Typical Crystal Characteristics - GE MASTR II Maintenance Manual

L'll-4896
CIRCUIT ANALYSIS
The ICOMs are enclosed in a dust-
proof, RF shielded can with the type ICOM
(5C-ICOM, EC-ICOM, or 2C-ICOM) printed on
the top of the can. Access to the
oscil~
lator trimmer is obtained 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 ICOM keying lead (termi-
nal 6) to A- by means of the frequency
se'Iector switch on the control unit. In
single-frequency radios, a jumper from
H9 to HlO in the control unit connects
terminal 6 of the ICOM to A-. The oscil-
lator is turned on by applying a keyed
+10 Volts to the external oscillator load
resistor. RF bypassing is provided for all
unused keying leads on eight frequency
radios. On two frequency radios, the six
unused keying leads are shorted to ground.
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CAUTION
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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 BC-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 SC-ICOM can pro-
vide 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
exhibit 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
temperature range (approximately 0°C to
+55°C), frequency decreases with increasing
temperature.
Since the rate of change is nearly lin-
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
2
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.
RC-2443
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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 t.he varactor ca-
pacity at a constant value over the entire
mid-temperature range. With no additional
compensation, all of the oscillators will
provide 2 PPM frequency stability from
0°C to 55°C (32°F to 131°F).
Compensator Circuits
Both the SC-ICOMs and 2C-ICOMs are
temperature compensated at both ends of the
temperature range to provide instant fre-
quency 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 de-
creases and the compensation voltage in-
creases.
The increase in compensation voltage
decreases the capacity of the varactor
in the oscillator, increasing the output
frequency of the ICOM.
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