written 6.9 years ago by | • modified 2.9 years ago |
Subject : Principle of Communication Engineering
Topic : Amplitude Modulation and Demodulation
Difficulty : High
written 6.9 years ago by | • modified 2.9 years ago |
Subject : Principle of Communication Engineering
Topic : Amplitude Modulation and Demodulation
Difficulty : High
written 6.7 years ago by |
The schematic diagram for a simplified medium-power transistor AM DSBFC modulator is shown in Fig1. The modulation takes place in the collector, which is the output element of the transistor.
Fig1. Schematic diagram of simplified medium power transistor AM DSBFC modulator
Class C amplifier is generally used by high and medium power AM modulators to achieve high power efficiency.
The Fig1 shows class C amplifier with two inputs: a carrier vc and a single frequency modulating signal vm. Class C amplifiers conduct for only a portion of the positive half cycle of the input carrier.
As the transistor is biased class C, it operates nonlinear and is capable of nonlinear mixing. Thus this circuit is also called as collector modulator because the modulating signal is applied directly to the collector.
The RFC component is a radio frequency choke that allows dc to pass and blocks high frequencies, thus isolating dc power supply from high frequency carrier and side frequencies, while still allowing low frequency modulating signals to modulate the collector of the transistor.
The modulator is a linear power amplifier that takes low level modulating signal and amplifies it to a high power level. The modulating output signal is coupled through modulation transformer T2 to class C amplifier. The secondary winding of the modulation transformer is connected in series with the collector supply voltage Vcc of the class C amplifier.
When the amplitude of the carrier exceeds the barrier potential (0.7 V for silicone transistor) of the base-emitter junction, Q1 turns on, and collector current flows. When the amplitude of the carrier drops below 0.7 V, Q1 turns off and collector current stops.
As a result, Q1 switches between saturation and cut off controlled by the carrier signal, collector current flows for less than 180° of each carrier cycle, and class C operation is achieved.
Each successive cycle of the carrier turns Q1 on for an instant and allows current to flow for a short time, producing negative-going waveform at the collector. The collector current and voltage waveforms are shown in Fig2 which resembles a repetitive half wave rectified signal.
With a zero modulation input signal, there is zero modulation voltage across the secondary of T2, the collector supply voltage is directly applied to the class C amplifier, and the output carrier is a steady sine wave.
Fig2. Collector waveforms with no modulating signal