Over modulated HF transmitter

Abstract

A pulsed HF, AM transmitter, utilizing extremely high levels of over modulation has been designed and tested. Over modulation of an AM, RF signal, produces a pulse whose frequency is the same as the applied audio modulation frequency. In its final form, the transmitter had an unmodulated carrier wave of 3.5 Watts with a modulated output of 45 Watts PEP into a 50 ohm load. Rise and fall times of the pulse envelope were approximately 1 microsecond each. Pulse rate capabilities were in excess of 160 kHz. Single frequency, square wave modulation produces no distortion of the audio when demodulated. Most, but not all, of the RF power in this transmitter is shifted to the side bands. The residual carrier eliminates the additional internal oscillator and mixer circuits necessary for side band receivers. The extreme levels of over modulation produce a demodulated wave of very high intensity which is not carrier wave power dependent. High harmonic levels, in conjunction with a variable audio input frequency, produces a wide spectrum of impedances which can be used in therapeutic applications. The output signal is extremely difficult to jam and has potential military applications. The transmitter has potential for PAM digital radio transmission in a spread spectrum mode.