Motorola ASTRO MT 1500, Service Manual

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Chapter 4.1 VHF Detailed Theories of Operation
4.1.1 Introduction
This chapter provides a detailed circuit description of the ASTRO XTS 2500/XTS 2500
I
/XTS 2250/
XTS 1500 radio board. When reading the theory of operation, refer to the appropriate schematic and
component location diagrams located in the back of this manual. This detailed theory of operation
can help isolate the problem to a particular component.
This manual is to be used in conjunction with the ASTRO XTS 2500, XTS 2500
I
, XTS 2250,
XTS 1500, MT 1500 Digital Portable Radios Basic Service Manual (Motorola part number
6816984H01), which uses the pass/fail service approach to radio problems.
4.1.2 Frequency Synthesis
The complete synthesizer subsystem consists of the following:
• The reference oscillator (Y200)
• Two voltage-controlled oscillators (TX and RX)
• One voltage-controlled oscillator buffer IC (U250)
• The synthesizer (U202)
The reference oscillator contains a temperature-compensated 16.8 MHz crystal. This oscillator is
digitally tuned and contains a temperature-referenced, six-bit, analog-to-digital (A/D) converter. The
output of the oscillator (pin 3 on Y200) is applied to pin K9 (XTAL1) of U202 through capacitor C214.
The voltage-controlled oscillators (VCOs) are varactor-tuned. The voltage (2–9.5V) varies as it is
being applied to varactors D251 to D252 and D270 of their respective VCO. The capacitance of the
varactors also varies, thereby changing the output frequency of the VCOs. Both VCOs are used to
cover the entire 38 MHz bandwidth of the VHF range (136–174 MHz):
• TX VCO covers the transmit frequencies from 136–174 MHz
• RX VCO covers the receive frequencies from 180.85–218.85 MHz
The TX and RX VCOs are selected using AUX 3 from U202. Both VCOs are connected to the same
prescaler filter (C292, C266, L266 and R266) feeding back to PREIN of the U202, through C293.
The synthesizer IC consists of the following:
• A prescaler
• A programmable loop divider
• A divider logic control
• 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
• A filter for the regulated 5 and 3 volts