2 Introduction Y Y We know a basic radio system consists of - a TransmitterThe Tx converts information(voice, pictures or digital code)into em radiation,which then radiates to the Rx,where is converted back into information.and a Reciever.TxYRxYInformationTransmitterReceiver
3 Introduction MODULATION We also know that ‘Em’ energy can be made to carry speechif low-frequency currents produced by speakingare combined with the high-frequency currentsthat produce radio waves.This combination process is calledMODULATION
4 λ = Aerials V F The first element in the process of receiving a radio message is the aerial.We know that the length of the aerialdictates the frequencyat which it will transmit and receive.An aerial can vary from a length of wireto a complex arrayselecting only certain frequencies.V=λF
5 Aerials but whatever its shape, its purpose is to detect the electromagnetic waves (‘em’)and convert them into tiny voltages.An aerial can vary from a length of wireto a complex arrayselecting only certain frequencies.
6 Aerials If an aerial in the form of a length of wire is placed into an electromagnetic field,tiny voltages are induced in it.These voltages alternatewith the frequency of the ‘em’ radiation,and are passed to the receiver circuitryfor processing.
7 Aerials Tuned Circuits. The signal strength the aerial inputs to the receiver is very tiny –( volts).So the receiver circuits have to beextremely sensitive,and they must also isolate the wanted signalfrom all the unwanted ones being received.This is achieved by usingTuned Circuits.which allows a single frequency to pass,filtering out all the unwanted signals.5 micro volts
8 Let’s examine a basic receiver The ReceiverBasic DiagramThe Receiver - Receives the signal fromthe aerial (Ae)Demodulator - Extracts the signal fromradio frequency RFto audio frequency AF.Audio Amplifier - increases the signal strengthfor output to the speaker.Let’s examine a basic receiverand what it consists of.YAeLoudSpeakerReceiverDemodulatorAudio Amplifier
9 The Receiver Basic Diagram Y In early models there were problems of - Poor/Limited Selection(ability to remain on station)Poor Amplification/Fidelity(strength & sound quality)and Noise(too much interference)ReceiverDemodulatorAudio AmplifierYAeLoudSpeaker
10 The Receiver The superheterodyne principle offers a way to overcome some of these problems.This involves the effect that one ‘sine wave’ hasover another adjacent ‘sine wave’,of a different frequency.For example -two waves in the sea meeting and interactingor the interaction of two AC electrical signalsof different frequencies
11 The Receiver When two notes of near equal frequency sound together - a periodic rise and fallin intensity can be heard - a beat.The same applies to radio waves,where the beat becomes an added frequency,known as an Intermediate Frequency (IF).This beat can be catastrophic !And has even resulted in old,badly designed propeller airlinersshaking themselves into fatigue failureand even destruction !
12 The Receiver f1 + f2 Sum Frequency Let’s take two frequency waves &f2The upper wave, f1 has a lower frequency than f2A composite wave would be –This resultant wave is theSum Frequencyf1 + f2
13 The Receiver f2 - f1 Difference Frequency Let’s take two frequency wavesf1&f2If we join up the peaks and troughsThe resultant wave is theDifference Frequencyf2 - f1
14 The Receiver f2 - f1 Difference Frequency Let’s take two frequency wavesf1&f2If we join up the peaks and troughsIf f1 = 248 kHzand f2 = 252 kHzThe resultant wave is theDifference Frequencyf2 - f1then this new wave gives a beat of 4 kHz
15 The Receiver To overcome the receiver problems (poor/limited selection,poor amplification/fidelity,excess noise and beating)the Super-Heterodyne (superhet)receiver was developed,making use of these ‘beats’by receiving lower frequency than Tuned Circuits.This lower frequency can be processedmore effectivelythan the higher radio frequencies.
16 The Superhet Receiver Basic Diagram Y Ae output1245631 RF Amplifier Amplifies and stabilises the signal.2 Mixer Changes frequency. With the Local Oscillator (LO)it combines to give Intermediate Frequency (IF).3 LO With the Mixer it produces a constant frequency.4 IF Amplifier Usually 2 or more stages. Amplifies the mixer output(gives most of gain).5 Demodulator Extracts the intelligence from RF to AF signal.6 AF Amplifier Increases signal to required levels of output.
