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Crystal Controlled Armstrong Oscillator

This circuit represents another version of an Armstrong Oscillator. This particular example uses a crystal in addition to the LC tank circuit used by the previous Series-Fed Armstrong Oscillator. By definition the Armstrong Oscillator uses a tickler coil for feedback, from the tuned circuit, as seen in the schematic. This oscillator is the shunt-fed type, using a separate path for the supply voltage and bypassing the tank circuit.

Series Tuned Crystal Controlled Armstrong Oscillator circuit schematic
Crystal Controlled Armstrong Oscillator

This circuit version uses a PNP transistor as the amplifier. Virtually any PNP transistor may be used as long as it exhibits gain at the desired frequency of operation. Either a 2N3904 PNP Transistor or 2N3906 PNP Transistor may be used and both offer operation over 100MHz. The only important criteria is that the transistor provides amplification in the frequency range of oscillation. Because this is a PNP transistor, the supply voltage [Vcc] is reversed from the previous circuit.

The frequency of the circuit operates at the frequency of the crystal, changing the crystal changes the frequency. Refer to this page for Crystal Oscillator Vendors.

Transformer T1 operates as both an inductor in the tank circuit [T1 & C1] and the tickler coil, providing feedback from the output circuit [collector] to the input circuit [base] of the transistor. Of course the windings of the transformer are coupled together providing the regenerative feedback the circuit requires to oscillate [180 degree phase shift]. Note that the crystal is in series with the ticker coil. The variable capacitor C1 makes the tank circuit tunable. There are a number of possible styles of Trimmer Capacitors, and a number of different Manufacturers making Trimmer Capacitors. The tank circuit is the frequency determining component of the oscillator. The frequency of oscillation is determined by the resonant frequency [Fr] of the components in the tank circuit, which is 1 / (2 x 2.1415927 x [LC]1/2). Trimmer capacitor C1 provides adjustments to the frequency by varying the capacitance in the tuned circuit.

A number of DC bias resistors are used in the circuit. The collector current is set by bias resistor Rc. Vcc is connected to resistor Rc, and bypasses the tank circuit, connecting directly to the collector of the transistor. In addition, capacitor C2 blocks any DC voltage from the tank circuit, only AC current flows into the tank circuit.

Resistor Re is the emitter bias resistor, also called self-bias. However the resistor only control the DC current, the AC current is shunted out of the circuit by capacitor Ce. Resistor Rb and Rf are the bias resistors for the Base circuit. The resistors form a voltage divider network between Vcc and ground, and the base voltage is determined by Vcc x (Rb/[Rf + Rb]) However the tickler coil is also in parallel with resistor Rb. Therefore the base voltage is effected by the oscillations of the tank circuit. Of course the crystal is also in series with the feedback coil. The crystal also changes impedance with frequency, increasing as the frequency deviates up or down. However at the series resonant frequency the impedance of the crystal is at its lowest.

Much of the circuit operation is identical to the previous Armstrong oscillator discussed. The Base resistors set the DC bias voltage to the Base circuit. With power applied a small amount of base current will flow through and forward bias the base of the transistor. With the base of the transistor forward biased, the transistor is turned on and collector current flows.

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