FM Receiver

What is the Radio?
Traditionally, a radio has been considered to be the ‘box’ that connects to the antenna and everything behind that, however, many system designs are segmented into two separate subsystems. The radio and the digital processor. With this segmentation, the purpose of the radio is to down convert and filter the desired signal and then digitize the information. Likewise, the purpose of the digital processor is to take the digitized data and extract out the desired information.

Digital Radio is the next generation of radio broadcasting. Digital radio receivers are able to receive and decode a digital program stream into a format that you can hear (and see, with program details on built in screens), offering you a superior listening experience. Digital radio is transmitted using digital signals instead of analogue.

Digital Radio has several advantages over the traditional analogue AM and FM services. As well as the clear audio signal, digital radio transmits additional information that your radio is able to display. Examples include program information to show what is on now and coming up next, weather forecasts, news headlines, and for music stations you can see artist and track names. .

but It takes a digital radio some time to process incoming signals—and you can hear that for yourself! Put a digital radio and an ordinary analog radio next to one another and tune them both into the same station. You'll find the sound from the digital radio lags noticeably behind the sound from the analog radio because of the time it takes to reassemble the digital signal. Give that some thought next time you set your watch by the time signal from a digital radio! .

Single-Carrier vs. Multi-Carrier
There are two basic types of radios under discussion. The first is called a single-carrier and the second a multi-carrier receiver Their name implies the obvious, however their function may not be fully clear. The single carrier receiver is a traditional radio receiver deriving selectivity in the analog filters of the IF stages. The multi-carrier receiver processes all signals within the band with a single rf/if analog strip and derives selectivity within the digital filters that follow the analog to digital converter. The benefit of such a receiver is that in applications with multiple receivers tuned to different frequencies within the same band can achieve smaller system designs and reduced cost due to eliminated redundant circuits. A typical application is a cellular/wireless local loop base station. Another application might be surveillance receivers that typically use scanners to monitor multiple frequencies. This application allows simultaneous monitoring of many frequencies without the need for sequential scanning.

Modulation types
Modulation is the process of adding baseband signal to a carrier wave. Two types of modulation are used in analog radio broadcasting systems; AM and FM.

In amplitude modulation (AM) the strength of the radio signal is varied by the audio signal. AM broadcasting is allowed in the AM broadcast bands which are between 148 and 283 kHz in the longwave range, and between 526 and 1706 kHz in the medium frequency (MF) range of the radio spectrum. AM broadcasting is also permitted in shortwave bands, between about 2.3 and 26 MHz, which are used for long distance international broadcasting.

In frequency modulation (FM) the frequency of the radio signal is varied slightly by the audio signal. FM broadcasting is permitted in the FM broadcast bands between about 65 and 108 MHz in the very high frequency (VHF) range. The exact frequency ranges vary somewhat in different countries.

FM stereo radio stations broadcast in stereophonic sound (stereo), transmitting two sound channels representing left and right microphones. A stereo receiver contains the additional circuits and parallel signal paths to reproduce the two separate channels. A monaural receiver, in contrast, only receives a single audio channel that is a combination (sum) of the left and right channels. While AM stereo transmitters and receivers exist, they have not achieved the popularity of FM stereo.

Most modern radios are able to receive both AM and FM radio stations, and have a switch to select which band to receive; these are called AM/FM radios.

Digital audio broadcasting (DAB)
is an advanced radio technology which debuted in some countries in 1998 that transmits audio from terrestrial radio stations as a digital signal rather than an analog signal as AM and FM do. Its advantages are that DAB has the potential to provide higher quality sound than FM (although many stations do not choose to transmit at such high quality), has greater immunity to radio noise and interference, makes better use of scarce radio spectrum bandwidth, and provides advanced user features such as electronic program guide, sports commentaries, and image slideshows. Its disadvantage is that it is incompatible with previous radios so that a new DAB receiver must be purchased. As of 2017, 38 countries offer DAB, with 2,100 stations serving listening areas containing 420 million people. Most countries plan an eventual switchover from FM to DAB. The United States and Canada have chosen not to implement DAB.

