radio receivers

RADIO RECEIVERS

ContentsIntroductionHistoryTypes Of Radio ReceiverSensitivity and SelectivityFrequency range

IntroductionA radio receiver is an electronic device that

receives radio waves and converts the information carried by them to a usable form.

HistoryAlexander Stepanovich Popov First radio receiver in 1896.It was based on electromagnetic waves, which were proven to exist by James Clerk

Maxwell only a few years earlier in 1887.

Types Of Radio ReceiverCrystal radio receiverTuned radio frequency receiverSuperheterodyne Receiver

Crystal radioA crystal radio is the simplest kind of radioIt needs no battery or power sourceIt gets all of its power only from the radio

wave.

Components of Crystal RadioAntennaTunerCoilGround wireDetectorCapacitorDiode

Working

Tuned radio frequency receiver one or more tuned radio frequency (RF) individually tuned to the station's frequency

Tuned Radio Frequency ReceiverA tuned radio frequency receiver (or TRF

receiver) is a type of radio receiver that is usually composed of one or more tuned radio frequency (RF) amplifier stages followed by a detector (demodulator) circuit to extract the audio signal and an audio frequency amplifier. Popular in the 1920s, it could be tedious to operate because each stage must be individually tuned to the station's frequency. By the mid 1930s it was replaced by the superheterodyne receiver invented by Edwin Armstrong.

How it worksThe classic TRF receivers of the 1920s and 30s

consisted of three sections:One or more tuned RF amplifier stages. These

amplify the signal of the desired station to a level sufficient to drive the detector, while rejecting all other signals picked up by the antenna

a detector, which extracts the audio (modulation) signal from the radio carrier signal by rectifying it

optionally, but almost always included, one or more audio amplifier stages which increase the power of the audio signal.

Advantages and disadvantagesDisadvantage.Since they used inductor and capacitor as

tunning the element, the circuit is bulky and costly.

They are not suitable to amplify audio frequencies

If the band of the frequency is increased, design become complex.

Advantages and disadvantagesAdvantagesThey amplify defined frequency.

Signal to noise ratio at output is good.

They are well suited for radio transmitters and receiver.

The band of frequency over which amplification is required can be varied.

Superheterodyne Receiver Antenna RF(Radio Frequency) Mixer Local Oscillator(LO) IF(Intermediate Frequency) Detector Audio amplifier Power amplifier Speaker

Architecture of Superheterodyne ReceiverAntennaThe antenna pick up all radiated signal and feeds them into the RF(Amplifier) .These signal are very small (usually only a few microvolts).

RF(Amplifier)This circuit can be adjusted (tuned) to select and amplify

any carries frequency within the AM Broad cost band . Only the selected frequency and its two side bands pass through the amplifier.(Some AM Receiver don’t have a Separate RF amplifier stage.)

Local OscillatorThis circuit generates a steady sine

wave at a frequency 455 khz above the selected RF

MixerIntermediate Frequency

fIF =  fRF ± fLO

RF=1000Khz

LO=1455Khz

fIF=2455, fIF=455

IF AmplifierIncrease the level of the signal .

DemodulationThe received signal is now processed by the demodulator stage where the audio signal (or other baseband signal) is recovered and then further amplified. 

Audio AmplifierThis circuit amplifies the detected audio signal and drive the speaker to drive sound

Sensitivity and SelectivitySensitivity

Weak signalnoise

SelectivityBand width filter1% of RC Frequency

How radio waves travel.Although radio waves are sometimes colloquially referred to as

"airwaves", they do not require air or any other medium in which to travel and can travel through a vacuum.

Like light waves, radio waves travel in straight lines unless something reflects or refracts them. Like light waves, radio waves may be obstructed by obstacles, which can cast a radio "shadow". Although radio waves can pass through many non-conductive material without much loss, they do suffer some loss when passing through walls, floors and roofs. This loss depends upon the building materials used and increases at higher frequencies. Metal, water, ground and other electrically conductive materials cause large losses to radio waves passing through them and in some cases no usable signal may pass through.

Frequency rangeA radio receiver may be designed to tune to a fixed

frequency, MF AM broadcast band, 535 kHz - 1605 kHzGeneral coverage MF/HF communications receiver 100 kHz -

30 MHzVHF FM broadcast band, 88 - 108 MHzUHF TV broadcast band (analogue or digital) 470 - 860 MHzScanning receiver 0.5 MHz - 1300 MHzGSM 900, GSM1800 or 3G mobile phone bandsWireless LAN band 2400 - 2483.5 MHz

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