GE MASTR II Maintenance Manual page 4

LBI4898
CIRCUIT ANALYSIS
The lCOMs are enclosed in an RF shielded
can with the type !COM (5C-ICOM, EC-ICOM or
2C-ICOM) printed on the top of the can.
Access to the oscillator trimmer is obtained
through a hole on the top of the can.
Frequency selectaon is accomplished by
switching the ICOM keying lead (terminal 6)
to A- by means of the frequency selector
switch on the control unit. In single-fre-
quency radios, a jumper from H9 to HlO in
the control unit connects terminal 6 of the
!COM to A-. The oscillator is turned on
by applying a keyed +10 Volts to the exter-
nal oscillator load resistor. RF bypassing
is provided for all unused keying loads in
eight frequency radios. In two frequency
radios the six unused keying leads are
shorted to ground.
r---------------
CAUTION --------------~
All ICOMs are individually compen-
sated at the factory and cannot be
repaired in the field. Any attempt
to repair or change an ICOM fre-
quency will void the warranty.
In standard 5 PPM radios using EC-ICOMs,
at least one 5C-ICOM must be used. The
5C-ICOM is normally used in the receiver Fl
position, but can be used in any transmit or
receive position. One 5C-ICOM can provide
compensation for up to 15 EC-ICOMs in the
transmit and receiver. Should the 5C-ICOM
compensator fail in the open mode, the
EC-ICOMs will still maintain 2 PPM frequency
stability from 0°C to 55°C (+32°F to 131°F)
due to the regulated compensation voltage
(5 Volts) from the 10-Volt regulator IC.
If desired, up to 16 5C-ICOMs may be used
in the radio.
The 2C-ICOMs are self-compensated at
2 PPM and will not provide compensation
for EC-ICOMs.
Oscillator Circuit
The quartz crystals used in ICOMs ex-
hibit the traditional "S" curve character-
istics of output frequency versus operating
temperature.
At both the coldest and hottest tem-
peratures, the frequency increases with
increasing temperature. In the middle tem-
perature range (approximately 0°C to +55°C),
frequency decreases with increasing temper-
ature.
Since the rate of change is nearly
linear over the mid-temperature range, the
output frequency change can be compensated
by choosing a parallel compensation capa-
citor with a temperature coefficient appro-
ximately equal and opposite that of the
crystal.
2
Fig~re
2 shows the typical performance
of an uncompensated crystal as well as the
typical performance of a crystal which has
been matched with a properly chosen compen-
sation capacitor.
::::!:
Cl..
Cl..
+5
-15" +10" 26.5" +42" 65"
DEGREES CENTIGRADE
Figure 2 - Typical Crystal Characteristics
At temperatures above and below the
mid-range, additional compensation must be
introduced. An externally generated com-
pensation voltage is applied to a varactor
(voltage-variable capacitor) which is in
parallel with the crystal.
A constant bias of 5 Volts (provided
from Regulator IC U901 in parallel with the
compensator) establishes the varactor capa-
city at a constant value over the entire mid-
temperature range. With no additional com-
pensation, all of the oscillators will pro-
vide 2 PPM frequency stability from 0°C to
55°C (+32°F to 131°F).
Compensator Circuits
Both the 5C-ICOMs and 2C-ICOMs are tem-
perature compensated at both ends of the
temperature range to provide instant freq-
uency compensation. An equivalent !COM cir-
cuit is shown in Figure 3.
The cold end compensation circuit does
not operate at temperatures above 0°C. When
the temperature drops below 0°C, the circuit
is activated. As the temperature decreases
the equivalent resistance decreases and the'
compensation voltage increases.
The increase in compensation voltage
decreases the capacity of the varactor in
the oscillator, increasing the output fre-
quency of the ICOM.
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