Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

.let472 FREQUENCY MODULATION, Volume IConsider a pair of parallel plates across which an alternatingpotential difference is applied, resulting in an electric field of amplitudeE and angular frequency a between the capacitor plates shown inFigure 3(a). The uniform magnetic field is in the direction shown.An electron is projected into the field between the plates along the lineab parallel to the magnetic field. Its velocity in this direction willremain constant so that at time t = the electron will be at position 1,and at time At seconds later it will be at position 2, etc. At position 1the electron has no velocity in the direction of E, but on encounteringthe electric field E between positions 1 and 2, it will be accelerated inthis instantaneous direction.If the frequency of the oscillating electricfield „ the electron would remainin phase with the electric fieldand would be continuously acceleratedso that it would receive more and more energy from the oscillating fieldand its motion would be a spiral of ever-increasing radius. This conditionwould produce an electron current in phase with E at allpositions1, 2, • • • andof current.would constitute a loading or resistive componentIf the capacitor plates were to form a part of a resonantcircuit, this "in-phase component" would have no effect on the frequencybut would reduce the Q of the circuit.The transit time of the electron through the capacitor platesthe relative values of « and a> ccurrent isandcan be chosen so that the net electronnot in the direction of E but in quadrature with E, so thatit constitutes a purely reactive current. To see how this comes about,o> cbe somewhat less than