SG-2000 MANUAL
132
The SGC Building, 13737 S.E. 26th St. Bellevue, WA. 98005 USA
©1995, SGC, Inc.
TEL: (206) 746-6310 FAX: (206) 746-6384
21.14
Master Oscillator with Oven
(Schematic J30100920)
The master oscillator is a self-contained crystal / FET 11.94 MHz oscillator within
a tightly controlled oven operating at 65°C. The oscillator produces a 4Vpp sine
wave signal at a frequency of 11.94 MHz ±3.4 Hz over a temperature range of -30
degrees C to +50 degrees C. The oscillator is coupled to the PLL chip IC610 and
is buffered to pin 14 of that chip. The master oscillator is used as a reference for
all the VCOs to establish the required frequencies determined by the micro-
processor assembly. The oscillator is limited to a deviation of 3.4 Hz due to the
division scheme utilized in the VCOs to correspond to a maximum deviation of
±10 Hz on the output of the unit in transmit and on the highest operating marine
frequency band of 25 MHz.
21.15
Crystal Oven
(Schematic J30100920)
The 11.94 MHz reference oscillator is maintained at a constant temperature by a
solid state regulated oven. The temperature regulating circuit includes a power
transistor (
Q601
) and a temperature sensing thermistor. On a cold start, the
control transistor is full on and a maximum current of approximately 300 milli-
ampers flows. When the equilibrium temperature is reached, the current drops
to a low level just sufficient to maintain the set temperature.
The complete oven assembly is enclosed in a thermal cover which retains the
heat, reducing the amount of power required and providing more precise control
of the oven temperature.