Analysis of Load-Impedance Modulation

A new type of modulation for television transmitters which was proposed recently is investigated. The relations referring to the operation of the network, of the modulator, and of the radio-frequency amplifier tubes are derived as well as the expressions for the sideband attenuation characteristic. It is concluded from the results that the claims made for the new modulation system are exaggerated. The modlator tubes are required to have a very high emission. This fact will limit the application of the new type of modulation to modulator tubes having thoriated-tungsten filaments and consequently to relatively small power, since at present tubes of this type are built for low or medium output power only. The efficiency of the new modulation system is essentially the same as that of grid-bias modulation. Contrary to expectations, the band width is not materially greater than that which is attainable with grid-bias modulation. This is due to the sideband clipping taking place in the quarter-wave line connecting the modulator tubes with the remainder of the network. The modulation characteristic is straighter than is customarily obtainable with grid-bias modulation; this is achieved by working the modulator grids in the positive region only, a mode of operation which requires high driving power and modulator grids capable of dissipating that power. The output capacitance of the modulator tubes is substantially smaller than the output capacitance of a grid-bias-modulated amplifier, but so is also the damping effect contributed by the internal resistance of the modulator.