DAB radio stations work differently from AM or FM stations: a single DAB station transmits a wide 1,500 kHz bandwidth signal that carries from 9 to 12 channels from which the listener can choose. Broadcasters can transmit a channel at a range of different bit rates, so different channels can have different audio quality. In different countries DAB stations broadcast in either Band III (174–240 MHz) or L band (1.452–1.492 GHz).

The signal strength of radio waves decreases the farther they travel from the transmitter, so a radio station can only be received within a limited range of its transmitter. The range depends on the power of the transmitter, the sensitivity of the receiver, atmospheric and internal noise, as well as any geographical obstructions such as hills between transmitter and receiver. AM broadcast band radio waves travel as ground waves which follow the contour of the Earth, so AM radio stations can be reliably received at hundreds of miles distance. Due to their higher frequency, FM band radio signals cannot travel far beyond the visual horizon; limiting reception distance to about 40 miles (64 km), and can be blocked by hills between the transmitter and receiver. However FM radio is less susceptible to interference from radio noise (RFI, sferics, static) and has higher fidelity; better frequency response and less audio distortion, than AM. So in many countries serious music is only broadcast by FM stations, and AM stations specialize in radio news, talk radio, and sports. Like FM, DAB signals travel by line of sight so reception distances are limited by the visual horizon to about 30–40 miles (48–64 km).

Radios are made in a range of styles and functions:
• Table radio - A self-contained radio with speaker designed to sit on a table.
• Clock radio - A bedside table radio that also includes an alarm clock. The alarm clock can be set to turn on the radio in the morning instead of an alarm, to wake the owner.
• Tuner - A high fidelity AM/FM radio receiver in a component home audio system. It has no speakers but outputs an audio signal which is fed into the system and played through the system's speakers.
• Portable radio - a radio powered by batteries that can be carried with a person. Radios are now often integrated with other audio sources in CD players and portable media players.

o Boom box - a portable battery-powered high fidelity stereo sound system in the form of a box with a handle, which became popular during the mid 1970s.
o Transistor radio - an older term for a portable pocket-sized broadcast radio receiver. Made possible by the invention of the transistor and developed in the 1950s, transistor radios were hugely popular during the 1960s and early 1970s, and changed the public's listening habits.

• Car radio - An AM/FM radio integrated into the dashboard of a vehicle, used for entertainment while driving. Virtually all modern cars and trucks are equipped with radios, which usually also includes a CD player.
• Satellite radio receiver - subscription radio receiver that receives audio programming from a direct broadcast satellite. The subscriber must pay a monthly fee. They are mostly designed as car radios.
• Shortwave receiver - This is a broadcast radio that also receives the shortwave bands. It is used for shortwave listening.
• AV receivers are a common component in a high-fidelity or home-theatre system; in addition to receiving radio programming, the receiver will also contain switching and amplifying functions to interconnect and control the other components of the system.

Radio transmitters

A radio transmitter consists of several elements that work together to generate radio waves that contain useful information such as audio, video, or digital data.

  • Power supply: Provides the necessary electrical power to operate the transmitter.
  • Oscillator: Creates alternating current at the frequency on which the transmitter will transmit. The oscillator usually generates a sine wave, which is referred to as a carrier wave.
  • Modulator: Adds useful information to the carrier wave. There are two main ways to add this information. The first, called amplitude modulation or AM, makes slight increases or decreases to the intensity of the carrier wave. The second, called frequency modulation or FM, makes slight increases or decreases the frequency of the carrier wave.
  • Amplifier: Amplifies the modulated carrier wave to increase its power. The more powerful the amplifier, the more powerful the broadcast.

    Antenna: Converts the amplified signal to radio waves.

Radio receivers
In radio communications, a radio receiver, also known as a receiver, a wireless or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. The antenna intercepts radio waves (electromagnetic waves) and converts them to tiny alternating currents which are applied to the receiver, and the receiver extracts the desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, and finally recovers the desired information through demodulation.

