Subject: Principles of Communication Engineering

Difficulty : Medium

Marks : 05

• The generating circuits for AM wave are called as amplitude modulator circuits.

• The modulator circuits are classified into two categories:

1. Low level modulation.

2. High level modulation.

Low level modulation: The block diagram of a low level AM transmitter is shown below.

• The RF crystal oscillator is most stable mechanical device suitable to generate high frequency carrier signal of particular frequency.

• The amplified modulating signal is applied to the modulator signal along with carrier. At the output of the modulator we get the AM wave.

• This AM signal is then amplified using a chain of linear amplifiers to raise its power level.

• The linear amplifiers can be class A, AB or B type amplifiers. The linear amplifiers are used in order to avoid the waveform distortion in AM wave.

• The amplitude modulated signal is then transmitted using transmitting antenna.

• The transistorized modulator circuits can be used for low level modulator due to the low power which is to be handled.

• The low level transmitter circuit does not require a large AF modulator power so its design is simplified.

• However the overall efficiency is much lower compared to high level modulation. This is due to the use of less efficient linear amplifiers.

High level modulation:

• Here the carrier generated by the stabilized crystal oscillator is first amplified to the adequate power level using class C RF amplifiers.

• The modulating signal also is amplified to a high power level before modulation takes place.If we want 100% modulation then the power of modulating signal must be 33% of the total power.So if 1500 W total power is to be transmitted, the modulating power will be 500 W.This highlights the need to amplify the modulating signal to an adequate power level.

• The modulation takes place in the last class C RF amplifier.The modulator output is Am wave which can be directly transmitted.
• The collector modulated transistorized circuit or plate modulated vacuum tube modulator is used as modulator stage.
• The advantage of high level modulation is its high due to the use of highly efficient efficiency class C amplifiers.
• The disadvantage is that a large AF power amplifier is needed to raise the modulating signal to the adequate power level.r