Abstract :
The paper is concerned with the development of a modulator capable of giving much lower stable levels of carrier leak than previously obtained. After consideration of various types of modulator, the conclusion is reached that the constant-current modulator, originally proposed by Cooper, is probably capable of giving the highest stable level of carrier rejection. This modulator consists, essentially, of a ring modulator whose input transformer is, ideally, replaced by infinite-impedance valve generators feeding each rectifier input terminal. The action of the constant-current modulator is analysed in detail on the basis of small differences in the characteristics of the rectifying elements, and associated topics are discussed. Experimental evidence, obtained using a carrier frequency of 3 kc/s, is given which, to a considerable degree, supports the theoretical analysis. The modulator, thus developed, is capable of giving carrier leak levels more than 90 dB below the carrier current from each source which are stable with temperature and time. Also, the action of the modulator is such that the fundamental component of the carrier leak is not affected by relatively large unbalances in the carrier currents from each source.
