CIRCUIT DESCRIPTION

800/900 MHz SYNTHESIZER DESCRIPTION

Over Current Shutdown

Current to final amplifier Q601 on the PA board is monitored by sensing the voltage drop across R680. Pins 5 and 6 of U500B are effectively connected across this resistor. As current increases, the voltage on U500B, pin 6 decreases which causes the output voltage on pin 7 to increase. The gain of each U500B input is set at ten by R509/R504 and R507/R502.

Emitter biasing for Q501 is provided by R506 and R511. Normally, the output voltage of U500B is not high enough to turn on Q501. However, if current becomes excessive, for example because of an antenna mismatch, Q501 begins turning on. This decreases the base voltage of Q502 which turns off Q500 slightly and cuts back power output.

3.10SYNTHESIZER CIRCUIT DESCRIPTION (800/900 MHz MODELS)

3.10.1 INTRODUCTION

The synthesizer block diagram is part of the RF/ PA board diagram shown in Figure 3-7.The synthesizer output signal is produced by a VCO (Voltage Controlled Oscillator) located on a separate module attached to the RF board. The frequency of the VCO is controlled by a DC voltage from the phase detector in synthesizer integrated circuit U804.

The phase detector senses the phase and frequency difference between a highly stable signal from the reference oscillator (fR) and a frequency produced by dividing down the VCO signal (fV). When the signal from the VCO is the same as the reference frequency, the VCO is on the correct frequency. If the VCO-derived signal is not the same, the VCO control voltage increases or decreases to change the VCO frequency until they are the same. The VCO is then “locked” on frequency.

The reference input (fR) to the phase detector is produced by dividing down the signal from reference oscillator U806. The fR input is 50 kHz for all 800 and 900 MHz channels. Therefore, with 900 MHz models, the reference divider in U804 divides the 14.850 MHz reference oscillator signal by 297, and with 800 MHz models, it divides the 17.500 MHz reference oscillator

signal by 350. The TCXO frequency stability is 1.5 PPM, so this is also the stability of the synthesizer (and the second injection signal which is derived from the TCXO frequency).

The VCO-derived input to the phase detector (fV) is the VCO frequency divided down by programmable dividers in synthesizer U804. The prescaler and main divider are programmed for each channel to produce an input frequency to the phase detector (fV) that is the same as the 50 kHz reference frequency (fR) when the VCO is oscillating on the correct frequency. Refer to Section 3.10.6 for more information on U804 operation.

3.10.2 VOLTAGE-CONTROLLED OSCILLATOR

Introduction

The VCO module is a separate assembly that is soldered directly to the RF board and covered by a metal shield. It uses a ceramic substrate that can easily be damaged by excessive heat; therefore, it is recommended that modules which have been removed using a standard soldering iron not be reused. In addition, the VCO center frequency is set by laser tuning ceramic resonator L907. Therefore, it is not possible to adjust this frequency if it changes as the result of changing a part. For these reasons, the VCO is considered not field serviceable.

Oscillator (Q902)

The VCO is formed by bipolar transistor Q902, ceramic resonator L907, and several capacitors and varactor diodes. It oscillates at the transmit frequency in the transmit mode and 52.950 (800 MHz) or 45 MHz (900 MHz) below the receive frequency in the receive mode (the first injection frequency).

Biasing of Q902 is provided by R906 and R908, and stabilization is provided by R912. Inductor L906 functions as an RF choke, and C924 is an AC bypass capacitor. An AC voltage divider formed by C913, C917, and C921 starts and maintains oscillation and matches Q902 to the tank circuit.

The tank circuit consists of laser tuned inductor L907, varactor diodes CR902-CR904, and several capacitors. Inductor L907 is laser trimmed to set the

 

February 2001

3-22

Part No. 001-9800-001