Armstrong Oscillator using a crystal

Posted on Feb 4, 2014

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-

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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...

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