Radio receivers are essential components of all systems that use radio. The information produced by the receiver may be in the form of sound, moving images (television), or digital data. A radio receiver may be a separate piece of electronic equipment, or an electronic circuit within another device. The most familiar type of radio receiver for most people is a broadcast radio receiver, which reproduces sound transmitted by radio broadcasting stations, historically the first mass-market radio application. A broadcast receiver is commonly called a "radio". However radio receivers are very widely used in other areas of modern technology, in televisions, cell phones, wireless modems and other components of communications, remote control, and wireless networking systems.

Broadcast radio receivers
The most familiar form of radio receiver is a broadcast receiver, often just called a radio, which receives audio programs intended for public reception transmitted by local radio stations. The sound is reproduced either by a loudspeaker in the radio or an earphone which plugs into a jack on the radio. The radio requires electric power, provided either by batteries inside the radio or a power cord which plugs into an electric outlet. All radios have a volume control to adjust the loudness of the audio, and some type of "tuning" control to select the radio station to be received.

Selectivity is the selection of a particular signal while rejecting the others. Sensitivity is the capacity of detecting RF signal and demodulating it, while at the lowest power level. Both Selectivity and Sensitivity should be high for an FM receiver. To overcome these drawbacks of the ordinary receiver, the superheterodyne receiver was invented. This FM receiver consists of 5 main stages.

Captures the radio waves. Typically, the antenna is simply a length of wire. When this wire is exposed to radio waves, the waves induce a very small alternating current in the antenna.

RF amplifier
A sensitive amplifier that amplifies the very weak radio frequency (RF) signal from the antenna so that the signal can be processed by the tuner.

RF Tuner Section
The modulated signal received by the antenna is first given to the tuner circuit through a transformer. The tuner circuit is nothing but an LC circuit which is also called as resonant or tank circuit. It selects the frequency, desired by the radio receiver. It also tunes the local oscillator and the RF filter at the same time.

so, the tuner is a circuit that can extract signals of a particular frequency from a mix of signals of different frequencies. On its own, the antenna captures radio waves of all frequencies and sends them to the RF amplifier, which dutifully amplifies them all.

Unless you want to listen to every radio channel at the same time, you need a circuit that can pick out just the signals for the channel you want to hear. That’s the role of the tuner.

The tuner usually employs the combination of an inductor (for example, a coil) and a capacitor to form a circuit that resonates at a particular frequency. This frequency, called the resonant frequency, is determined by the values chosen for the coil and the capacitor. This type of circuit tends to block any AC signals at a frequency above or below the resonant frequency.

You can adjust the resonant frequency by varying the amount of inductance in the coil or the capacitance of the capacitor. In simple radio receiver circuits, the tuning is adjusted by varying the number of turns of wire in the coil. More sophisticated tuners use a variable capacitor (also called a tuning capacitor) to vary the frequency.

RF Mixer
The signal from the tuner output is given to the RF-IF converter, which acts as a mixer. It has a local oscillator, which produces a constant frequency. The mixing process is done here, having the received signal as one input and the local oscillator frequency as the other input. The resultant output is a mixture of two frequencies produced by the mixer, which is called the Intermediate Frequency (IF)

The production of IF helps in the demodulation of any station signal having any carrier frequency. Hence, all signals are translated to a fixed carrier frequency for adequate selectivity.

IF Filter
Intermediate frequency filter is a bandpass filter, which passes the desired frequency. It eliminates any unwanted higher frequency components present in it and noise. IF filter helps in improving Signal to Noise Ratio (SNR)

Responsible for separating the audio information from the carrier wave. For AM signals, this can be done with a diode that just rectifies the alternating current signal. What’s left after the diode has its way with the alternating current signal is a direct current signal that can be fed to an audio amplifier circuit. For FM signals, the detector circuit is a little more complicated.

Audio Amplifier
This component’s job is to amplify the weak signal that comes from the detector so that it can be heard. This can be done using a simple transistor amplifier circuit.

it is the power amplifier stage which is used to amplify the detected audio signal. The processed signal is given strength to be effective. This signal is given to the loudspeaker to get the original sound signal.

This superheterodyne receiver is well used because of its advantages like better SNR, sensitivity, and selectivity.

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