An Inductively Coupled Frequency Modulator

The frequency modulator described here allows the engineer to design a frequency-modulated oscillator that has more power output and will operate on a higher fundamental frequency than conventional modulators in use at present. It consists of an inductance, capacitance, and resistance in parallel coupled inductively to the frequency-controlling circuit of the self-excited oscillator. Variations in the parallel resistance refect reactance and resistance changes into the oscillator inductance thereby causing frequency variations. In the experimental setup used, the parallel resistance is the plate circuit of a 6F6 tube. Variations in the alternating-current plate resistance of the 6F6 tube are accentuated by the proper placement of a resistor in the plate circuit. Calculated curves are given showing the variations of the introduced reactance and resistance as the parallel resistance is varied. Experimental curves are given showing the performance of the system at 2.5 megacycles.