A wide-band oscillator using a conical-helix tuning inductor

Abstract

The paper describes a wide-band oscillator (frequency variation, 1:45) using a single tank circuit between the grid and the anode of a triode. The frequency control is made flexible by the use of conical helices (instead of the usual cylindrical ones) as inductance. Two conical helices connected at their apices by a large capacitor are connected to a pair of variable cylindrical capacitors. For the high-frequency end, the leads connecting the coils to the grid and the anode are used as transmission lines, the main tuning circuit being automatically isolated. The use of the conical form of inductance makes the transition from the transmission-line mode of operation to LC-mode of operation and vice versa smooth, and also keeps the undesirable coupling between the used and unused portions of the inductance small. Methods of calculating the oscillation frequency in terms of the circuit parameters are described. In an experimental set tested conical helices of seven turns each were used (diameter of the largest turn, 6.0 cm) and the components were so mounted that by turning a single control the frequency could be varied from 10 to 450 Mc/s. The speed of variation of the frequency with the turn of the control can be regulated by adjusting suitably the geometry of the tank circuit (coil and capacitor). The output power can be smoothed to any desired extent throughout the range by means of a variable output coupling.