AM receiver vs FM receiver | Difference between AM receiver and FM receiver
This page describes operation and function of AM/FM radio system and compares AM receiver vs FM receiver to dictate difference between AM receiver and FM receiver types.
As we know the major blocks in any wireless communication system are modulator and demodulator. The modulator modulates the baseband information and demodulator demodulates the modulated signal to get back baseband. The modulator employs different modulation schemes to function. They are divided into linear modulation and angle modulation. The linear modulation types include DSB, AM, SSB and VSB. Angle modulation types include FM and PM. AM, FM and PM is the short form of Amplitude Modulation, Frequency Modulation and Phase Modulation respectively. Refer AM vs FM vs PM which differentiate AM, FM and PM modulation types.
AM/FM radio system
There are two main principles behind AM/FM radio system:
• To share the frequency spectrum i.e. many transmitters will use the same one medium.
• Demodulates desired signal and rejects all other signals transmitted simultaneously.
As we know source signal in AM/FM radio system is audio information. Different sources of voice information such as speech, music, hybrid signal(i.e. singing) will have different spectrum. Hence they will occupy different bandwidth. Speech occupies 4KHz, high quality music specifies 15KHz, AM radio limits baseband bandwidth to about 5KHz and FM radio limits baseband bandwidth to 15KHz.
There are two main components in a radio system.
1. Radio Station Transmitter
2. Radio Receiver
Radio system i.e. radio receiver should be able to receive any type of audio source simultaneously. Different radio stations will share the frequency spectrum using AM and FM modulation types. Each radio station within a certain geographical area is assigned a carrier frequency around which it need to transmit. Sharing of AM/FM radio spectrum is achieved using FDM i.e. Frequency Division Multiplexing. Refer FDM vs TDM for more information.
Following are the requirements of a radio receiver.
• It should be cost effective, so a common man can afford.
• It should work with both AM and FM signals.
• It should tune to and amplify the desired radio station
• It should filter out all other stations
• Demodulator part has to work with all radio stations regardless of the carrier frequency.
AM receiver/FM receiver
In a AM radio system, each station occupies maximum bandwidth of 10KHz. Hence carrier spacing is 10KHz. In a FM radio system, each station occupies bandwidth of 200KHz. Hence carrier spacing is 200KHz.
Figure depicts the combined block schematic of AM/FM receiver. Let us understand working of AM/FM radio receiver.
For the demodulator to work with any radio signal, we convert carrier frequency of any radio signal to IF (Intermediate frequency). Radio receiver is optimized to work with these IF frequencies. To achieve this, suitable IF filters and demodulators at those IF frequencies for AM and FM is designed.
As both AM and FM have different radio frequency spectrum ranges as mentioned below, there are two different
IF frequencies for each of them.
|Frequency Range||540 to 1600 KHz||88 to 108 MHz|
|IF Frequency||455 KHz||10.7 MHz|
As mentioned in figure-1 a radio receiver consists of following modules:
• RF Section: Tunes to desired RF frequency Fc. Includes RF BPF centered around Fc with desired baseband bandwidth. It passes desired radio station as well as nearby stations.
• RF to IF converter: It converts carrier frequency to IF frequency. A local oscillator with variable frequency which varies with RF carrier frequency is used. This helps in tuning all the carrier frequencies to the same IF frequency. Here while tuning to the desired channel, we are tuning LO and RF filter simultaneously. In the mixing process, two frequencies are generated. The higher component is eliminated using filtering and we are left with IF filtering. The problem with this receiver is generation of image frequency at (Fc+2*FIF). This image frequency is also present at the output of RF-to-IF converter along with desired signal. This image frequency is eliminated using rf filtering. RF to IF is done in two stages in the radio receiver, it is known as super heterodyne receiver.
• IF filter: Depending upon the type of received signal whether AM or FM appropriate IF filter is selected.
• Demodulator: Output of IF filter is demodulated either using AM or FM demodulators. For AM,
• Audio Amplifier: This module amplifies the demodulated baseband information.
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