17 FM Receivers Reception on the AM bands is limited in both quality of reproductionand bandwidth availability.FM systems are less likelyto be affected by "noise"and give increased signal performance.
18 FM Receivers Y Y Remember the AM receiver The FM circuitry is similar to the AM systembut uses a discriminator(also called a ratio detector)in place of a demodulator.ReceiverYAeAudio AmplifierLoudSpeakerDemodulatorCarrierInputYAeReference SourceDiscriminatorRatio Detector
19 FM Receivers Y The discriminator circuit has been designed to detect small differences in frequencies.These differences are converted to a voltage outputthat represents the AF component input.The FM circuitry is similar to the AM systembut uses a discriminator(also called a ratio detector)in place of a demodulator.CarrierInputYAeReference SourceDiscriminatorRatio DetectorRecoveredSignalLoudSpeakerAmplifier for Output
20 FM Receivers Y Y AM Receiver FM Receiver Ae Recovered Carrier Signal DemodulatorAudio AmplifierYLoudSpeakerAM ReceiverCarrierInputRecoveredSignalFM ReceiverAmplifier for OutputLoudSpeakerRecoveredSignalReference SourceDiscriminatorRatio DetectorCarrierInputYAe
21 Check of Understanding A tuned circuit is used to . . .Filter out all unwanted signals.Amplify all unwanted signals.Select only unwanted signals.Attenuate all unwanted signals.
22 Check of Understanding What do FM receivers useto demodulate signals?ModulatorDiscriminatorAmplifierMixer
23 Check of Understanding What is the purpose of an aerial on a receiver?To convert the electromagnetic waves (em)into constant voltages.To convert the electromagnetic waves (em)into tiny voltages.To convert the electromagnetic waves (em)into large voltages.To convert the electromagnetic waves (em)into amplified voltages.
24 Check of Understanding The superheterodyme receiver is used . . .To operate a lounspeaker withoutaudio frequency amplificationFor the reception of lower frequency signals than is possible with the tuned circuit receiverWhen intermediate frequency amplificationis not requireFor improved sensitivity and selectivity
25 Check of Understanding In a radio receiver,what is the process of convertingthe radio signal frequency into audio frequencyknown as?SuperhetrodyningRe-amplificationLocal OscillationDemodulation
26 Check of Understanding This diagram shows a discriminator(ratio detector)what does the item ‘P’ represent?Carrier InputReference SourceRecovered SignalOutput Amplifier
27 Check of Understanding This diagram shows a discriminator system,what does the item ‘W’ represent?ReceiverRatio DetectorOutput AmplifierCarrier Input
28 Check of Understanding This diagram shows a discriminator(ratio detector)what does the item ‘X’ represent?Carrier InputReference SourceRecovered SignalOutput Amplifier
29 Check of Understanding This diagram shows a discriminator system,what does the item ‘Y’ represent?ReceiverRatio DetectorOutput AmplifierCarrier Input
30 Check of Understanding This diagram shows a discriminator system,what does the item ‘Z’ represent?ReceiverRatio DetectorOutput AmplifierCarrier Input
31 Check of Understanding In a receiver,what is the purpose ofa radio frequency amplifier?Amplify the signal after demodulationConvert the signal to a lower frequencyAmplify and stabilise the signalDemodulate the signal
32 Check of Understanding In this diagram,what does the block marked ‘R’ represent?AF AmplifierIF AmplifierRF AmplifierCarrier Input
33 Check of Understanding In this diagram,what does the block marked ‘S’ represent?AmplifierMixerLocal OscillatorDemodulator
34 Check of Understanding In this diagram,what does the block marked ‘T’ represent?DemodulatorRF AmplifierMixerIF Amplifier
35 Check of Understanding In this diagram,what does the block marked ‘U’ represent?Local OscillatorMixerIF AmplifierDemodulator
36 Check of Understanding In this diagram,what does the block marked ‘V’ represent?RF AmplifierLocal OscillatorDemodulatorAmplifier
37 Check of Understanding In this diagram,what does the block marked ‘W’ represent?OscillatorRF AmplifierIF AmplifierAF Amplifier
38 Check of Understanding What type of circuit is used in a receiverto recover FM signals?DiscriminatorModulatorDemodulatorLocal Oscillator