Armstrong Oscillator using a crystal

Description: This circuit represents a variant of an Armstrong Oscillator. This specific example incorporates a crystal along with the LC tank circuit utilized in the previous Series-Fed Armstrong Oscillator. By definition, the Armstrong Oscillator employs a tickler coil for feedback from the tuned circuit, as illustrated in the schematic. This oscillator is of the shunt-fed type, utilizing a separate path for the supply voltage while bypassing the tank circuit. In this circuit version, a PNP transistor serves as the amplifier. Any PNP transistor can be used, provided it exhibits gain at the desired frequency of operation. Both the 2N3904 and 2N3906 PNP transistors are suitable for operation above 100 MHz. The critical criterion is that the transistor must provide amplification within the oscillation frequency range. Since a PNP transistor is used, the supply voltage (Vcc) is inverted compared to the previous circuit. The circuit operates at the frequency of the crystal, and changing the crystal alters the frequency. Transformer T1 functions as both an inductor in the tank circuit (T1 & C1) and as the tickler coil, providing feedback from the output circuit (collector) to the input circuit (base) of the transistor. The transformer windings are coupled together, delivering the regenerative feedback necessary for oscillation (180-degree phase shift). It is noteworthy that the crystal is in series with the tickler coil. The variable capacitor C1 allows for tuning of the tank circuit. Various styles and manufacturers of trimmer capacitors are available. The tank circuit serves as the frequency-determining component of the oscillator, with the oscillation frequency dictated by the resonant frequency (Fr) of the components in the tank circuit, calculated as 1 / (2 x π x √(LC)). Trimmer capacitor C1 enables frequency adjustments by varying the capacitance in the tuned circuit. Several DC bias resistors are employed in the circuit. The collector current is regulated by bias resistor Rc, which connects Vcc and bypasses the tank circuit directly to the collector of the transistor. Additionally, capacitor C2 blocks any DC voltage from the tank circuit, allowing only AC current to flow into it. Resistor Re serves as the emitter bias resistor, also known as self-bias. This resistor controls the DC current, while capacitor Ce shunts the AC current out of the circuit. Resistors Rb and Rf act as bias resistors for the base circuit, forming a voltage divider network between Vcc and ground. The base voltage is determined by the equation Vcc x (Rb / [Rf + Rb]). The tickler coil is also in parallel with resistor Rb, thus influencing the base voltage due to the oscillations of the tank circuit. The crystal is in series with the feedback coil, and its impedance varies with frequency, increasing as the frequency deviates. At the series resonant frequency, the crystal's impedance reaches its minimum. Much of the circuit's operation mirrors that of the previously discussed Armstrong oscillator. The base resistors establish the DC bias voltage for the base circuit. When power is applied, a small amount of base current flows through, forward biasing the transistor's base. With the base forward biased, the transistor activates, allowing collector current to flow.

The Armstrong Oscillator circuit exemplifies a classic design in oscillator technology, leveraging the unique properties of LC circuits and the feedback mechanisms provided by the tickler coil and crystal. The use of a PNP transistor as the active element is critical, as it allows for the necessary gain and phase inversion required for sustained oscillation. The design emphasizes stability and frequency accuracy, crucial for applications in radio frequency (RF) transmission and signal generation.

The tank circuit, composed of the inductor (T1) and capacitor (C1), is pivotal in establishing the desired resonant frequency. The tuning capability provided by C1 allows for fine adjustments, enabling the oscillator to lock onto specific frequencies dictated by the crystal. The interaction between the crystal and the tickler coil is essential for maintaining oscillation, as it ensures that the feedback remains within the required phase shift to sustain the oscillatory state.

The biasing network comprising resistors Rc, Rb, and Rf is designed to ensure proper operating conditions for the transistor, allowing for reliable performance across varying temperatures and supply voltages. Capacitors C2 and Ce play crucial roles in isolating the AC and DC components of the circuit, ensuring that the oscillations are not dampened by DC biasing effects.

Overall, this circuit exemplifies the fundamental principles of oscillator design, integrating feedback mechanisms, frequency determination, and transistor amplification to achieve stable and tunable oscillations.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. 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.