SUPER HETERODYNE RECEIVER

SUBMITTED BYT.LASYA REKHA

SHIVANGI SRIVASTAVA

Intercept the electromagnetic waves in the receiving antenna to produce the desired RF modulated carrier.

Select the desired signal and reject the unwanted signal.

Amplify the RF signal.

Detect the RF carrier to get back the original modulation frequency voltage.

Amplify the modulation frequency voltage

Receivers

Simplicity of operation

Good fidelity

Good selectivity

Average sensitivity

Adaptability to different types of aerials

Features of receivers

If receiver has poor selectivity…….

If receiver has poor sensitivity….

If receiver has poor fidelity…

A good solution to overcome these problems is using superheterodyne receiver……….it is the major breakthrough in the communication field……..

Pre selector

RF amplifier

Mixer

IF Amplifier

Band passfilter

AM Detector

Audio Amplifier

Super Heterodyne Receiver

LocalOscillator

Mixer / Converter

SectionRF Section IF Section

Audio detector Section

Audio amplifier Section

speaker

Gang tuning

RF signal IF signal

Audio Frequencies

Heterodyne receiver has five sections

RF section

Mixer/converter section

IF section

Audio detector section

Audio amplifier section

Sections

Preselector 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 conversion

Preselector

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

The adjustment for the center frequency of the preselector and the adjustment for local oscillator are gang tuned.

The two adjustments are mechanically tied together and

singleadjustment will change the center frequency of the

preselector andthe local oscillator

Gang tuning

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.

Local oscillator tracking:

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.

Tracking error

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 RFSF

IF IM

fif fif

2fif

frequency

Image frequency rejection ratioIt 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(

Choice of IF : Factors

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.

Detector section

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

Amplifier section

Although the basic idea for the superheterodyne receiver works very well, to ensure the optimum performance under a number of situations, an extension of the principle, known as the double superheterodyne radio receiver may be used.

Improves image rejection ratio and adjacent channel filter performance.

Double superheterodyne receiver

After invention of superheterodyne,there are

many inventions but it is the best receiver…..

Conclusion