Harmonic Generation by Means of Grid-Circuit Distortion

The usual vacuum tube harmonic generators are fundamentally high distortion amplifiers operated under conditions of grid bias and plate voltage which, to distort the input voltage wave, utilize the non-linear relation between plate voltage and plate current. This paper gives the result of an investigation into the possibilities of using the non-linear grid voltage-current characteristic to produce harmonics. Grid distortion can be utilized by operating the tube so that the grid goes positive for a part of each fundamental frequency cycle and then inserting in series with the grid lead an impedance across which the grid current impulses produce a voltage drop rich in harmonics, which is then amplified in the plate circuit. The grid circuit distortion harmonic generator has the advantages of producing the harmonic in one circuit and developing the power in another, combined with the very high distortion properties of the grid circuit. The result is that the grid-distortion harmonic generator develops from 100 to over 1,000 times as much power on the higher harmonics as does the usual plate dist rtion generator. Thus, when using a suitable parallel resonant circuit as the distorting grid impedance, a steady output of 135 volts effective has been obtained on the 22nd harmonic from a radio receiving tube. When the grid impedance consists of a large inductance it is possible to generate simultaneously at least 1/20th milliwatts of power on every harmonic up to the 50th.