AMPLITUDE MODULATION(AM)

ObjectivesTo describe the principles of AMTo define and analyze the modulation index To analyze the spectral analysis and bandwidth calculationTo analyze the power distribution of AM

Lecture overviewPrinciples of amplitude modulation (AM)Modulation indexSpectral analysis and bandwidth calculationPower analysis of AM

Principles of AMDefinitions:The process of changing the amplitude of a relatively high frequency carrier signal in proportion with the instantaneous value of modulating signal (information)A process of translating information signal from low band frequency to high band frequency.

ContdInformation signal cannot travel far. It needs carrier signal of higher frequency for long distance destination.Inexpensive, low quality form of modulation

ContdAmplitude of the carrier signal varies with the information signal. The modulated signal consist of carrier signal, upper sideband and lower sideband signalsThe modulated AM signal (figure 1 & figure 2) needs to go through demodulation process to get back the information signal.

Contd

The AM EnvelopeAM double-sideband full carrier (AM DSBFC) is the most commonly used and the oldest and simplest form of AM modulation. Sometimes called conventional AM or simply AM.The outline of the positive and negative peaks of the carrier frequency re-create the exact shape of the modulating signal known as envelope.Note that the repetition rate of the envelope is equal to the frequency of the modulating signal.

The Generation of AM Envelope

AM Frequency Spectrum and BandwidthAn AM modulator is a non-linear device.Nonlinear mixing results in a complex output envelope consists of the carrier frequency and the sum (fc + fm) and difference (fc fm) frequencies (called cross-products).The cross-products are displaced from the carrier frequency by fm on both sides of it.AM modulated wave contains no frequency component of fm.

Frequency spectrum of an AM DSBFC Wave

Bandwidth (BW)The BW of an AM DSBFC wave is equal to the difference between the highest upper side frequency and lowest lower side frequency:BW = [fc + fm(max)] [fc fm(max)] = 2fm(max)For efficiency transmission the carrier and sidebands must be high enough to be propagated thru earths atmosphere.

Example 1For a conventional AM modulator with a carrier freq of fc = 100 kHz and the maximum modulating signal frequency of fm(max = 5 kHz, determine:Freq limits for the upper and lower sidebands.Bandwidth.Upper and lower side frequencies produced when the modulating signal is a single-freq 3-kHz tone.Draw the output freq spectrum.

Modulation Index and Percent of ModulationUsed to describe the amount of amplitude change (modulation) present in an AM waveform.Percentage modulation (%m) is simply the modulation index (m) stated as a percentage.More specifically percent modulation gives the percentage change in the amplitude of the output wave when the carrier is acted on by a modulating signal.

ContdMathematically, the modulation index is

And the percentage of modulation index ism = modulation indexEm = peak change in the amplitude output waveform (sum of voltages from upper and lower side frequencies)Ec = peak amplitude of the unmodulated carrier

Determining modulation index from Vmax and Vmin

ContdIf the modulating signal is a pure, single-freq sine wave and the process is symmetrical then the modulation index can be derived as follows:

Therefore,

ContdSince the peak change of modulated output wave Em is the sum of the usf and lsf voltages hence,

Then

Eusf = peak amplitude of the upperside frequency (volts)Elsf = peak amplitude of the lower side frequency (volts)

ContdFrom the modulated wave displayed in the previous slide, the maximum and minimum values of the envelope occurs at +Vmax = Ec + Eusb + Elsb +Vmin = Ec Eusb Elsb-Vmax = -Ec - Eusb - Elsb -Vmin = -Ec + Eusb + Elsb

Modulation Index for trapezoidal patternsModulation index, m can be calculated using the equation: m = Emax Emin/ Emax + Emin = Em / Ec = (A - B) / (A + B)

Contd

% modulation of AM DSBFC envelope

ContdFor proper AM operation, Ec > Em means that 0 m 1.If Ec < Em means that m > 1 leads to severe distortion of the modulate wave.If Vc = Vm the percentage of modulation index goes to 100%, means the maximum information signal is transmitted. In this case, Vmax = 2Vc and Vmin = 0.

Example 2Suppose that Vmax value read from the graticule on an oscilloscope screen is 4.6 divisions and Vmin is 0.7 divisions. Calculate the modulation index and percentage of modulation.

Example 3For the AM waveform shown in Figure below, determinePeak amplitude of the upper and lower side frequencies.Peak amplitude of the unmodulated carrier.Peak change in the amplitude of the envelope.Modulation index.Percent modulation.

