Vackar Oscillator Circuit

L2 is an RFC with a self-resonant frequency above the design frequency of the oscillator, it could be replaced with a resistor or an RFC in parallel with a resistor.

Cₐ has a very high value for an RF circuit and masks the FET drain capacitance and RFC self-capacitance resistance. It also limits the output voltage swing if the output is taken from the drain.

L1 is the tuning inductor and resonates with the combined capacitance of all the shown capacitors:

(Cₒ in parallel with (Cᵥ in series with C₉)) in series with Cₐ. A rough approximation is Cₒ in parallel with Cᵥ.

Cᵥ in series with C₉ forms a capacitor voltage divider circuit with a 7:1 ratio (100/(600+100)).

This creates a 49:1 impedance match effectively hiding R1 and the FET gate capacitance from the turned circuit.

Cₒ is an additional tuning capacitance and could be made variable.

Overall the tuned circuit is highly decoupled from passive component parameters and the FET semiconductor capacitances making the frequency of oscillation very stable.

The FET can be replaced with a BJT transistor or MosFET with suitable circuit changes.

You can take the output from the FET drain or you can wind a few turns around L1 to extract a higher amplitude output.

Overall the Vackar oscillator circuit has:

• Very good stability: The Vackar oscillator is a stable oscillator that is not prone to frequency drift or other problems.

• Wide frequency range: The Vackar oscillator can typically cover a frequency range of 2.5 to 1 or more, which is significantly wider than the range of typical Colpitts and Clapp oscillators.

• Near constant output amplitude: The Vackar oscillator's output amplitude remains relatively constant over its frequency range, unlike other VCO designs that can exhibit significant variations in amplitude.

• Easy to construct: The Vackar oscillator is a relatively simple circuit that is easy to construct using readily available components.

• Is a good choice for regenerative receiver design.