CIRCUIT DESCRIPTION

DISPLAY DESCRIPTION

The functions controlled by U2 are as follows:

•Display controller U1 programming

•Backlight control

•Transmit/Busy indicator CR4 control

•Front panel option switch detection

•Front panel Select switch detection

•Microphone hanger off-hook detection.

The use of a separate microcontroller on the display board minimizes the number of interconnections required with the audio/logic board because separate lines are not required for each of the preceding functions.

When the front panel Select switch is pressed, the two PBNO lines are shorted together. Then when it is rotated clockwise, low pulses appear on the CW line, and when it is rotated counterclockwise, low pulses appear on the CCW line. The contrast and viewing angle of the display are set by potentiometer R46.

3.7SYNTHESIZER CIRCUIT DESCRIPTION (UHF MODELS)

3.7.1 INTRODUCTION

The synthesizer block diagram is part of the RF/ PA board diagram shown in Figure 3-4.The synthesizer output signal is produced by a VCO (Voltage Controlled Oscillator) located in 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 UHF channels. Therefore, the reference divider in U804

divides the 14.850 MHz reference oscillator signal by

297.The TCXO frequency stability is 2.0 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 3.7.5 for more information on U804 operation.

3.7.2 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 L101. 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 (Q102)

The VCO is formed by bipolar transistor Q102, ceramic resonator L101, and several capacitors and varactor diodes. It oscillates at the transmit frequency in the transmit mode and 45 MHz below the receive frequency in the receive mode (the first injection frequency).

Biasing of Q102 is provided by R108 and R112, and stabilization is provided by R114. Inductor L102 functions as an RF choke, and an AC voltage divider formed by C112, C115, and C119 starts and maintains oscillation and matches Q102 to the tank circuit.

The tank circuit consists of laser tuned inductor L101, varactor diodes CR101, CR103-CR106, and several capacitors. Inductor L101 is laser trimmed to

 

February 2001

3-11

Part No. 001-9800-001