the divider ratio via the modulus
control line.
3. The output of the prescaler is then applied to the loop divider.
4. The output of the loop divider then feeds the phase detector. The phase detector will compare
the loop divider's output signal form the VCOs. The VCO signal is divided down after it is
applied to pin 32 of U202. The result of the comparison is a pulsed DC signal which is applied
to the charge pump.
5. The charge pump outputs a current that will be present at pin 43 of U202. The loop filter
consisting of R202, C202, R203, C203, R201, C201, C236, C237, C238, C239 and C240, will
convert this current into a voltage that will be applied to varactors D251, D252, D253, D254,
D271, D272, D273 and D274, to alter the output frequency of the VCO.
In order to modulate the PLL, the following two-port modulation method is utilized: The analog
modulating signal is applied to the (1) A/D converter as well as to the (2) balance attenuator:
1. The A/D converter converts the low-frequency analog modulating signal into a digital code.
When the digital code is applied to the loop divider, it causes the carrier to deviate.
2. The balance attenuator is used to adjust the VCO’s deviation sensitivity to high-frequency
modulating signals.
4.2.3 Antenna Switch
An electronic PIN diode switch steers RF between the receiver and transmitter. Switching is derived
from the interaction of PIN diodes D170 and D171 as well as the quarter-wavelength components
(L176, C178 & C192).
1. In transmit mode:
a. RF is routed to the anode of diode D170.
b. PIN Diodes D170 and D171 are forward biased by turning on transistor Q197 via RX pin
of U102 (pulled low). Due to the characteristics of the quarter-wavelength circuit (L176,
C178 and C192), the receiver path appears "disconnected."
2. In receive mode:
a. RF signal is routed via C171 through L176 and coupling capacitor C179.
b. With Q197 turned OFF, both PIN diodes are not biased and the transmit path through
D170 becomes "Open."