HP 85662A, Руководство по устранению неисправностей и ремонту

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18.4 MHz Oscillator
The 18.4 MHz Oscillator is basically a Colpitts oscillator with a crystal in the feedback
path. A simplified schematic is shown in Figure 4.
Figure 4. 18.4 MHz Oscillator, Equivalent Circuit
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
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
temperature-compensating capacitor
C7, and FREQ ZERO COARSE air-variable capacitor Cl0 are in series with the crystal and
can pull it several
either side of 18.4 MHz.
The voltage output is determined by the current through times the collector load. If the
output is too low, 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:
0.13
Buffer Amplifiers
The Buffer Amplifiers are two almost identical amplifier circuits. The top amplifier on the
schematic
provides the 18.4 MHz LO signal to Up Converter, while the
bottom amplifier (Q12, Q13) provides the signal to Down Converter.
4