The Heising method was the first practical method of generating an Amplitude Modulated radio signal. This method used the application of an audio signal superimposed on the anode supply voltage of a class-C RF amplifier.
Outphasing modulation was invented by Henry Chireix in 1935. This method obtains an AM signal by the use of counter-phase modulation and vector addition of two radio frequency signals. It was marketed by RCA under the trade name "Ampliphase."
One of the first, workable, solid state modulation techniques for very high power use for Low, Medium and Shortwave transmitters running from 100 kw to 600 kw. In this type, the modulation steps are broken up into 24 modules. In this method, low voltage power supplies are stacked in series and switched into and out of operation to generate an anode modulation voltage. To further refine the linearity, each power supply step is pulse-width modulated, creating a smooth transition from one step to the next. Numbering the modules from 1-28, the low numbered modules handle the lower levels of modulation. This would mean that the lower the level of modulation the unit handles, the more it is used. However, the PSM controller attempts to distribute the usage among the vaious levels by rotating the units through different levels of modulation levels. This gives longer overall life of each individual unit.
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Graphic illustration of how output envelope wave (blue) is produced by pulse-step input (yellow). |
