Motorola ASTRO XTS 3000 R Detailed Service Manual page 48

5-2
both be high;
• for talkaround TX frequencies 851-870MHz, pin 6 must be low
while pin 7 must be high.
The synthesizer IC (U302) 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 5 volts.
This filtered five volts is present at pin 19 of U302, pin 9 of U307, and
pins 2, 3, 4, and 15 of U303. It is also applied directly to resistors R309,
R315, and R311. Additionally, the 13V, being generated by the positive
voltage multiplier circuitry, should be present at pin 35 of U302. The
serial interface (SRL) is connected to the microprocessor via the data
line (pin 2 of U302), clock line (pin 3 of U302), and chip-enable line
(pin 4 of U302).
The complete synthesizer subsystem works as follows:
• The output of the VCO, pin 4 on U307, is fed into the RF input
port (pin 9) of U303. In the TX mode, the RF signal will be present
at pin 4 of U303; in the RX mode, the RF signal will be present at
pin 3 of U303.
• The output of the prescaler buffer, pin 15 of U303, is applied to the
PREIN port (pin 21) of U302. The prescaler in U302 is a dual-
modulus type with selectable divider ratios. This divider ratio is
controlled by the loop divider, which in turn receives its inputs
from the SRL. The loop divider adds or subtracts phase to the
prescaler divider by changing the divide ratio via the modulus
control line.
• The output of the prescaler is then applied to the loop divider.
• The output of the loop divider is then applied to the phase
detector. The phase detector will then compare the loop divider's
output signal with the signal from U304 (that is divided down
after it is applied to pin 14 of U302). The result of the signal
comparison is a pulsed dc signal which is applied to the charge
pump.
• The charge pump outputs a current that will be present at pin 32
of U302. The loop filter (which consists of capacitors C322, C317,
C318, C329, C324, and C315, and resistors R307, R305, and R314)
will transform this current into a voltage that will be applied to
pins 1 and 7 of U307, and alter the VCO's output frequency.
In order to modulate the PLL, the two-port modulation method is
utilized. The analog modulating signal is applied to the A/D converter
as well as the balance attenuator, via U302, pin 5. The A/D converter
converts the low-frequency analog modulating signal into a digital
code that is applied to the loop divider, thereby causing the carrier to
deviate. The balance attenuator is used to adjust the VCO's deviation
sensitivity to high-frequency modulating signals.