Relations of Carrier and Side-Bands in Radio Transmission

This paper discusses generally the characteristics of carrier transmission as applied in radio a n d in carrier current c o m m u n i c a t i o n over wires a n d analyses the factors which affect the faithfulness with which such systems reproduce the signals imparted to t h e m . M o d u l a t i o n is shown to generate t w o side b a n d s which, with respect to frequency, lie just a b o v e a n d just below the carrier frecpiency, the frequency w i d t h of each side b a n d being the same as the frequency width of the original signals. U p o n detection, currents of frequencies corresponding to the difference frequencies between all the possible pairs of component frequencies of the side b a n d s a n d carrier are produced a n d , in general, are all found in the received message. It is therefore impossible to transmit messages, either telephone or telegraph, by carrier which will be absolutely free of distortion, but since the a m p l i t u d e of a n y particular difference frequency is proportional to the product of the a m p l i t u d e s of its t w o generating frequencies the distortion can be reduced below a troublesome value by m a i n t a i n i n g the a m p l i t u d e of the original carrier sufficiently large with respect to the a m p l i t u d e s of the signal components. The distortion which arises from phase shifts between the component frequencies of the t r a n s m i t t e d message a n d carrier is also considered. The paper discusses single side-band transmission a n d carrier suppression w i t h h o m o d y n e detection a n d their various merits are pointed o u t .