Detection Of Amplitude Modulated Wave

Amplitude modulation (AM) is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. In AM, the amplitude of the carrier wave is varied in proportion to the waveform being sent. The detection of amplitude-modulated waves is a crucial process in communication systems, allowing the retrieval of the original information signal from the modulated carrier.

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This Story also Contains

1. Detection of Amplitude Modulated Wave
2. Limitation of Amplitude Modulation
3. Frequency Modulation
4. Solved Example Based On Detection of Amplitude Modulated Wave
5. Summary

Amplitude modulation detection is fundamental to various everyday technologies and industries. For instance, AM radio broadcasting relies on this principle to transmit audio signals over long distances, enabling listeners to tune into their favourite stations. In aviation, AM is used for air traffic control communications, ensuring clear and reliable exchanges between pilots and ground control. Additionally, amplitude modulation detection plays a vital role in radar systems, which are used for navigation, weather forecasting, and even speed enforcement by law enforcement.

Detection of Amplitude Modulated Wave

The transmitted message gets attenuated in propagating through the channel. The receiving antenna is, therefore, to be followed by an amplifier and a detector. In addition, to facilitate further processing, the carrier frequency is usually changed to a lower frequency by what is called an intermediate frequency (IF) stage preceding the detection. The detected signal may not be strong enough to be made use of and hence is required to be amplified. A block diagram of a typical receiver is shown in fig. below

Detection is the process of recovering the modulating signal from the modulated carrier wave. We just saw that the modulated carrier wave contains the frequencies In order to obtain the original message signal m(t) of angular frequency , a simple method is shown in the form of a block diagram below

The modulated signal of the form given in the above figure (a) is passed through a rectifier to produce the output shown in (b). This envelope of a signal (b) is the message signal. In order to retrieve m(t), the signal is passed through an envelope detector.

So the detector actually removes these frequencies from the signal using diodes for an analog signal or uses digital means to obtain the natural frequency of the signal. Thus the detector generates the original frequency of the signal.

An important point to note is that in the above process, a simple RC circuit can be additionally used along with the detector to generate the original frequency of the signal. This is known as a Detector Envelope which can be used to differentiate the incoming signal from the IF stage signal.

Limitation of Amplitude Modulation

(1) Noisy reception

(2) Low efficiency

(3) Small operating range

(4) Poor audio quality

Some types of amplitude modulation

• Pulse amplitude modulation (PAM)-The amplitude of the pulse varies in accordance with the modulating signal
• Pulse width modulation (PWM)-The pulse duration varies in accordance with the modulating signal.
• Pulse position modulation (PPM)-The position of the pulses of the carrier wave train is varied in accordance with the instantaneous value of the modulating signal.

Frequency Modulation

Frequency modulation deviation The amount by which carrier frequency is varied from its unmodulated value.

The deviation is proportional to the instantaneous value of the modulating voltage.

Value of frequency deviation =δ=f−fcdmax=fmax−fc=±KEmEm= modulating amplitude

The modulation index of frequency modulation

It is defined as the ratio of maximum frequency deviation to the modulating frequency.

mf=δmaxfm=±KEmfm

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Solved Example Based On Detection of Amplitude Modulated Wave

Example 1: Which among the following is true about frequency modulation deviation?

1) Frequency modulation deviation shows the deviation of carrier frequency from its unmodulated value after frequency modulation

2) It is proportional to the instantaneous amplitude of the modulating signal

3) It is proportional to the frequency of carrier wave

4) All of the above

Solution:

Frequency modulation deviation

The amount by which carrier frequency is varied from its unmodulated value.

wherein

The deviation is proportional to the instantaneous value of the modulating voltage.

Frequency Modulation deviation, δ=f−fc
Where fc= carrier frequency
f=frequency of the modulated wave
Further δ∝Vm
And δ∝fc
Since δmax=KVmfc

Hence, the correct answer is the option (4).

Example 2: An audio signal of frequency 500 Hz and voltage 2.4 V is frequency modulated with a carrier wave, and a frequency deviation of 4.8 kHz occurs. If the audio voltage is increased to 5 V, the frequency deviation (in kHz) will be :

1)10

2)9.6

3)4.8

4)5

Solution:

Value of frequency deviation (d)
δ=f−fc

wherein

dmax=fmax−fc=±KEmEm= modulating amplitude

We know for frequency modulation, frequency deviation δ∝Em

δ=KEm
Here 4.8KHz=K(2.4 V)

K=2kHz/V

Now the audio voltage is increased to 5V since the deviation is proportional to audio voltage, it shall also increase accordingly

δnew =KEm|new =(2kHz/V)∗5 Vδnew =10kHz

Hence, the correct answer is the option (1).

Example 3: An audio signal of frequency 1 KHz and voltage 5 V is frequency modulated to a carrier, and a maximum frequency deviation of 4.8 KHz takes place. The modulation index for the above frequency modulation is

1) 4.8

2) 0.2

3) 0.96

4) 5

Solution:

The modulation index for frequency modulation is the ratio between the maximum frequency deviation and frequency of modulating signals

mf= maximum frequency deviation frequency of modulating signals mf=δmaxfmmf=4.8KHz1KHz=4.8

Hence, the correct answer is the option (1).

Example 4: Which among the following waveforms represents pulse Amplitude modulated wave for the given sinusoidal signal?

1)

2)

3)

4)none

Solution:

Pulse amplitude modulation (PAM)

The amplitude of the pulse varies in accordance with the modulating signal.

In pulse amplitude modulation signal is sampled at regular intervals and each sample is made proportional to the amplitude of the signal

Only (1) shows the amplitude of the pulse in proportion to the value of the analogue signal at that instant Hence (1)

Hence, the correct answer is the option (1).

Example 5: In AM modulation, a signal is modulated on a carrier wave such that maximum and minimum amplitudes are found to be 6 V and 2 V respectively. The modulation index is :
1) 100%
2) 80%
3) 60%
4) 50%

Solution:
Vmax=Vc+Vm=6 V Vmin=VC−Vm=2 V Vc=4 V, Vm=2 V
Modulation index =μ=VmVcμ=12 or μ=50%

Hence, the correct answer is the option (4).

Summary

Amplitude modulation (AM) is a technique used for transmitting information by varying the amplitude of a carrier wave. Detection of AM involves recovering the original signal from the modulated wave, typically using a rectifier and an envelope detector. While AM is widely used in radio broadcasting and aviation, it has limitations such as noisy reception and low efficiency. Pulse modulation techniques like PAM, PWM, and PPM vary different pulse characteristics to encode information. Frequency modulation (FM) involves varying the carrier frequency based on the modulating signal's amplitude, with specific calculations determining the modulation index and frequency deviation.