Chapter 4.4 700/800 Mhz Detailed Theories Of Operation; Introduction; Frequency Synthesis - Motorola XTS 2500 Service Manual

Chapter 4.4 700/800 MHz Detailed Theories of

4.4.1 Introduction

This chapter provides a detailed circuit description of the ASTRO XTS 2500/XTS 2500
XTS 1500/XTS 2000/ATS 3000 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, XTS 2000, ATS 3000 MT 1500 Digital Portable Radios Basic Service Manual (Motorola
part number 6816984H01), which uses the pass/fail service approach to radio problems.

4.4.2 Frequency Synthesis

The complete synthesizer subsystem consists of the following:
• The reference oscillator (Y200)
• The voltage-controlled oscillators (Q210,Q212, and Q214)
• 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 (2 V – 11 V) varies as it
is being applied to varactors D210, D211, and D212 of their respective VCO. The capacitance of the
varactors also varies, thereby changing the output frequency of the VCOs. The three VCOs are used
to cover the entire 700/800MHz band.
• VCO1 covers the RX (downlink) 800 MHz band and the talkaround 700 MHz (764 MHz – 776 MHz)
band.
• VCO2 covers the TX (uplink) 700/800MHz (794 MHz – 824 MHz) band.
• VCO3 covers the talkaround 800 MHz (850 MHz
band.
VCO1, VCO2, and VCO3 are selected using aux1, 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 Q217.
The synthesizer IC (U201) consists of the following:
• A prescaler
• A programmable loop divider
• A divider logic control
Operation
870 MHz) band and the RX (downlink) 700 MHz
–
I
/XTS 2250/
I
, XTS 2250,