Armstrong oscillator (the figure below) uses the series-tuned mode of
operation. It works much the same as the Hartley oscillator except that
frequency stability is improved by the crystal (in the feedback path).
To operate the oscillator at different frequencies, you simply change crystals (each crystal operates at a different frequency).
Crystal controlled Armstrong oscillator.
Variable capacitor C1 makes the circuit tunable to the selected crystal frequency. C1 is capable of tuning to a wide band of selected crystal frequencies. Regenerative feedback from the collector to base is through the mutual inductance between the transformer windings of T1. This provides the necessary 180-degree phase shift for the feedback signal. Resistors R B, RF, and RC provide the base and collector bias voltage. Capacitor CE bypasses ac variations around emitter resistor RE.
frequencies above and below the series-resonant frequency of the
selected crystal, the impedance of the crystal increases and reduces the
amount of feedback signal. This, in turn, prevents oscillations at
frequencies other than the series-resonant frequency.
Crystal-Controlled Pierce Oscillator
The crystal-controlled PIERCE OSCILLATOR uses a crystal unit as a parallel-resonant circuit. The Pierce oscillator is a modified Colpitts oscillator. They operate in the same way except that the crystal unit replaces the parallel-resonant circuit of the Colpitts.
The figure below shows the common-base configuration of the Pierce oscillator. Feedback is supplied from the collector to the emitter through capacitor C1. Resistors RB, RC, and RF provide the proper bias conditions for the circuit and resistor RE is the emitter resistor. Capacitors C1 and CE form a voltage divider connected across the output.
Since no phase shift occurs in the common-base circuit, capacitor C1 feeds back a portion of the output signal to the emitter without a phase shift. The oscillating frequency is determined not only by the crystal but also by the parallel capacitance caused by capacitors C1 and CE. This parallel capacitance affects the oscillator frequency by lowering it. Any change in capacitance of either C1 or CE changes the frequency of the oscillator.
Pierce oscillator common base configuration.
The figure below shows the common-emitter configuration of the Pierce oscillator. The resistors in the circuit provide the proper bias and stabilization conditions. The crystal unit and capacitors C1 and C2 determine the output frequency of the oscillator. The signal developed at the junction between Y1 and C1 is 180 degrees out of phase with the signal at the junction between Y1 and C2. Therefore, the signal at the Y1-C1 junction can be coupled back to the base of Q1 as a regenerative feedback signal to sustain oscillations.
Pierce oscillator common emitter configuration.