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Difference between AM, FM and PM

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Last updated date: 15th Jul 2024
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What is AM, FM, and PM?

In FM, a radio wave known as the "carrier" or "carrier wave" is modulated in frequency by the signal that is to be transmitted. However, the amplitude and the phase constant remain constant in the case of frequency modulation. While amplitude modulation or AM is a technique that we use in electronic communication, most commonly for transmitting information through a radio carrier wave. 


Additionally, there is a term that is one of the two principal forms of angle modulation alongside frequency modulation, and it is called phase modulation. PM is a modulation pattern that we use for conditioning communication signals for transmission.


This page will help you distinguish between amplitude modulation and frequency modulation. Also, you will get to learn the difference between FM, AM, and PM modulation in tabular form. 

 

How does FM Work?

Frequency modulation works by continuously changing the strength of the transmitted signal in relation to the information being sent.


FM radio uses frequency modulation. However, to understand frequency modulation, assume a signal with a low frequency and amplitude. As the signal passes, its frequency remains unchanged or unmodulated. We find that a signal carries very little information.


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So, when you introduce information to this signal, there's a variation to the frequency that varies directly with the information. Also, when the frequency is modulated between low and high, the carrier frequency transmits music or voice. Therefore, we find that the frequency changes as a result, but the amplitude remains constant the entire time.


For example, the changes in the strength of the signal may be utilized to specify the sounds to be reproduced by the speaker, or the light intensity of television pixels.

How does an AM Work?

While receiving input signals, an AM receiver discovers amplitude variations in the radio waves at a specific frequency. It amplifies changes in the signal voltage to operate a loudspeaker or earphone.


Working principle of an AM receiver in reality:


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In this, a radio receiver is present in the opposite of a radio transmitter that uses an antenna to capture radio waves. Further, it processes those waves to elicit only those waves that are vibrating at the needed frequency and filters the audio signals that were added to those waves, following, amplifying the audio signals, and finally plays them on a speaker.


How does PM Work?

Phase modulation encodes a message or an input signal as variations in the instantaneous phase of a carrier wave.


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In phase modulation, the working principle is that the instantaneous amplitude of the baseband signal modifies the phase of the carrier signal thereby keeping the constant amplitude and frequency. The phase of a carrier signal is modulated by phase modulation to follow the altering signal level or amplitude of the message signal. 


Furthermore, the peak amplitude and the frequency of the carrier signal are also kept constant. However, as the amplitude of the message signal alters, the phase of the carrier also changes.


Difference between AM and FM

Below is the table of AM versus FM:

S.No.

Parameters

AM

FM

1.

Full-form

Amplitude modulation

Frequency modulation

2.

Origin

The AM method of audio transmission was successfully carried out in the mid-1870s.

FM radio was developed in the United States in the 1930s by Edwin Armstrong.

3.

Modulating differences

In AM, a radio wave known as the "carrier" or "carrier wave" is modulated in amplitude by the signal that is to be transmitted. 

In FM, a radio wave known as the "carrier" or "carrier wave" is modulated in frequency by the signal that is to be transmitted.

4. 

Constant parameters

The frequency and phase remain the same.

The amplitude and phase remain the same.

5.

Quality

AM has poorer sound quality, and a lower bandwidth but is cheaper. It can be transmitted over long distances as it has a lower bandwidth, which is why it can hold more stations available in any frequency range.

FM is less affected by interference, but FM signals are impacted by physical barriers. They have a better sound quality due to higher bandwidth.


6.

Frequency range

AM radio ranges from 535 to 1700 kHz or up to 1200 bits per second.


FM radio ranges in a higher spectrum from 88.1 to 108.1MHz or up to 1200 to 2400 bits per second.

7.

Bandwidth BW

BW  is much less than FM.


B.W. =  2 fm

BW is large. Hence a wide channel is required.


B.W. = 2 x (δ + fm)

8.

Bandwidth requirements

Bandwidth is less than FM or PM and doesn’t depend upon the modulation index.