AM Envelope for Example 3

The Mathematical Representation and Analysis of AM Representing both the modulating signal Vm(t) and the carrier signal Vc(t) in trigonometric functions.The AM DSBFC modulator must be able to produce mathematical multiplication of these two analog signals

ContdSubstituting Vm = mVc gives:Constant + mod. signalUnmodulated carrier

ContdThe constant in the first term produces the carrier freq while the sinusoidal component in the first term produces side bands frequencies

Upper side frequency signal (volts)Lower side frequency signal (volts)Carrier frequency signal (volts)

ContdFrom the equation it is obvious that the amplitude of the carrier is unaffected by the modulation process.The amplitude of the side frequencies depend on the both the carrier amplitude and modulation index.At 100% modulation the amplitudes of side frequencies are each equal to one-half the amplitude of the carrier.

Generation of AM DSBFC envelope showing the time-domain of the modulated wave, carrier&sideband signals

Voltage spectrum for an AM DSBFC wave

Example 4One input to a conventional AM modulator is a 500-kHz carrier with an amplitude of 20 Vp. The second input is a 10-kHz modulating signal that is of sufficient amplitude to cause a change in the output wave of 7.5 Vp. DetermineUpper and lower side frequencies.Modulation index and percentage modulation.Peak amplitude of the modulated carrier and the upper and lower side frequency voltages.Maximum and minimum amplitudes of the envelope.Expression for the modulated wave.

AM Power DistributionIn any electrical circuit, the power dissipated is equal to the voltage squared (rms) divided by the resistance.Mathematically power in unmodulated carrier isPc = carrier power (watts)Vc = peak carrier voltage (volts)R = load resistance i.e antenna (ohms)

ContdThe upper and lower sideband powers will be

Rearranging in terms of Pc,

ContdThe total power in an AM wave is

Substituting the sidebands powers in terms of PC yields

Since carrier power in modulated wave is the same as unmodulated wave, obviously power of the carrier is unaffected by modulation process.

Power spectrum for AM DSBFC wave with a single-frequency modulating signal

ContdWith 100% modulation the maximum power in both sidebands equals to one-half the carrier power.One of the most significant disadvantage of AM DSBFC is with m = 1, the efficiency of transmission is only 33.3% of the total transmitted signal. The less wasted in the carrier which brings no information signal.The advantage of DSBFC is the use of relatively simple, inexpensive demodulator circuits in the receiver.

Example 5For an AM DSCFC wave with a peak unmodulated carrier voltage Vc = 10 Vp, a load resistor of RL = 10 and m = 1, determinePowers of the carrier and the upper and lower sidebands.Total sideband power.Total power of the modulated wave.Draw the power spectrum.

Transmitter EfficiencyTransmitter efficiency, = average power from sideband/total power absorbed. = m/ ( 2+m )

Modulation by a complex information signalPrevious examples are all using a single frequency modulation signal. In practice, however, modulating signal is very often a complex waveform made up from many sine waves with different amplitudes and frequencies. Example: if a modulating signal contains three frequencies(fm1, fm2, fm3), the modulated signal will contain the carrier and three sets of side frequencies, spaced symmetrically about the carrier:

Contd..frequency spectrum for complex information signalfcFc-fm1Fc-fm2Fc-fm3Fc+fm1Fc+fm2Fc+fm3

Contd..modulation index for complex information signalWhen several frequencies simultaneously amplitude modulate a carrier, the combined coefficient of modulation is defined as:

mt=total modulation index/coefficient of modulationm1, m2, m3, mn= modulation index/coefficient of modulation for input 1, 2 ,3 , n

Contd..Power calculation for complex information signalThe combined coefficient of modulation can be used to determine the total sideband power and transmitted power, using:

Example 6For an AM DSBFC transmitter with an unmodulated carrier power, Pc= 100W that is modulated simultaneously by three modulating signals, with coefficients of modulation m1=0.2, m2= 0.4, m3=0.3, determine:

Total coefficient of modulationUpper and lower sideband powerTotal transmitted power

Low Level AM Transmitter

High Level AM Transmitter

At the end of this chapter, you should be ableTo describe the principles of AMTo define and analyze the modulation index To analyze the spectral analysis and bandwidth calculationTo analyze the power distribution of AM

*********************************************