EXPERIMENT NUMBER 1
Amplitude Modulation & Demodulation
AIM: To perform amplitude modulation and demodulation and calculate the modulation index for various
modulating voltages and plot the relevant waveforms.
LEARNING OBJECTIVE:
To provide a familiarization to generate an amplitude-modulated (AM) signal, with an adjustable
modulation factor (m).
To examine both time & frequency displays of an AM signal.
To measure the percentage modulation (m %), and the percentage of total power in both sidebands and
in the carrier versus the modulation index (m).
To investigate the use (& limitation) of envelope detection in demodulating AM signals.
PRIOR CONCEPTS: Modulation and its types, Frequency and Amplitude varia nce, Sampling Theorem.
EQUIPMENT REQUIRED COMPONENTS REQUIRED
THEORY: Modulation is defined as the process by which some characteristics of a carrier signal is varied in
accordance with a modulating signal. The base band signal is referred to as the modulating signal and the output of the
modulation process is called as the modulated signal. Amplitude modulation is defined as the process in which is the
amplitude of the sinusoidal carrier wave is varied in accordance with the base band signal. The envelope of the
modulating wave has the same shape as the base band signal provided the following two requirements are satisfied.
1. The carrier frequency fc must be much greater than the highest frequency components fm of the
message signal m (t) i.e. fC>> fm.
2. The modulation index must be less than unity. If the modulation index is greater than unity, the
carrier wave becomes over modulated.
PROCEDURE
1. Connections are made as shown in the circuit diagram.
2. The power supply is connected to the collector of the transistor.
3. Modulated Output is taken from the collector of the transistor.
4. Carrier signal is set to 3.2V (p-p), 10 KHz using function generator.
5. Modulating signal is set to around 3.2V (p-p), 1 KHz (with a +2V dc - offset) and amplitude is varied
around the carrier voltage.
6. Calculate Fmax and Fmin from the output waveform.
7. Calculate the modulation index using the formula
