Analysis of diode modulators having frequency-selective terminations using computers

Frequency translators or modulators using passive non-linear resistances give rise to problems when connected in cascade with frequency-dependent networks due to unwanted multiple-modulation products which must be suppressed by filters. The dimensioning is discussed of networks preceding and following a four-terminal modulator in order to obtain a specific frequency response in the pass-band whilst retaining the requirements for unwanted modulation products at the input and output. The network is defined in terms of differential mesh equations that may ultimately be solved as a differential equation of the nth degree. The solution is simple if the network contains only time-invariant components but no systematic synthesis is known for the steady-state time-variant networks as the number of unknowns and equations becomes infinite. Practical experience has proved, however, that valuable results could be obtained with as few as five equations. Examples have shown that the loss in the pass-band of a modulator may be reduced almost to zero by keeping the ohmic loss in the diodes very small and by giving the pass-band of the filter conjugated-complex characteristics. The transmission-curve ripple can in all cases be held within the required limits through appropriate filter dimensioning. Evaluation of the theoretical relations for practical analysis and design is possible only with the aid of computers.