Amplitude Modulation-Voltage Distribution

ANALOG AND DIGITAL COMMUNICATION Presentation by: KARTHIKA S. AP,ECE SSN College of Engineering

UNIT I FUNDAMENTALS OF ANALOG COMMUNICATION

Learning Objectives On completion of this session you will be able to:

• Understand AM voltage distribution

AM VOLTAGE DISTRIBUTION • Unmodulated carrier can be described mathematically as Vc(t) = Ec sin(2π f ct) • Instantaneous amplitude of the modulated wave can be expressed as Vam(t) = [Ec + Em sin(2π f mt)][sin(2π f ct)] • Em = m Ec, Vam(t) = [Ec + m Ec sin(2π f mt)][sin(2π f ct)]

AM VOLTAGE DISTRIBUTION • Factoring Ec, Vam(t) = [1 + m sin(2 π f mt)][Ec sin(2π f ct)] [1 + m sin(2π f mt)] – constant + modulating signal [Ec sin(2π f ct)] – unmodulated carrier

AM VOLTAGE DISTRIBUTION • Multiplying, Vam(t) = [Ec sin(2π f ct)] + [m sin(2π f mt)][Ec sin(2π f ct)]

• Therefore, Vam(t) = [Ec sin(2π f ct)] – (m Ec /2)cos[2π(f c+f m)t] + (m Ec /2)cos[2π(f c-f m)t]

AM VOLTAGE DISTRIBUTION • From the above equation, [Ec sin(2π f ct)] – carrier signal(volts) – (m Ec /2)cos[2π(f c+f m)t] - upper side freq (volts) + (m Ec /2)cos[2π(f c-f m)t] – lower side freq (volts)

AM VOLTAGE DISTRIBUTION From the AM voltage distribution analysis, several interesting characteristics can be noted 1. Amplitude of carrier is unaffected by modulation process 2.amp of upper and lower side freq depend on Ec and m

AM VOLTAGE DISTRIBUTION

PROBLEMS 1.

One input to a conventional AM modulator is a 500kHz carrier with an amplitude of 20 Vp. The second input is a 10kHz modulating signal that is of sufficient amplitude to cause a change in the output wave of 7.5 Vp. Determine (a) upper and lower side frequencies (b) modulation coefficient and percent modulation (c) peak amplitude of the modulated carrier and upper and lower side frequency voltages (d) maximum and minimum amplitudes of the envelope (e) expression for the modulated wave (f) draw the output spectrum and sketch the output envelope