A method and arrangement for digital transmission with amplitude-modulated transmitters with modulation transformer

摘要

Amplitude-modulated radio broadcasting transmitters operating in the long, medium and short-wave range can be used for digital transmission provided they comprise a modulator which transmits a direct component (mean envelope signal). AM-transmitters with modulation transformers, which remain widely used worldwide, cannot be converted to digital transmission because the modulation transformers do not transmit a direct component and their lower cut-off frequency of approximately 30 hertz is far too high. According to the invention, digital transmission using AM-transmitters with modulation transformers is made possible by transmitting the mean envelope signal by constant adjustment to the dc voltage of the transmitter output stage and compensating the lower cut-off frequency of the modulation transformer by means of an adapted transmission network. The same method can be used to compensate the pre-stages of the modulation amplifier by using, for each stage, a transmission network with a low frequency response. Alternatively it is also possible to replace the low-output pre-stages with dc-coupled amplifiers.