The bandwidth requirement is twice the highest modulating frequency.



Bandwidth requirement is greater and depends upon the modulating.

The bandwidth requirement is twice the sum of the modulating signal frequency and the frequency deviation.

9.

The frequency required for broadcasting

In AM radio broadcasting, if the modulating signal has a bandwidth of 15 kHz, then the bandwidth of an amplitude-

The modulated signal is 30 kHz. 

Let’s say, if the frequency deviation is 75kHz and the modulating signal frequency is 15kHz, the bandwidth required is 180kHz.

10.

No of Sidebands

The number of sidebands is constant and equal to 2.

The number of sidebands having significant amplitude depends upon the modulation index

11.

Zero crossings in modulating signal

Equidistant

Not equidistant

12.

Complexity

AM transmitters and receivers are less complex than FM and PM, but synchronization is needed in the case of SSBSC carriers.

FM (or PM) transmitters are more complex than AM because the variation of modulating signal has to be converted and detected from the corresponding variation in frequencies. 

13.

Noise

AM receivers are very less susceptible to noise because noise affects the amplitude, which is where information is stored in AM signals.

FM receivers are better immune to noise and it is possible to decrease noise by further deviation.

14.

Efficiency

Power is wasted in transmitting the carrier.

All transmitted power is useful so that’s why FM is very efficient. 

15.

Application

MW (Medium wave), SW (short wave) band broadcasting, video transmission in T.V.

Broadcasting FM, audio transmission on T.V.


Difference between AM, FM, and PM

S.No.

Parameters

FM

AM

PM

1.

Definition

Frequency modulation is a technique of modulation, in which the frequency of the carrier varies in accordance with the amplitude of the modulating signal. The amplitude and phase are constant.

Amplitude modulation is a technique of modulation in which the amplitude of the carrier wave varies in accordance with the amplitude of the modulating signal. The frequency and phase are constant.

Phase modulation is a technique of modulation in which the phase of the carrier wave varies in accordance with the amplitude of the modulating signal. The amplitude and frequency are constant.

2.

Noise

Noise immunity of FM is superior to AM and PM.

AM receivers are very susceptible to noise.

Noise immunity is better than AM but not FM.

3.

Function

The frequency of the carrier wave deviates as per the voltage of the modulating signal input.

The amplitude of a carrier wave in AN diverges as per amplitude or voltage of modulating signal input.

A phase of the carrier wave varies as per the voltage of modulating signal input.

4.

Constant parameter


The amplitude of the carrier wave is kept changeless.

The frequency of the carrier wave is kept invariable.

The amplitude of the carrier wave is kept changeless.

5.

Types

Digital FM types: FSK, GFSK, offset PSK, etc.

AM types: DSB-SC, SSB, VSB, etc.

Digital PM types: QPSK, BPSK, QAM (the combination of amplitude and phase, modulation).

6.

Waveforms

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For a radio signal to carry audio or other information for broadcasting or for two-way radio communication, signals must be modulated or changed in some way. Though we have several ways in which a radio signal may be modulated, one of the easiest is to change its amplitude in line with variations of the sound. Here, we discussed three types of modulation, viz: FM, AM, and PM, which will help you understand the basics of the modulation along with the difference between each. 

FAQs on Difference between AM, FM and PM

1. What is the modulation?

Modulation is a technique in electronics that is used for impressing a piece of information such as voice, music, picture, or data on a radio-frequency carrier wave by changing one or more features of the wave in accordance with the intelligence signal. The most commonly altered features include amplitude, frequency, phase, pulse sequence, and pulse duration. 

2. Write the advantages and disadvantages of FM.

Advantages

  • The noise can be reduced by increasing the deviation that’s why FM is more immune to noise.

  • Due to higher efficiency most, the power is utilized and the FM transmissions can be used for the stereo sound transmission due to a large number of sidebands.

Disadvantages

  • The bandwidth of FM is 10 times larger than the AM. Hence, a wide channel would be required for FM transmission.

  • The transmitting and receiving equipment are very complicated in FM.