Theory and Application of Resistance Tuning

The oscillation frequency of a resonant circuit can be adjusted over a wide range by inserting resistance in series with one or both of its capacitive and inductive arms. Series resistance in the capacitive arm tends to reduce the effective shunt capacitance and thereby to increase the frequency while series resistance in the inductive arm tends to increase the effective shunt inductance and thereby to decrease the frequency. A large variation in frequency requires so much resistance that the wave form becomes distorted. The nonlinear factors which influence the frequency are outlined in theory. Tests conducted with a pentode oscillator based on the reverse transconductance from the suppressor to the screen show that if good wave form is not essential a frequency variation in the ratio of 50 to 1 is possible. The simultaneous opposite variation of resistance in both arms permits a frequency variation of 20 to 1 with some distortion in wave form or 1.5 to 1 with very little distortion and uniform amplitude. In the latter case a linear relationship between resistance and frequency is possible. This device is useful in any applications where it is desired to convert a resistance variation to a frequency variation.