The impulse modulator. Initial studies in a strobe and negative feed-back method applied to the design of an amplitude modulator of great linearity

Abstract

A feed-back circuit is discussed wherein the anode current flowing in a valve is made almost exactly proportional to its anode potential. Sharp pulses at r.f. are now applied suitably so that the valve becomes intermittently cut-off. The amplitude of each anode current pulse is proportional to the instantaneous anode potential which is controlled by the modulating signal. A resonant load in series with the valve extracts the modulated r.f. signal. A simple circuit elaboration is discussed which permits 100 per cent, depth without the necessity for residual carrier neutralization. Since the r.f. oscillator and the modulator are quite separate, there is zero frequency modulation.A circuit of the above type is treated both theoretically and experimentally, with special reference to harmonic distortion in the output carrier, modulation efficiency and linearity, and the effect of pulse mark/space ratio.A complete quantitative theory, enabling the degree of modulation linearity to be computed, has not yet been worked out but the paper gives a clear physical treatment of this matter and indicates how an estimate of the distortion may be made.The importance of this modulator lies in the fact that its linearity may be made arbitrarily high by processes which are physically very clear and not dependent on subtle aspects of valve characteristics. The efficiency falls as the linearity is increased but not to such an extent as to deprive the circuit of practical use for many purposes.Furthermore it seems probable that circuit constants can be adjusted so that fairly high modulation linearity can be achieved at high efficiency by cancellation of two opposing forms of distortion. This development is not pursued here.