The Two Methods of Amplitude Modulation and how They Work
Methods of amplitude modulation can be put in the two categories namely Linear modulation methods and Square law modulation methods. Linear modulation method utilizes the linear region of the current voltage characteristics of the amplifying device that is transistor or electron tube. Square law modulation method utilizes the square law region of some current voltage characteristics of a diode or transistor or electron tube. A large number of linear modulation methods have been devised and have been used to varying degree. These methods are namely linear shunt plate modulation or anode choke modulation, linear series plate modulation, grid bias modulation, cathode modulation, suppressor grid modulation, screen grid modulation, collector modulation.
Square law modulation circuits make use of non linear current voltage characteristics of diodes or triodes and are in general suited for use at low voltages. Important square law modulation methods are square law diode modulation and balanced modulator.
In linear shunt plate modulation the modulation voltage is inserted in the plate circuit of a class C amplifier tuned to the carrier frequency. The plate currents of modulated amplifier tube and the modulating amplifier tube find parallel paths or shunt paths. The carrier voltage is applied at the input of the class C modulated amplifier. Linear series plate modulation makes use of a linear class C tunes amplifier as modulated amplifier. This tuned amplifier is so designed that a linear relation exists between the root mean square value of peak tank current and the plate supply voltage for constant grid excitation. The modulating voltage appears in series with the plate supply voltage of the tuned modulated amplifier.
The merits of linear series plate modulation are small distortion, high plate circuit efficiency and easy adjustments. Grid bias modulation uses a class C amplifier in which audio modulated voltage is placed in series with the fixed grid bias. In Cathode modulation the modulation voltage is introduced in the cathode circuit of the class C modulator amplifier. It has characteristics intermediate between those of grid bias modulation and plate modulation.
Suppressor grid modulation uses a pent code in a class C modulated amplifier wherein the modulating voltage is applied to the suppressor which is biased sufficiently negatively. It provides linear modulation up to eighty percent modulation depth. Screen grid modulation is applied at the screen grid. The distortion produced is low. In the collector modulation method the modulating voltage is applied in the collector circuit of tuned class C amplifier in push pull arrangement. It has the merits of high linearity of modulation, high collector circuit efficiency and high power output per transistor.
Square law modulation methods makes use of non linear current voltage characteristics of diodes or triodes and are suited only for use at low voltages. Square law diode modulation uses non linear region of the current voltage dynamic characteristics of a diode
Tymon Hytem has worked in the electronics feild for the past 15 years. He enjoys helping people decide on electronic gadgets from telephones to XM Radio and choosing the perfect XM Satellite Radio system for their needs.