HP 10811A, Operating & Service Manual

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Model 10811A/B
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8-13. MAIN OSCILLATOR THEORY OF OPERATION
8-14. The 10811A/B oscillator is a Colpitts-type crystal oscillator which uses the crystal as the series
inductor. Figure 8-3A shows the basic block diagram of a Colpitts-type oscillator. Figure 8-3B
shows the basic equivalent components of the oscillator.
Figure 8-3A. Basic Colpitts- Type Oscillator
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Figure 8-3B. Main Oscillator Schematic Design
8-15. Comparing Figure 8-3A and 8-3B, we find Y1 is the inductor, capacitor CA is C8 and CB is the
combination of C5, L2, C6, and L3. C5 is a dc blocking capacitor. Its reactance is very low, so the
combination of these components can be redrawn as in
Figure 8-4 . The crystal Y1 is a "third
overtone" crystal and is operated at 10 MHz. To keep the circuit from oscillating at the crystal's
fundamental, or at a different overtone, the mode suppression network of C5, L2, C6, L3 appears
capacitive only at frequencies between 9 MHz and 10.5 MHz. Below and above this frequency
range, the network appears inductive. This does not allow the proper phase shift around the loop
and thus suppresses oscillations at all frequencies other than 10 MHz. It should be noted that any
reactance in series with the crystal will cause a change in frequency.