Chapter 4.5 900 Mhz Detailed Theories Of Operation; Introduction; Frequency Synthesis - Motorola ASTRO XTS 2500 Service Manual

Chapter 4.5 900 MHz Detailed Theories of Operation

4.5.1 Introduction

This chapter provides a detailed circuit description of the ASTRO XTS 2500/XTS 2500
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
XTS 1500, MT 1500 Digital Portable Radios Basic Service Manual (Motorola part number
6816984H01), which uses the pass/fail service approach to radio problems.

4.5.2 Frequency Synthesis

The complete synthesizer subsystem consists of the following:
• The reference oscillator (Y200)
• The voltage-controlled oscillators (Q213 and Q215)
• The voltage-controlled oscillator buffer IC (U250)
• The synthesizer (U201)
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 U201 through capacitor C243
and resistor R217.
The voltage-controlled oscillators (VCOs) are varactor-tuned. The voltage (2V–11V) varies as it is
being applied to varactors D211, and D212 of their respective VCO. The capacitance of the varactors
also varies, thereby changing the output frequency of the VCOs. The two VCOs are used to cover
the entire 900MHz band.
• VCO1 covers the TX band (896-902MHz and 935-941MHz).
• VCO2 covers the RX 900MHz band (935-941MHz).
VCO1 and VCO2 are selected using aux2, and aux4, respectively. TX and RX mode are controlled
using aux3: High for RX; Low for TX.
The buffer IC (U250) includes a TX, RX, and prescaler buffer. The main purpose of the prescaler
buffer is to individually maintain a constant output and provide isolation, The TX buffer is chosen by
setting pin 19 of U250 high; the RX buffer is chosen by setting pin 19 of U250 low. The prescaler
buffer is always on. An additional external TX buffer is used with Q216.
The synthesizer IC (U201) 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
I
/XTS 2250/
I
, XTS 2250,