Presented by;

Deep A Patel

Sem : 5th

Content…

INTRODUCTION…

A radio receiver is an electronic device that receive radio waves and converts the information carried by them to a usable form.

• Select desired signal and reject unwanted

signal

• Amplify the R.F. signal

• Demodulate the selected signal

FUNCTION

Simplicity of operation

Good fidelity

Good selectivity

Average sensitivity

If receiver has poor selectivity…….

It refers to the

ability of a receiver

to select a signal of

desired frequency

while reject all

others.

The bandwidth of a tuned circuit is

a measure of the selectivity

If receiver has poor sensitivity….

The ability of receiver to detect the weakest

possible signal is known as sensitivity

If receiver has poor fidelity…

The ability of receiver to reproduce faithfully all

the frequency components in the baseband signal is

called fidelity

TYTES OF RADIO RECEIVERS….

Super heterodyne Receiver

Tuned Radio Frequency (T.R.F) Receiver

AM Receiver

FM Receiver

SPEAKER

RF

AMP

ANTENNA

XIF

AMPDEMOD

AF

AMP

MIXER

cf cf

LO c

LO c

f f

f f

+

-

IFf

LOfRFB B>

IFB B=

AVC

LO

BASEBAND

(a)

Super heterodyne Receiver

A super heterodyne receiver converts all incoming radio

frequency (RF) signals to a

lower frequency known as an intermediate frequency (IF).

Pre

selector

RF

amplifier

Mixer

IF

Amplifier

Band pass

filter

AM

Detector

Audio

Amplifier

Local

Oscillator

Mixer / Converter

Section

RF Section IF Section

Audio detector

Section

Audio amplifier

Section

speaker

Gang tuning

RF signalIF signal

Audio Frequencies

Heterodyne receiver has five sections

RF section

Mixer/converter section

IF section

Audio detector section

Audio amplifier section

Preselected stage

Broad tuned band pass filter with adjustable frequency that is tuned to

carrier frequency

Provide initial band limiting to prevent specific unwanted radio

frequency called image frequency from entering into receiver.

Reduces the noise bandwidth of the receiver and provides the initial

step toward reducing the over all receiver bandwidth to the minimum

bandwidth required to pass the information signal.

RF section

It determines the sensitivity of the receiver.

RF amplifier is the first active device in the network it is

the primary contributor to the noise. And it is the

predominant factor in determining the noise figure.

Receiver may have one or more RF amplifier depending

on the desired sensitivity.

Amplifier stage

Greater gain and better sensitivity

Improved image frequency rejection

Better signal to noise ratio

Better selectivity.

Due to RF amplifier

It consists of two components

Mixer

Local oscillator

Mixer stage :

Heterodyning takes place in the mixer stage.

Radio frequencies are down converted to intermediate frequency

Carrier and sidebands are translated to high frequencies without

effecting the envelope of message signal.

Mixer or conversion section

Local oscillator

frequency is tuned above

RF

High side injection Low side injection

Local oscillator

frequency is tuned below

RF

f LO = fRf + fIF f LO = fRf - fIF

Frequency conversion

The difference between the RF and Local oscillator frequency is

always constant IF.

RF-to-IF conversionPreselector

535 - 565

kHz

Mixer

IF filter

450 – 460

kHz

Oscillator

1005 kHz

Receiver RF input (535 – 1605 kHz)

565 kHz535 545 555

470 kHz440 450 460

450 460 kHz IF Filter output

high-side

injection

(fLO > fRF)

lo RF IFf f f= +

TRACKING:

It is the ability of the local oscillator in a receiver to oscillate either above or

below the selected radio frequency carrier by an amount equal to the IF frequency

through the entire radio frequency band.

Tracking error: the difference between the actual local oscillator frequency

to the desired frequency.

The maximum tracking error 3KHz + or -.

Tracking error can be reduced by using three point tracking.

The preselector and local oscillator each have trimmer capacitor in parallel

with primary tuning capacitor that compensates for minor tracking errors in

the high end of AM spectrum.

The local oscillator has additional padder capacitor in series with the

tuning coil that compensates for minor tracking errors at the low end of

AM spectrum.

Image frequency : It is any frequency other than the selected radio

frequency carrier that is allowed to enter into the

receiver and mix with the local oscillator will

produce cross product frequencies that is equal to

the intermediate frequency.

flo =fsi+fif → fsi=flo-fif when signal frequency is mixed with oscillator

frequency one of the by products is the difference frequency which is

passed to the amplifier in the IF stage.

The frequency fim= flo+fsi the image frequency will also produce fsi

when mixed with fo .

For better image frequency rejection a high IF is preferred.

If intermediate frequency is high it is very difficult to design stable

amplifiers.

LO RF

SF

IF IM

fif fif

2fif

frequency

Image frequency rejection ratio

It is the numerical measure of the ability of the preselector to reject the image frequency.

Single tuned amplifier the ratio of the gain at the desired RF to the gain at the image

frequency.

-

=

+=

im

RF

RF

im

f

f

f

f

QIFRR

221(

If the IF is too high

Poor Selectivity and Poor adjacent channel rejection.

Tracking Difficulties.

If the IF is too low

Image frequency rejection becomes poorer.

Selectivity too sharp and cutting off sidebands

Instability of oscillator will occur.

It contains detector and AGC or AVC

Detector: Rectifies the modulated signal, then filters out the

455 KHz. Leaving only the audio frequency or intelligence of

50 Hz – 20 KHz Which is sent to the AF amplifiers.

Automatic Volume Control or gain control is taken at the

detector (demodulated and fed back to the first IF amplifier

base). Required to overcome atmospheric and terrain

conditions that adversely affect signal strength between the

transmitter & receiver.

The resultant audio signal is amplified in this section

and fed into the

output device(ex: loudspeaker)……

In this section we have

Audio preamp stage

Audio driver stage

Audio push pull stage

If the receiver gain is const. then receiver o/p will fluctuate

according to the strength of i/p signal.& this is not desirable .

AGC is used to adjust the receiver gain automatically so as to

keep the receiver O/P constant irrespective of the strength of

I/P Signal.

Automatic gain control (A.G.C)

Advantages of SHRR

No variation in bandwidth. It remains constant

over the entire frequency range.

High selectivity & sensitivity

High adjacent channel rejection

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

MF AM broadcast band, 535 kHz - 1605 kHz

General coverage MF/HF communications receiver 100 kHz - 30

MHz

VHF FM broadcast band, 88 - 108 MHz

UHF TV broadcast band (analogue or digital) 470 - 860 MHz

Scanning receiver 0.5 MHz - 1300 MHz

GSM 900, GSM1800 or 3G mobile phone bands

Wireless LAN band 2400 - 2483.5 MHz

The super heterodyne receiver concept is the basis for most modern receiver designs.

The selectivity of a super heterodyne receiver is established in the IF stage.

Tuning of a super heterodyne receiver consists of tuning the LO such that the desired mixer output falls in the center of the IF band.