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18.4 MHz Oscillator @
The 18.4 MHz Oscillator is basically a Colpitts oscillator with a crystal Yl in the feedback
path. A simplified schematic is shown in Figure 4.
If the crystal were replaced with a large capacitor, the circuit would oscillate at the resonant
frequency of the parallel resonant circuit made up of Ll, C13, C14, and C15. When the
crystal is inserted, the feedback path is broken except at the series resonance of the crystal
(18.4 MHz). The parallel combination of varactor CRl, temperature-compensating capacitor
C7, and FREQ ZERO COARSE air-variable capacitor Cl0 are in series with the crystal and
can pull it several kHz either side of 18.4 MHz.
The voltage output is determined by the current through RlO* times the collector load. If the
output is too low, RlO* can be decreased in size.
The collector voltage is divided by C13, C14, and Cl5 and goes to the Buffer Amplifiers. The
division ratio is:
Buffer Amplifiers @J
The Buffer Amplifiers are two almost identical amplifier circuits. The top amplifier on the
schematic (QlO, QS) provides the 18.4 MHz LO signal to A4A6Al Up Converter, while the
bottom amplifier (Q12, Q13) provides the signal to A4A6A2 Down Converter.
4 A4A5
Figure 4. 18.4 MHz Oscillator, Equivalent Circuit
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