Phasitron tube FM transmitter

A phasitron tube consists of a cathode, an electrostatic focus and deflection system, and an anode structure. The electrons, drawn from the cathode surface to the anode assembly are acted on by the focus elements to form a tapered, thin edge disk, whose axis is the cathode and whose focus is at anode .1 of the anode assembly. The deflection system has 36 rigidly mounted elements whose active portions lie in a radial plane below the electron dish and a solid neutral plane located above the disk. Every third deflector is connected together and to one phase of a three- phase excitation source. The three-phase potential source has crystal oscillator and phase-splitting network. The action of the deflection system is such that portions of the electron disk are deflected above or below the normal plane by the magnetic field of the three-phase system.

Phasitron Tube FM Transmitter
Phasitron Tube FM Transmitter

A phasitron tube consists of a cathode, an electrostatic focus and deflection system, and an anode structure. The electrons, drawn from the cathode surface to the anode assembly are acted on by the focus elements to form a tapered, thin edge disk, whose axis is the cathode deflection system has 36 rigidly mounted elements whose active portions lie in a redial plane below the electrons dish and a solid neutral plane located above the disk. Every third deflector is connected together and to one phase of a three phase excitation source. The three phase potential source has a crystal oscillator and phase splitting network. The action of the deflection system is such that portions of the electron disk are deflected above or below the normal plane by the magnetic field of the three phase system.

The disk is rotated at a rate determined by the crystal oscillator.

Phasitron Tube FM Transmitter Anode Development Section
Phasitron Tube FM Transmitter Anode Development Section

Anode 1 is a cylinder with 24 holes punched alternately above and below normal plane of disk. Electrons striking the surface of the cylinder are collected by it, while those which pass through the holes are collected by the solid anode 2. Fig 2 shows a developed section of anode 1. The solid line curve represents the edge of the electron disc at the time the maximum number of electrons passes through the openings to anode 2. The dotted curve indicates the situation after one-half cycle, and almost no electrons pass through the openings to anode 2. If, therefore, the two anodes are connected to opposite ends of a resonant circuit, the circuit get excited at the crystal frequency and in a time-phase, since that is determined by the phase of the anode current pulses.

Frequency modulation of the resonant anode circuit is obtained by phase modulation of the electron disk. A schematic of the phasitron FM transmitter is shown in fig. 1. Phase modulation is obtained by applying the audio signal to a solenoid which surrounds the phasitron. The axial magnetic field that is so produced causes the electron disk to be advanced or retarded about its axis relative to its zero signal position consequently, the phase of the oscillator gets shifted, with a resultant production of PM waves. Moreover, since the magnetic field is produced by a solenoid which is essentially a pure reactance at audio frequencies, then for a constant potential input, the current and hence magnetic field that is produced, varies inversely with the frequency of the impressed voltage. Thus, the output form the oscillator is on FM wave. A deviation of 175 Hz is possible with this system, requiring a multiplication of 424 times to produce 75 KHz deflection.