Syllabus Sections:-


4b.1 Understand the function of the components in typical VFO and crystal oscillators.

a crystal oscillator

the variable oscillator

The oscillators shown above in figs 1 & 2 are very similar. In effect, fig 1 shows a crystal oscillator, with the transistor being used as an amplifier with POSITIVE feedback (provided by C2 and C3) at one particular frequency (determined by the frequency for which the crystal has been cut). In Fig 2, the crystal has been replaced with a parallel tuned circuit (consisting of the inductor L1 and C1). This achieves exactly the same result, giving the advantage that either the capacitor or the inductor value may be varied to achieve a change in frequency.

The crystal has a much higher "Q" value than the tuned circuit, and as a result, although the crystal frequency cannot practically be changed, it has the advantage that the output is very stable, being only slightly affected by changes in temperature etc.

Note the use of C6 in the variable oscillator. The function of this is to couple the tuned circuit to the transistor, whilst preventing the DC voltage on the base of the transistor being short circuited through the inductor. (The crystal does not pass DC, and hence this capacitor is not required in the crystal oscillator).

C4 is provided to ensure that:-

  1. Any signals appearing on the voltage supply line do not get into the oscillator as they will safely pass to earth through the capacitor.

  2. C4 also ensures that signals generated by the oscillator do not get onto the voltage supply line.

C5 couples the oscillator output signal to the next stage, whilst preventing the DC voltage on the emitter of the transistor being passed onward as well.

The resistors in the circuits are used to set the DC operation of the transistor. (i.e. biasing the transistor for use as an amplifier).

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