Motorola Astro Digitalport Saber Detailed 68p81076c10 A Manual

4-6
Notes 

5-1
VHF/UHF Transceiver Board 
Detailed Theory of Operation
5
IntroductionThis section of the manual provides a detailed circuit description of an 
ASTRO Digital SABER VHF and UHF Transceiver Board. When reading 
the theory of operation, refer to your appropriate schematic and 
component location diagrams located in the back section of this 
manual. This detailed theory of operation will help isolate the 
problem to a particular component. However, first use the ASTRO 
Digital SABER Portable Radios Basic...

5-2
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...

5-3
Antenna SwitchTwo antenna switches are part of the radio circuitry. One of the 
switches which is located in the radio casting is mechanical. It 
switches between the radio antenna and a remote antenna. Switching 
is accomplished by a plunger located on the accessory connector. With 
a remote antenna installed, continuity between the radio antenna and 
the RF input line is broken; continuity is made from the remote 
antenna to the radio RF line. The second switch is a current device. It 
is a pair of...

5-4
the first IF frequency. The first IF frequency of VHF and UHF bands are 
45.15MHz and 73.35MHz respectively. The 1st LO signal for VHF is 
45.15MHz higher than the carrier frequency while that for the UHF is 
73.35MHz lower than the carrier frequency. The 1st IF signal output, 
at U2 pins 4 and 6, is routed through transformer T2 and impedance 
matching components, and applied to a 2-pole crystal filter (FL1), 
which is the final stage of the receiver front end. The 2-pole crystal 
filter removes unwanted...

5-5
TransmitterThe transmitter consists of three major sections:
•Harmonic Filter
•RF Power Amplifier Module
•ALC Circuits
Harmonic FilterRF from the power amplifier (PA) module (U105) is routed through the 
coupler (U104), passed through the transmit antenna switch (CR108), 
and applied to a harmonic filtering network in UHF. In the case of a 
VHF transceiver board, RF from the PA module (U105) is routed also 
through the coupler (U104), then through the harmonic filtering 
network, and on to the antenna...

5-6
Notes 

6-1
800MHz Transceiver Board 
Detailed Theory of Operation
6
IntroductionThis section of the manual provides a detailed circuit description of an 
ASTRO Digital SABER 800MHz Transceiver Board. When reading the 
theory of operation, refer to your appropriate schematic and 
component location diagrams located in the back section of this 
manual. This detailed theory of operation will help isolate the 
problem to a particular component. However, first use the ASTRO 
Digital SABER Portable Radios Basic...

6-2
6 and 7 must both be high. For talkaround TX frequencies 851-
870MHz, pin 6 must be low while pin 7 must be high.
The synthesizer IC (U303) 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...

6-3
CR102, biasing them to a low-impedance state. Bias current returns to 
ground through U504 pin 20. In receive, U504 pin 21 is pulled down 
to ground and pin 20 is pulled up to B+, reverse biasing diodes CR104 
and CR102 to a high impedance.
Receiver Front EndFor the purposes of this discussion, the receiver front end is defined to 
be the circuitry from the antenna switch to the output of the IF crystal 
filter. The 800MHz front end is designed to convert the received RF 
signal to the 1st IF frequency...