Super Heterodyne Receiver

SUBMITTED BY:

TANUJ KUMAR (12104025)

ANKIT SOTI (12104069)

Receivers

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

Features of receivers

Simplicity of operation

Good fidelity

Good selectivity

Average sensitivity

Adaptability to different types of aerials

Why a Super heterodyne receiver?

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

Sections Heterodyne receiver has five sections

RF section

Mixer/converter section

IF section

Audio detector section

Audio amplifier section

RF section

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.

Amplifier stage

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.

Due to RF amplifier

Greater gain and better sensitivity

Improved image frequency rejection

Better signal to noise ratio

Better selectivity.

Mixer or conversion section

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.

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

Gang tuning

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 single

adjustment will change the center frequency of the preselector and

the local oscillator

Local oscillator tracking: 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

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.

It is difficult to keep stray radiation from the local oscillator below the level that a nearby receiver can detect. The receiver's local oscillator can act like a low-power CW transmitter. Consequently, there can be mutual interference in the operation of two or more super heterodyne receivers in close proximity.

Intelligence operations, local oscillator radiation gives a means to detect a covert receiver and its operating frequency. The method was used by MI-5 during Operation RAFTAR. This same technique is also used in radar detector detectors used by traffic police in jurisdictions where radar detectors are illegal.

A method of significantly reducing the local oscillator radiation from the receiver's antenna is to use an RF amplifier between the receiver's antenna and its mixer stage.

Local Oscillator Radiation

Local oscillators typically generate a single frequency signal that has negligible amplitude modulation but some random phase modulation. Either of these impurities spreads some of the signal's energy into sideband frequencies. That causes a corresponding widening of the receiver's frequency response, which would defeat the aim to make a very narrow bandwidth receiver such as to receive low-rate digital signals.

Care needs to be taken to minimize oscillator phase noise, usually by ensuring that the oscillator never enters a non-linear mode.

Local Oscillator Sideband Noise

One major disadvantage to the super heterodyne receiver is the problem of image frequency. In heterodyne receivers, an image frequency is an undesired input frequency equal to the station frequency plus twice the intermediate frequency. The image frequency results in two stations being received at the same time, thus producing interference. Image frequencies can be eliminated by sufficient attenuation on the incoming signal by the RF amplifier filter of the super heterodyne receiver.

Image Frequency

For example, an AM broadcast station at 580 kHz is tuned on a receiver with a 455 kHz IF. The local oscillator is tuned to 580 + 455 = 1035 kHz. But a signal at 580 + 455 + 455 = 1490 kHz is also 455 kHz away from the local oscillator; so both the desired signal and the image, when mixed with the local oscillator, will also appear at the intermediate frequency. This image frequency is within the AM broadcast band. Practical receivers have a tuning stage before the converter, to greatly reduce the amplitude of image frequency signals; additionally, broadcasting stations in the same area have their frequencies assigned to avoid such images.

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.

Detector section

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.

Amplifier 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

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

In the case of modern television receivers, no other technique was able to produce the precise band pass characteristic needed for vestigial sideband reception, similar to that used in the NTSC system first approved by the U.S.

This technique is already in use in certain designs, such as very low-cost FM radios incorporated into mobile phones, since the system already has the necessary microprocessor.

It is cost effective.

Advantages

Disadvantages Image Frequency

Local Oscillator Radiation

Local Oscillator Sideband noise

Conclusion

After invention of superheterodyne,there are

many inventions but it is a great receiver…..

References

IEEE paper 3.1.023 on electronic communication systems receivers.

IEEE paper on superheterodyne.

www.hyperphysics.com/superheterodyne

www.Wikipedia.com/superheterodyne

MCQ’s10 MULTIPLE CHOICE QUESTIONS WITH ANSWERS.

Q.1 The tuned circuits prior to the mixer in a superheterodyne receiver are called the

a) Front End

b) Tuner

c) Preselector

d) All of the above

Ans. C. Preselector

Q:2 Why are image frequencies somewhat less of a problem in FM receivers than they are in SSB or AM receivers?

a) SSB uses less bandwidth than does FM.

b) FM signals have a capture effect characteristic.

c) FM mixer stages are square-law devices.

d) FM recievers don’t use the superheterodyne design

Ans. B. FM signals have a capture effect characteristic.

Q.3 Frequency conversion which is a form of amplitude modulation is carried out by a mixer by a process known as:

a) Phasing

b) Heterodyning

c) Demodulating

d) Multiplexing

Ans. B. Heterodyning

Q.4 One way to obtain selectivity while eliminating the image problem is to use:

a) a single-conversion superheterodyne receiver

b) a dual-conversion superheterodyne receiver

c) more tuned circuits

d) a TRF receiver

Ans. B. a dual-conversion superheterodyne receiver

Q.5 A special version of the superheterodyne that converts the incoming signal directly to baseband is known as the

a) indirect conversion receiver

b) zero-IF receiver

c) dual-conversion receiver

d) special conversion receiver

Ans. A. indirect conversion receiver

Q.6 Main advantange of superhetrodyne receiver is:

a) Improvement in sensitivity and selectivity

b) Simple tracking alignment

c) High fidelity

d) Better image rejection

Ans. A. Improvement in sensitivity and selectivity

Q.7 A superhetrodyne receiver receives desired signal at 1000khz frequency, assuming IF=455khz, the corresponding image signal:

a) Depends on modulating frequency

b) Depends on modulating index

c) Is within it’s medium band

d) Is outside it’s medium band

Ans. C. Is within it’s medium band

Q.8 The image frequency rejection in a superhetrodyne receiver is achieved by

a) RF stage only

b) IF stage only

c) RF and detector stage only

d) RF, IF and detector stage only

Ans. B. IF stage only

Q.9 When a superheterodyne receiver is tuned to 555khz, it’s local osciallator provides the mixer signal at 1010khz. The image frequency is ____khz:

a) 1465

b) 1010

c) 555

d) 455

Ans. A. 1465 khz

Q.10 Which section is used in the Preselector Stage?

a) RF section

b) Mixer/converter section

c) IF section

d) Audio detector section

Ans. A. RF Section.