GE Mastr II Maintenance Manual page 4

LBI-4732
CIRCUIT ANALYSIS
by prying up the plastic tab on the top of
the can. The tabs can also be used to pull
the ICOMs out of the radio.
Frequency selection is accomplished by
switching the !COM keying lead (termianl 6)
to A-. The oscillator is turned on by
applying a keyed +10 Volts to the external
oscillator load resistor.
CAUTION
All ICOMs are individually compen-
sated at the factory and cannot be
repaired in the field. Any attempt
to repair or change an !COM fre-
quency will void the warranty.
In transmitter types KT-56-A, C using
EC-ICOMs, at least one 5C-ICOM must be
used. The 5C-ICOM is normally used in the
receiver Fl position, but ('<Jn be used in
any transmit or receivr rn~ition. One 5C-
ICOM can provide cnmpensatiqn for up to 8
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 com-
pensation voltage (5 Volts) from the 10-
Volt regulator IC. In transmitter types
KT-57-A, C and KT-58-A, C at least one 5C
!COM is required for the transmitter and
at least one 5C-ICOM is required for the
receiver. If desired, up to 8 5C-ICOMs
may be used in the station.
RC-2443
-10
L.__L__-L~f-----+--+---+---f--.;..._-=-.....c:.....J
-15" +10° 26.5° +42° 65°
DEGREES CENTIGRADE
Figure 2 - Typical Crystal Characteristics
The 2C-IC0Ms are self-compensated at
2 PPM and will not provide compensation for
EC-ICOMs.
2
Oscilla~or
Circuit
The quartz crystals used in ICOMs ex-
hibit the traditional "S" curve character-
istics of output frequency versus operating
temperature.
At both the coldeot and hottest temper-
atures, the frequency increases-with increas-
ing temperature. In the middle temperature
range (approximately 0°C to +55°C), frequency
decreases with increasing temperature.
Since the rate of change is nearly lib-
ear over the mid-temperature range, the out-
put frequency change can be compensated by
choosing a parallel compensation capacitor
with a temperature coefficient approximately
equal and opposite that of the crystal.
Figure 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.
At temperatures above and below the mid-
range, additional compensation must be intro-
duced. An externally generated compensation
voltage is applied to a varactor (voltage-
variable capacitor) which is in parallel with
the crystal.
In transmitter types KT-56-A, C a con-
stant bias of 5 Volts (provided from Regula-
tor IC U901 in parallel with the compensator)
establishes the varactor capacity at a con-
stant value over the entire mid-temperature
range. With no additional compensation, all
of the oscillators will provide 2 PPM fre-
quency stability from 0°C to 55°C (+32°F to
131°F).
Compensator Circuits
Both the 5C-IC0Ms and 2C-ICOMs are tem-
perature compensated at both ends of the tem-
perature range to provide instant frequency
compensation. An equivalent ICOM circuit 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.
The hot end compensation circuit does
not operate at temperatures below +55°C,
When the temperature rises above +55°C, the
circuit is activated. As the temperature
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