Communications Systems - Modulation (Amplitude Modulation)

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COMMUNICATIONS SYSTEMSREVIEWER

MODULATION Amplitude ModulationPrepared by: Mark B. Salgado, ECE

Modulation

What is Modulation?

Modulation can be defined as the transformation of superimposition of the signal containing the information on a highModulating frequency Modulation If we transmit a audio carrier.frequency of 4KhzVoice Signal or Baseband Signal

Signal

Modulated Signal

Analog Radio

Low frequencyCarrier Signal

Using a typical carrier frequency of 100Mhz for FM

High frequency

Analog Modulation

Two types of Analog Communication through Modulation: Amplitude Modulation Frequency ModulationAmplitude Modulation Carrier

Modulating signal Frequency Modulation (Baseband)

Amplitude Modulation

What is Amplitude Modulation? Is defined as a system of modulation in which the amplitude of the carrier is made proportional to the instantaneous amplitude of the modulating voltage.wc = 2fcec = Ec sin wct ec = Instantaneous Carrier Signal Ec = Carrier Voltage (Amplitude), V fc = Carrie Frequency (Hz): w c/2 Sin (wct) = Sinusoidal waveform: rads/sec em = Instantaneous Modulating Signal Em = Modulating Voltage (Amplitude), V fm = Modulating Frequency (Hz): w m/2 Sin (wmt) = Sinusoidal waveform: rads/sec

wm = 2fmem = Em sin wmt

Amplitude Modulation

What is Amplitude Modulation? Amplitude Modulation is the simplest and earliest form of transmitters. AM applications include broadcasting in medium- and high-frequency applications, CB radio, and aircraft communications.

Amplitude Modulation

Analog Modulation Characteristics: The information signal varies the instantaneous amplitude of the carrier. Amplitude Modulation is a nonlinear process. Sum and difference frequencies are created that carry the information.

Amplitude Modulation

Amplitude Modulation:E

The Phase angle of is unchanging in Amplitude Modulation. The Amplitude (Voltage) varies along the process

Em Ec A

Emsin Wmt

tfSB = fc nfm

fSB = sideband frequency fc = carrier frequency fm = modulating frequencym = modulating index Ec = Carrier Voltage, V Em = Modulating Voltage, V

m = Em/Ec

Amplitude Modulation

Modulating Index:

In order for proper AM to occur, the modulating signal voltage Em must be less than the carrier voltage Ec. Modulation index is also called modulation factor, modulation coefficient , or the degree of modulation. The modulation index should be a number between 0 and 1.

If: m = 1 perfectly modulated; m = 0 the signal is a constant amplitude carrier; m = < 0 under modulation and m = > 1 over modulation

Amplitude Modulation

Percentage Modulation Index:

Amplitude Modulation

Amplitude Modulation Equation:E e = A sin , thus e = A sin wctAmplitude Modulation Equation: Em Emsin Wmt e = Ec sinwct + (mEc/2) cos(wc-wm)t (mEc/2) cos(wc+wm)t

m = Em/Ec

Ec

A

e = Ec (1 + m sinwmt) sin wct Using trigonometric t function of: sinxsiny = [cos (x-y) cos (x+y)]

A = E c + em A = Ec + Em sin wmt A = Ec + mEc sin wmt A = Ec (1 + m sinwmt)

Amplitude Modulation

AM Waveform:Amplitude Modulation Equation: e = Ec sinwct + (mEc/2) cos(wc-wm)t (mEc/2) cos(wc+wm)t

Modulating Signal

Carrier

Amplitude Modulation

AM Waveform:Amplitude Modulation Equation: e = Ec sinwct + (mEc/2) cos(wc-wm)t (mEc/2) cos(wc+wm)t Carrier Signal: Ec sinwct Lower Sideband Signal: (mEc/2) cos(wcwm)t Upper Sideband Signal: (mEc/2) cos(wc+wm)t

LSB

USB

B 2 Fmfm fm

f usb f c f m f lsb f c f m E lsb E usb mE c 2

fc - f m

fc

fc + fm

Amplitude Modulation

AM Waveform:

m = Vmax Vmin Modulating index for Vmax + Vmin multiple modulating frequencies:

m m1 m2 2 2

Amplitude Modulation

Power Relation in AM: Power in a transmitter is important, but the most important power measurement is that of the portion that transmits the information. AM carriers remain unchanged with modulation and therefore are wasteful.P = E2/R; where: P = Power, Watts (W); for Using the AM Wave formula E = Voltage, (V) Power: 2 Pc Ohms R = Resistance,= Ec /R () PLSB = ELSB2/R Pt = E2carr/R + E2LSB/R + E2USB/R PUSB = EUSB2/R Pc = Carrier Power, W Pt = Total Power, W PLSB = Lower Sideband Power, W PUSB = Upper Sideband Power, W Ec = Carrier Voltage, V ELSB = Lower Sideband Voltage, V EUSB = Upper Sideband Voltage, V R = Antenna Resistance, Ohhms,

Amplitude Modulation

Power Relation in AM:Get the RMS Value: Pc = Ec2/R = Pc = (Ec/2)2 / R Pc Ec /2R Get=the2RMS Value: PLSB = PUSB = ESB2/R = [(mEc/2)2 / 2] / R = Substitute in the AM Power PLSB = PUSB Equation: = m2Ec2/8R Pt = Pc + PLSB + PUSB Pt = Ec2/2R + m2Ec2/8R + m2Ec2/8R Pt = Pc + (m2/4)Pc + (m2/4)Pc Pt = Pc(1+(m2/2)

ERMS = For maximum power divide by 2 of AM wave:Pt = 1.5Pc; where m=1

Pc = Carrier Power, W Pt = Total Power, W PLSB = Lower Sideband Power, W PUSB = Upper Sideband Power, W Ec = Carrier Voltage, V ELSB = Lower Sideband Voltage, V EUSB = Upper Sideband Voltage, V R = Antenna Resistance, Ohhms,