Some aspects of the design of balanced rectifier modulators for precision applications

Abstract

The design of balanced rectifier modulators of ring and Cowan types is discussed, and it is shown how the performance of the modulator depends on rectifier characteristics, the circuit impedance in which the modulator operates, the resistance of the carrier generator and the carrier voltage.The main performance features discussed are efficiency, stability, production of unwanted modulation products, impedance, and carrier leak.It is shown that there are advantages in designing a modulator for maximum efficiency by choosing a circuit impedance of optimum value and a carrier supply of high resistance. Variations in efficiency due to variations in carrier voltage, circuit impedance and temperature are then a minimum although it may be necessary to have each rectifier shunted with a constant resistance to achieve this. The production of modulation products of higher order is greatest in a modulator of maximum efficiency in which the circuit providing the carrier is of high resistance, and is reduced considerably if inefficient working and a low-resistance circuit are used. Further reduction, if desired, can be obtained by the use of a large bias on the rectifiers, and it has been found possible to reduce the 3fc±f sidebands to over 26 db below the fc + f sidebands (where fc = carrier frequency and f = signal frequency). This method is of value only in ring modulators, as with the Cowan circuit it introduces products of the type 2fc + f.It is shown that the ordinary potentiometer adjustment for carrier leak compensates only the unbalances between the low forward resistances of the rectifiers, and there remains an out-of-balance current at small values of the instantaneous carrier voltage. This effect can be reduced by using a high-resistance carrier supply and by appropriate selection of the rectifiers; a further improvement, in the case of ring modulators, can be obtained by the use of large biases on the rectifiers. Carrier leaks as low as − 60 db relative to 1 volt in 600 ohms, or 40 db below sideband-level, when high-level signals are applied, can readily be obtained and maintained in modulators with at least 1 volt carrier peak voltage across the rectifiers.It is assumed throughout that the rectifiers are purely resistive, as with the diodes and crystal valves now available, the effect of the capacitance is negligible up to frequencies of the order of 1 Mc/s.A method of design of a ring modulator is described in which the input impedance remains relatively constant over the cycle of carrier voltage; this may have important applications in precision circuits.