communication system eeeb453 chapter 2 amplitude modulation

COMMUNICATION SYSTEM EEEB453Chapter 2

AMPLITUDE MODULATION

Dept of Electrical EngineeringUniversiti Tenaga Nasional

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AM Transmitter

Transmitter Requirements: Generate a signal of correct freq with the

right type of modulation at the right carrier freq.

Provide sufficient power amplification to ensure that signal level is high enough to be carried over a long distance.

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Amplitude Modulated Transmitter

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Amplitude Modulator Amplitude Modulator – signal source and carrier

are “multiplied” resulting in the modulation envelope. The AM signal varies according to information signal but maintains the frequency.

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Output Amplifier (Power Amplifier)

Power amplifier takes modulator output and increases its magnitude.

It also increases the signal strength to ensure it can go the distance.

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Antenna

Antenna converts a current or a voltage signal to an electromagnetic signal which is radiated throughout space.

A transmitter needs an antenna to send its radio signal, and a receiver needs an antenna to pick up that radio signal.

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AM Receiver In a broadcasting system, beside demodulating

the incoming modulated signal, the receiver is also required to perform some other functions:

Carrier Freq. Tuning – to select the desired signal among the unwanted ones.

Filtering – to separate/extract the desired signal from other modulated signals.

Amplification – to compensate lost of signal during transmission.

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Sensitivity

Receiver parameter to measure the minimum RF signal level that can be detected at the input to the receiver and still produce a usable demodulated information signal (also called receiver threshold)

It is usually stated in microvolt of received signal (eg. two-way mobile radio is between 0.1uV and 10uV)

Sensitivity of an AM receiver depends on the noise power present at the input to the receiver – the best way to improve sensitivity of a Rx is to reduce noise level

AM Receiver - Parameters

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AM Receiver - Parameters Selectivity

Measure the ability of the receiver to accept a given band of frequencies and reject all others (eg. in AM broadcasting, each channel is allocated a 10kHz bandwidth – BPF is of 10kHz to pass only the specific channel)

Reject unwanted signals at frequencies different from that of the desired signal.

Shape Factor (SF) – used to measure selectivity (ideal SF = 1)

Fidelity The ability of the receiver to produce an exact replica of the

original information signal at the receiver output.

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AM Receiver

Figure shows a basic version of AM Receiver

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AM Receiver

Antenna - EM waves strike the antenna and generate a small voltage (weak signal) in the antenna.

RF Amplifier – Detects, bandlimits and amplifies RF signals. Selects the wanted station and rejects the unwanted ones.

Mixer – Converts the RF signal to Intermediate Frequency (IF) signal.

IF Amplifier – Amplification and selectivity (tuning)

AM Detector – Demodulates the AM wave and converts it back to the original signal

Audio Amplifier – Amplifies the recovered information.

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Basic Principles Basic Principles of Amplitude Modulationof Amplitude Modulation

AM in the Time Domain – Linear Component Amplitude modulation voltage is produced by a circuit that can

multiply the carrier by the modulating signal and then add the carrier.

Block diagram of a circuit to produce AM.

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Amplitude Modulation –Linear Component Taken from Beasley &

Miller

The output (d) is not suitable for Tx (the Rx antenna only detect the carrier signal due to the low freq signal cannot be propagate efficiently as a radio wave)

Solution – using non-linear device

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AM in the Frequency Domain – Nonlinear Component The product of the carrier and modulating signal can be

generated by applying both signals to a nonlinear component such as a diode.

A square-law function is one that varies in proportion to the square of the input signals. A diode gives a good approximation of a square-law response.

A square-law circuit for producing AM.


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The tuned circuit filters out the modulating signal and carrier harmonics,leaving only the carrier and sidebands.

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Amplitude ModulatorsAmplitude Modulators Two types of amplitude modulators : I) Low-level modulators:

- Generate AM with small signals and must be amplified before transmission.

- Diode Modulator,Transistor Modulator, PIN Diode Modulator, Differential Amplifier

II) High-level modulators:

- Produce AM at high power levels, usually in the final amplifier stage of a transmitter.

- Collector Modulator, Series Modulator

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Amplitude modulation with a diode.

Amplitude Modulator – Diode Modulator

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Amplitude Modulator – Diode Modulator Diode modulation consists of a resistive mixing network, a diode

rectifier, and an LC tuned circuit. The carrier is applied to one input resistor and the modulating signal

to another input resistor. This resistive network causes the two signals to be linearly mixed –

added (c) The waveform is applied to a diode rectifier +(ve) potion of input wave : Forward biased -(ve) potion of input wave : Diode cut off and no signal passes Current through diode is a series of +(ve) pulses whose amplitude

varies in proportion to the amplitude of modulating signal (d) The coil and capacitor repeatedly exchange energy, causing an

oscillation or ringing at the resonant frequency –creates one –(ve) half cycle for every +(ve) input pulse (e)

The resulting waveform across the tuned circuit is an AM signal The Q of the tuned circuit – High enough to eliminate harmonics - filter out modulating signal -low enough that its bandwidth

accomodates the sidebands generated.

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Amplitude Modulator – Transistor Modulator

Simple transistor modulator.

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AM Demodulator

The Demodulator also called the Detector, is part of the receiver that recovers the original signal, Vm from the modulated signal, Vam.

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AM Demodulator

Two basic types of radio receivers

1. Coherent – locally generated carrier is synchronized to the transmitter’s carrier frequency (rarely used in practice)

2. Non-Coherent - carrier is not locally generated at the receiver i.e the frequencies used for demodulation are completely independent from the transmitter’s carrier frequencies

Two non-coherent methods of AM demodulation are1. Rectifier detection2. Envelope detection

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Rectifier Detection

Low-pass Filter

This detector is simply rectify the incoming signal to remove half of the envelope and then use a low pass filter to remove the high freq components of the signal.

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Rectifier Detection

At point A, the modulated waveform at the detector input.

At point B, demonstrates the use of the diode rectifier in removing half the envelope.

At point C, shows the combined effects of the rectifier and low-pass filter – this is the baseband signal with a dc offset.

The offset can be removed with a series capacitor, producing the output shown at point D.

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Envelope Detection

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Envelope Detection

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Envelope Detection

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Envelope Detection

The result is an output containing 3 components: The wanted information signal Some ripple at the IF frequency A positive DC voltage level

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Exercise

1. In Figure1, A is 220 mVp-p and B is 350 mVp-p. The percent modulation of the AM signal is:

a. 62.9% b. 1.59%

c. 22.8% d. 4.38%

2. In Figure 1, A is 1.5Vp-p and B is 3.3Vp-p. If the carrier signal is set at 1.2V peak, the modulating signal is:

a. 3.2V peak. b. 0.55V peak.

c. 0.9V peak. d. 0.45V peak.

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Exercise3. A receiver’s sensitivity is:

a. the extent to which a receiver is capable of differentiating between the desired signal and other signals.

b. its ability to drive the output speaker to an acceptable level.

c. the ability of the receiver to demodulate a modulated signal.

d. the ability of a receiver to attenuate noise signals.

4. The diode detector:

a. is one of the simplest and most effective AM detectors.

b. consists of a nonlinear diode and low-pass filter.

c. is sometimes referred to as an envelope detector.

d. all of the above

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Exercise5. Draw a circuit diagram illustrating how to demodulate a conventional AM signal using envelope/diode detection and briefly explain.

6. Explain what happen to the bandwidth of AM if the frequency of the carrier signal is doubled?

7. Draw a block diagram of an AM transmitter.

8. Explain what happen to the modulated waveform if the frequency of the information signal decreases by half.