Bi-carrier modulator

Abstract From the Canadian patent

To minimize carrier leak in the modulation of voice frequencies to the desired line frequency for a carrier system a, first balanced modulator is operated at a low frequency where low carrier-leak modulators may be constructed and a second balanced modulator, directly coupled to the first is employed The, modulation frequency of said second modulator is very high compared to that of said low-frequency modulator. The sideband obtained from the second modulation stage is remote from said high-frequency carrier and a band-pass filter selects the desired sideband and substantially eliminates the high-frequency carrier and the other sideband. Thus, only a low amplitude of carrier leak is present …