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EENG 3810 Chapter 4

Amplitude Modulation

(AM)

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Chapter 4 Homework

1. For an AM DSBFC modulator with a carrier frequency

fc = 200KHz and a maximum modulating signal frequency fm(max) = 10 KHz, determine :

a. Frequency limits for the upper and lower sidebands.

b. Bandwidth.

b. Upper and lower side frequencies produced when the modulating signal is a single-frequency 6 KHz tone.

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Homework Continued

2. For the AM wave form above determine:

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Homework Continued

3. 400 2556

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Homework Continued

4. Repeat steps (a) through (d) in Example 4 in these lecture slides for a modulation coefficient of 0.5.

5. For an AM DSBFC wave with a peak unmodulated carrier voltage Vc = 20 Vp, a load resistance RL = 20 , and a modulation coefficient m = 0.8, determine the power of the modulated wave

Homework Continued

6.Determine the noise improvement for a receiver with an RF bandwidth equal to 100 KHz and an IF bandwidth equal to 20 KHz.

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Amplitude Modulation Transmission

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AM Generation

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Frequency Spectrum of An AM Double Sideband Full Carrier (DSBFC) Wave

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Example 1

For an AM DSBFC modulator with a carrier frequency

fc = 100KHz and a maximum modulating signal frequency fm(max) = 5 KHz, determine :

a. Frequency limits for the upper and lower sidebands.

b. Bandwidth.

c. Upper and lower side frequencies produced when the

modulating signal is a single-frequency 3 KHz tone.

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Example 1 Solution

a.

b.

c.

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Example 1 d. The Output Spectrum For An AM DSBFC Wave

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Phasor addition in an AM DSBFC envelope

• For a single-frequency modulating signal, am AM envelop is produced from the vector addition of the carrier and upper and lower side frequencies. Phasors of the carrier,

• The upper and lower frequencies combine and produce a resultant component that combines with the carrier component.

• Phasors for the carrier, upper and lower frequencies all rotate in the counterclockwise direction.

• The upper sideband frequency rotates faster than the carrier. (usf > c)

• The lower sideband frequency rotes slower than the carrier. (usf < c)

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Phasor addition in an AM DSBFC envelope

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Modulation Coefficient

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If the modulating signal is pure, single frequency sine wave and the modulation process is symmetrical, the % modulation can be derived as follows:

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Peak Amplitudes of Upper and Lower Sidebands

The peak change in amplitude of the output wave (Em) is equal to the sum of the voltages from the

upper and lower sideband frequencies. Therefore,

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Percent Modulation of An AM DSBFC Envelope (a) modulating signal; (b) unmodulated carrier; (c) 50% modulated wave;

(d) 100% modulated wave

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Example 2

For the AM wave form above determine:

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Example 2

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Voltage Spectrum for an AM DSBFC Wave

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Generation of an AM DSBFC Envelope Shown in The Time Domain

sin(225t)

–½ cos(230t)

+ ½ cos(220t)

summation of (a), (b), and (c)

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Voltage of an AM DSBFC Envelope In The Time Domain

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Example 3

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Example 3 Continued

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Output Spectrum for Example 3

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AM envelope for Example 3

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Power for Upper and Lower Sideband

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Total Power for an AM DSBFC Envelop

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Power Spectrum for an AM DSBFC Wave with a Single-frequency Modulating Signal

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Example 4

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Power Spectrum for Example 4

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Single Transistor, Emitter Modulator

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Single Transistor, Emitter Modulator (output waveforms )

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Medium-power Transistor AM DSBFC Modulator

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High-power AM DSBFC Transistor Modulator

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Linear Integrated-circuit AM Modulator

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Block Diagram of a Low-level AM DSBFC Transmitter

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Block Diagram of a High-level AM DSBFC Transmitter

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Single-Sideband

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Conventional DSFC-AM

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Single-side Band Full Carrier (SSBFC)

The carrier is transmitted at full power and only one sideband is

transmitted.

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SSBFC waveform, 100% modulation

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Single-Sideband Suppressed Carrier (SSBSC)

The carrier is suppressed 100% and one sideband is removed. Only one

sideband is transmitted.

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SSBSC waveform

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Single-Sideband Reduced Carrier(SSBRC)

One sideband is removed and the carrier voltage is reduced to 10%

of its un-modulated amplitude.

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Independent Sideband(ISB)

A single carrier is independently modulated by two different modulating signals.

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ISB waveform

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Vestigial Sideband(VSB)

The carrier and one complete sideband are transmitted, but only part of the

other sideband is transmitted.

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Single-Sideband Generation

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Balanced modulator waveforms

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FET Balanced Modulator

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AM DSBSC modulator using the LM1496/1596 linear integrated circuit

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Amplitude Modulation Reception

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Simplified Block Diagram of an AM Receiver

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Simplified Block Diagram of an AM Receiver

• Receiver front end = RF section– Detecting the signal– Band-limiting the signal

– Amplifying the Band-limited signal• Mixer/converter

– Down converts the RF signal to an IF signal

• Intermediate frequency (IF) signal– Amplification– Selectivity

• Ability of a receiver to accept assigned frequency

• Ability of a receiver to reject other frequencies

• AM detector demodulates the IF signal to the original signal

• Audio section amplifies the recovered signal.

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Noncoherent Tuned Radio Frequency Receiver Block Diagram

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AM Superheterodyne Receiver Block Diagram

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Bandwidth Improvement (BI)

• Noise reduction ratio

• BI = BRF / BIF

• Noise figure improvement

• NFIMP = 10 log BI

• Determine the noise improvement for a receiver with an RF bandwidth equal to 200 KHz and an IF bandwidth equal to 10 KHz.– BI = 200 KHz / 10 KHZ = 20

– NFImp = 10 log 20 = 13 dB

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Sensitivity

• Sensitivity: minimum RF signal level that the receiver can detect at the RF input.

• AM broadcast receivers– 10 dB signal to noise ratio– ½ watt (27 dBm) of power at the audio output– 50 uV Sensitivity

• Microwave receivers– 40 dB signal to noise ratio– 5 mw (7 dBm) of power at the output

• Aa61

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Dynamic Range

• Dynamic Range– Difference in dB between the minimum input level and

the level that will over drive the receiver (produce distortion).

– Input power range that the receiver is useful.– 100 dB is about the highest posible.

• Low Dynamic Range– Causes desensitizing of the RF amplifiers– Results in sever inter-modulation distortion of weaker

signals

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Fidelity

• Ability to produce an exact replica of the original signal.• Forms of distortion

– Amplitude• Results from non-uniform gain in amplifiers and filters.• Output signal differs from the original signal

– Frequency: frequencies are in the output that were not in the orginal signal

– Phase• Not important for voice transmission

• Devastating for digital transmission

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SSBRC Receiver

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SSBFC Receiver