Electronic Transformers Modulation


Posted on Feb 6, 2014

Wave-filter principles are applied in many circuits other than filters. An example is the design of modulation transformers for high-level amplitude-modulated radio transmitters. Some of these transformers are large. In plate-modulated power amplifiers, the modulator power required to produce 100 per cent modulation is half of the power amplifier


Electronic Transformers Modulation
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input. Improved audio quality and reduction in size of components are achieved through the use of what may be called the pi-filter method. For low modulation frequencies, this method may be illustrated by means of the circuit diagram of Fig. 145(a). The modulator usually is a class B amplifier. Output transformer OCL, coupling capacitor, and modulation reactor combine to form a pi-section high-pass filter, Fig. 145(b). The elements are proportioned for characteristic impedance equal to the equivalent plate input resistance EB/IB of the modulated power amplifier. Formerly these components were made as large as was considered practical. Transformer secondary and reactor reactances were each 3 to 4 times the power amplifier plate input resistance, and the coupling capacitor reactance was a fraction of this resistance, at the lowest modulation frequency. Advantages of the pi-filter are a reduction of the two inductive reactances to 1. 41 times the terminating load resistance, and increase in capacitive reactance to the same value, at a low frequency f1, which is 1. 41 times cut-off frequency of the filter. Down to f1 the filter maintains a tube load of almost 100 per cent power factor, although the ohmic value rises to 190 per cent of the terminating load resistance at f1. The voltage required for constant output rises to 138 per cent of normal. Partly compensating for this defect is the tendency of class B amplifier tubes to...




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