Methods for generating Amplitude Modulation

Posted on Feb 6, 2014

Amplitude Modulation (AKA AM) was the first modulation type to impress audio on an RF carrier. Prior to this, information was transmitted via on/off keying of a continuous wave transmitter using Morse code or some equivalent. 1. Low level modulation. The modulation is developed in the first stage RF section, then amplified by subsequent stages to

Methods for generating Amplitude Modulation
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full power. Simple and easy to implement, especially for mobile transmitters and SSB installations. Disadvantages come from the need for linear amplification through all the stages requiring class A or AB amplifiers and does not reproduce wide band AM well. 2. Doherty modulation. William Doherty came up with an ingenious way to use a low level linear modulator with good to excellent efficiency. Under full carrier, no modulation conditions, the carrier tube is generating the RF carrier and the peak tube is mostly cut off (very little current). When modulation is applied, the peak tube then begins to conduct, the output of this tube is combined with the output of the carrier tube through a 90 ° LC network, which is the same as 1/4 wave length transmission line. The effect of this is to lower the output impedance, thus allowing the carrier tube to modulate 100 percent. 3. High level or plate modulation. The RF and Audio sections are developed separately with in the transmitter, then combined in the final stage of the transmitter. Older systems used a modulation transformer. Advantages are all the amplifiers can be run class C or greater, which reduced electrical consumption and power supply requirements. Much higher power levels are achievable with this design. These transmitters also reproduce wide band audio much better than low level modulated units. They are also extremely rugged. Disadvantages are the system requires...

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