455 Khz Local Oscillator; Clarifier - Codan X-2 Service Manual

Technical Description 4-12
4.4.4

455 kHz Local Oscillator

USB
For upper sideband the modulator/demodulator local oscillator is derived
from the 7304 kHz reference oscillator divided by 16 (456.5 kHz).
The 7304 kHz reference oscillator (USB) is taken from the oscillator output
pin 3 of IC11 and applied to the input of a ÷ 2 divider IC16A
The output pin 6 is applied to a second ÷ 2 divider IC16B pin 11 and the
clock input of the microcontroller IC403 pin 16.
In USB mode, the second divider IC16B is enabled by the microcontroller
selecting the enable line pin 10 high, at the same time disabling the 1814 kHz
oscillator IC14/C pin 8.
The 7304 kHz ÷ 4 reference signal at pin 8 of IC16/B is fed via OR gate
IC14/B to two cascaded ÷ 2 dividers IC15/A and IC15/B. This produces a
balanced output at pins 8 and 9 at a frequency of 456.5 kHz. From here it is
applied to the balanced local oscillator input of modulator IC301 (sheet 2)
and demodulator IC302 pins 8 and 10.
LSB
For lower sideband a separate 1814 kHz crystal oscillator divided by 4 is
used. In LSB mode, the output from divider IC16/B is disabled by the
microcontroller at the same time enabling IC14/C
oscillator.
The output from pin 10 of the oscillator is fed via OR gate IC14/B and
dividers IC15A and IC15B to the modulator and demodulator for the local
oscillator frequency of 453.5 kHz.
4.4.5

Clarifier

Operation of the clarifier on the front panel produces a code which is
converted to serial data and read by the microcontroller. For each clarifier
step detected, the microcontroller reprogrammes IC11, shifting VCO2 in
10 Hz steps from the nominal frequency. The clarifier range is ± 50 Hz for
channels 2 MHz to 5 MHz and ± 10 ppm above 5 MHz. For example, the
clarifier range at 18 MHz is ± 180 Hz.
The operation of the clarifier control varies the frequency of VCO2 in 10 Hz
steps. This is done by varying both VCO2 and the reference frequency
dividers according to a 'look-up' table in the transceiver's operating system.
The phase comparison frequency varies from 1.1 to 2.0 kHz.
An audible indication is given when the clarifier reaches its upper or lower
limit. The clarifier automatically resets to mid frequency when the channel is
changed.
X-2 Technical Service Manual
[H8] at pin 3.
, the 1814 kHz crystal
[F9]
Publication No: 15-02047