Motorola ASTRO Digital Saber, Service Manual

Page: 38 / 128

6
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1
800MHz Transceiver Board
Detailed Theory of Operation
6
Introduction
This 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 Service Manual to troubleshooting
the problem to a particular board.
Frequency Synthesis
The complete synthesizer subsystem consists of the reference oscillator
(U304), the voltage controlled oscillator (VCO), U307, a buffer IC
(U303), and the synthesizer (U302).
The reference oscillator contains a temperature-compensated
16.8MHz crystal. This oscillator is digitally tuned and contains a
temperature-referenced 5-bit analog-to-digital (A/D) converter. The
output of the oscillator (pin 10 on U304) is applied to pin 14 (XTAL1)
on U302 via capacitor C309 and resistor R306.
Module U307 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 frequencies (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
U307. When radio-to-radio or talk-around operation is necessary, the
high band VCO (851-870MHz) is selected. This is accomplished by
pulling pin 3 low and pin 8 high on U307.
The buffer IC (U303) 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 U303
high; the RX buffer is chosen by setting pin 7 of U303 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 U303 (or pin 38 of U302) and pin 7 of U303 (or pin 39 of
U302). For the first LO injection frequencies 777-797MHz, pins 6 and
7 must both be low; for the TX repeater frequencies 806-825MHz pins