Motorola ASTRO Digital Saber, Service Manual

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is divided into one of three pre-determined frequencies. A time-based
algorithm is used to generate the fractional-N ratio.
If the two frequencies in the synthesizer’s comparator differ, a control
(error) voltage is produced. The phase detector error voltage (V
control) at pin 31 and 33 of U204, is applied to the loop filter
consisting of resistors R211, R212, and R213, and capacitors C244,
C246, C247 and C275. The filtered voltage alters the VCO frequency
until the correct frequency is synthesized. The phase detector gain is
set by components connected to U204 pins 28 and 29.
In the TX mode, U204 pin 38 goes high and U201 pin 14 goes low,
which turns off transistor Q202 and turns on the internal TX VCO
transistor in U204 and the external TX VCO buffer Q203 on the UHF
circuit. The TX VCO feedback capacitors are C219 and C220. Varactor
diode CR203 sets the TX frequency while varactor CR202 is the TX
modulation varactor. The modulation of the carrier is achieved by
using a 2-port modulation technique. The modulation of low
frequency tones such as DPL/TPL is achieved by injecting the tones
into the A/D section of the fractional-N synthesizer. The digitized
signal is modulated by the fractional-N divider, generating the
required deviation. Modulation of the high frequency audio signals is
achieved by modulating the varactor (CR203) through a frequency
compensation network. Resistors R207 and R208 form a potential
divider for the higher frequency audio signals.
In order to cover the very wide bandwidths, positive and negative V-
control voltages are used. High control voltages are achieved using
positive and negative multipliers. The positive voltage multiplier
circuit consists of components CR204, C256, C257 and reservoir
capacitor C258.The negative multiplier circuit consists of components
CR205, CR206, C266, C267 and reservoir capacitor C254 in VHF and
UHF radios. Out-of-phase clocks for the positive multiplier appear at
U204 pins 9 and 10. Out-of-phase clocks for the negative multiplier
appear at U204 pins 7 and 8, and only when the negative V-control is
required (i.e., when the VCO frequency exceeds the crossover
frequency). When the negative V-control is not required, transistor
Q201 is turned on, and capacitor C259 discharges. The 13V supply
generated by the positive multiplier is used to power-up the phase
detector circuitry. The negative V-control is applied to the anodes of
the VCO varactors.
The TX VCO signal is amplified by an internal buffer in U201, routed
through a low pass filter and routed to the TX PA module, U105 pin 1.
The TX and RX VCOs and buffers are activated via a control signal
from U204 pin 38.
The reference oscillator supplies a 16.8MHz clock to the synthesizer
where it is divided down to a 2.1MHz clock. This divided-down clock
is fed to the ABACUS IC (U401), where it is further processed for
internal use.