AM Receiver vs FM Receiver: Key Differences Explained

This article explains the workings of AM and FM radio systems and compares AM receivers with FM receivers, highlighting the key differences between them. As we know, any wireless communication system relies on modulators and demodulators.

The modulator encodes the baseband information onto a carrier wave, and the demodulator recovers the original baseband signal from the modulated carrier.

Modulation techniques fall into two primary categories: linear modulation and angle modulation. Linear modulation includes techniques like Double-Sideband (DSB), Amplitude Modulation (AM), Single-Sideband (SSB), and Vestigial Sideband (VSB). Angle modulation encompasses Frequency Modulation (FM) and Phase Modulation (PM).

AM, FM, and PM are abbreviations for Amplitude Modulation, Frequency Modulation, and Phase Modulation, respectively.

The AM/FM Radio System: A Shared Spectrum

The AM/FM radio system operates on two core principles:

• Frequency Spectrum Sharing: Allowing multiple transmitters to utilize the same communication medium.
• Selective Demodulation: Isolating and demodulating the desired signal while rejecting all other simultaneously transmitted signals.

In an AM/FM radio system, the source signal is audio information. Different sources of voice information, such as speech, music, or a hybrid signal (like singing), will possess different frequency spectrums and, consequently, occupy varying bandwidths.

Speech typically occupies 4 kHz, high-quality music requires 15 kHz, AM radio limits the baseband bandwidth to approximately 5 kHz, and FM radio extends it to 15 kHz.

A radio system consists of two primary components:

1. Radio Station Transmitter
2. Radio Receiver

Ideally, a radio receiver should be able to receive any type of audio source simultaneously. Different radio stations share the frequency spectrum by employing AM and FM modulation techniques. Each radio station within a specific geographical area is assigned a carrier frequency around which it transmits its signal.

Sharing the AM/FM radio spectrum is achieved using Frequency Division Multiplexing (FDM).

Radio Receiver Requirements

Here are the key requirements of a radio receiver:

• Cost-Effectiveness: Affordable for the average consumer.
• AM and FM Compatibility: Able to process both AM and FM signals.
• Tunability and Amplification: Able to tune to and amplify the desired radio station’s signal.
• Filtering: Able to filter out signals from other stations.
• Carrier Frequency Independence: The demodulator should work effectively regardless of the carrier frequency of the radio station.

AM receiver vs FM receiver

In an AM radio system, each station occupies a maximum bandwidth of 10 kHz, resulting in a carrier spacing of 10 kHz. In contrast, an FM radio system allocates a bandwidth of 200 kHz per station, leading to a carrier spacing of 200 kHz.

The figure above illustrates a combined block diagram of an AM/FM receiver.

Understanding the AM/FM Radio Receiver

For the demodulator to operate with any radio signal, the carrier frequency of the incoming signal is converted to an Intermediate Frequency (IF). The radio receiver is optimized to perform with these IF frequencies. This is accomplished by designing appropriate IF filters and demodulators for AM and FM at their respective IF frequencies. Since AM and FM utilize different radio frequency spectrum ranges, they have different IF frequencies:

As depicted in the figure, a radio receiver typically comprises the following modules:

• RF Section: Tunes to the desired radio frequency (Fc). It includes an RF Band-Pass Filter (BPF) centered around Fc with the required baseband bandwidth, allowing the desired radio station and nearby stations to pass through.

• RF to IF Converter: Transforms the carrier frequency to the intermediate frequency (IF). This involves a local oscillator with a variable frequency that adjusts with the RF carrier frequency, enabling the tuning of all carrier frequencies to the same IF frequency. Tuning to the desired channel involves simultaneous adjustment of the LO and RF filter. The mixing process generates two frequencies; the higher component is filtered out, leaving the IF filtering.

  A potential issue with this receiver is the generation of an image frequency at (Fc + 2 * FIF). This image frequency is also present at the output of the RF-to-IF converter, along with the desired signal. RF filtering is used to eliminate this image frequency.

  In most radio receivers, the RF-to-IF conversion is performed in two stages, a design known as a superheterodyne receiver.

• IF Filter: Selects the appropriate IF filter based on whether the received signal is AM or FM.

• Demodulator: Demodulates the output of the IF filter using either an AM or FM demodulator.

• Audio Amplifier: Amplifies the demodulated baseband information.