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

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…….. Mixer / Converter Section

RF Section Mixer IF Section

Pre RF Band pass IF selector amplifier filter Amplifier

RF signal IF signal

Local Oscillator

Gang tuning Audio amplifier Audio detector Section Section

Audio AM speaker Amplifier Detector 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.

Frequency conversion

 The difference between the RF and Local oscillator frequency is always constant IF.

High side injection Low side injection

Local oscillator frequency Local oscillator frequency is tuned above RF is tuned below RF

f LO = fRf + fIF f LO = fRf - fIF Receiver RF input (535 – 1605 kHz) RF-to-IF conversion

Preselector 535 - 565 kHz

535 545 555 565 kHz Mixer Oscillator 1005 kHz high-side injection 440 450 460 470 kHz IF filter (fLO > fRF) 450 – 460 kHz flo f RF f IF

450 460 kHz IF Filter output 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.

Local Oscillator Radiation

 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 Sideband Noise

 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. Image Frequency

 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.  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. 2fif

fif fif

IF RF LO IM SF 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. IFRR (1 Q2 2

 fim   fRF         fRF   fim  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 Double superheterodyne receiver

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. Advantages

 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. 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