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Advanced Radioand Radar
Part 4Receivers
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IntroductionWe know a basic radio system consists of -
a Transmitter
The Tx converts information (voice, pictures or digital code)
into em radiation,which then radiates to the Rx,
where is converted back into information.
Transmitter Receiver
Tx
Y
Rx
Y
and a Reciever.
Information
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IntroductionWe also know that ‘Em’ energy
can be made to carry speech
if low-frequency currents produced by speaking
are combined with the high-frequency currents
that produce radio waves.
This combination process is called
MODULATION
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AerialsThe first element in the process of receiving
a radio message is the aerial.
We know that the length of the aerial dictates the frequency
at which it will transmit and receive.
An aerial can vary from a length of wire to a complex array
selecting only certain frequencies.
V=
λF
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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 wire to a complex array
selecting only certain frequencies.
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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 alternate with the frequency of the ‘em’ radiation, and are passed to the receiver circuitry
for processing.
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AerialsThe signal strength the aerial inputs
to the receiver is very tiny – (0.000005 volts).
So the receiver circuits have to be extremely sensitive,
and they must also isolate the wanted signal from all the unwanted ones being received.
This is achieved by using
Tuned Circuits. which allows a single frequency to pass,
filtering out all the unwanted signals.
5 micro volts
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The Receiver
The Receiver - Receives the signal from the aerial (Ae)
Demodulator - Extracts the signal from radio frequency RF to audio frequency AF.
Audio Amplifier - increases the signal strengthfor output to the speaker.
Receiver DemodulatorAudio
Amplifier
YAe
Basic Diagram
LoudSpeaker
Let’s examine a basic receiverand what it consists of.
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The Receiver
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)
Basic Diagram
Receiver DemodulatorAudio
Amplifier
YAe
LoudSpeaker
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The ReceiverThe 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 interacting
or the interaction of two AC electrical signals of different frequencies
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The ReceiverWhen two notes of near equal frequency sound together - a periodic rise and fall
in 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 airliners
shaking themselves into fatigue failure and even destruction !
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The ReceiverLet’s take two frequency waves
f1
f2
&
The upper wave, f1 has a lower frequency than f2
A composite wave would be –
f1 + f2
This resultant wave is the
Sum Frequency
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The ReceiverLet’s take two frequency waves
f1
f2
&
If we join up the peaks and troughs
The resultant wave is the
Difference Frequency f2 - f1
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The ReceiverLet’s take two frequency waves
f1
f2
&
If we join up the peaks and troughs
The resultant wave is the
Difference Frequency f2 - f1
If f1 = 248 kHz and f2 = 252 kHz
then this new wave gives a beat of 4 kHz
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The ReceiverTo 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 processed more effectively
than the higher radio frequencies.
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The Superhet Receiver
1 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.
1
Y
4
3
5 6
Basic DiagramAe
2 output
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FM Receivers
Reception on the AM bands is limited in both quality of reproduction
and bandwidth availability.
FM systems are less likely to be affected by "noise"
and give increased signal performance.
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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.
FM Receivers
Receiver
YAeAudio
AmplifierLoud
SpeakerDemodulator
Reference SourceDiscriminator
Ratio DetectorCarrier
Input
YAe
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FM Receivers
The FM circuitry is similar to the AM systembut uses a discriminator (also called a ratio detector)
in place of a demodulator.
The discriminator circuit has been designed to detect small differences in frequencies.
These differences are converted to a voltage output that represents the AF component input.
Amplifier for Output
LoudSpeaker
RecoveredSignal
Reference SourceDiscriminator
Ratio DetectorCarrier
Input
YAe
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FM Receivers
Receiver DemodulatorAudio
Amplifier
Y
LoudSpeaker
AM ReceiverCarrier Input
RecoveredSignal
FM Receiver
Amplifier for Output
LoudSpeaker
RecoveredSignal
Reference SourceDiscriminator
Ratio DetectorCarrier
Input
YAe
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A tuned circuit is used to . . .
Amplify all unwanted signals.
Filter out all unwanted signals.
Attenuate all unwanted signals.
Select only unwanted signals.
Check of Understanding
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Check of UnderstandingWhat do FM receivers use
to demodulate signals?
Mixer
Modulator
Amplifier
Discriminator
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Check of UnderstandingWhat is the purpose of an aerial on a receiver?
To convert the electromagnetic waves (em) into amplified voltages.
To convert the electromagnetic waves (em) into constant voltages.
To convert the electromagnetic waves (em) into large voltages.
To convert the electromagnetic waves (em) into tiny voltages.
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The superheterodyme receiver is used . . .
For improved sensitivity and selectivity
To operate a lounspeaker without audio frequency amplification
When intermediate frequency amplification is not require
For the reception of lower frequency signals than is possible with the tuned circuit receiver
Check of Understanding
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In a radio receiver, what is the process of converting
the radio signal frequency into audio frequencyknown as?
Demodulation
Superhetrodyning
Local Oscillation
Re-amplification
Check of Understanding
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This diagram shows a discriminator(ratio detector)
what does the item ‘P’ represent?
Reference Source
Carrier Input
Output Amplifier
Check of Understanding
Recovered Signal
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This diagram shows a discriminator system,what does the item ‘W’ represent?
Carrier Input
Receiver
Output Amplifier
Check of Understanding
Ratio Detector
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This diagram shows a discriminator(ratio detector)
what does the item ‘X’ represent?
Carrier Input
Output Amplifier
Check of Understanding
Recovered Signal
Reference Source
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This diagram shows a discriminator system,what does the item ‘Y’ represent?
Carrier Input
Receiver
Output Amplifier
Check of Understanding
Ratio Detector
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This diagram shows a discriminator system,what does the item ‘Z’ represent?
Carrier Input
Receiver
Output Amplifier
Check of Understanding
Ratio Detector
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Demodulate the signal
Amplify the signal after demodulation
Amplify and stabilise the signal
Convert the signal to a lower frequency
Check of UnderstandingIn a receiver,
what is the purpose of a radio frequency amplifier?
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In this diagram,what does the block marked ‘R’ represent?
Carrier Input
AF Amplifier
RF Amplifier
Check of Understanding
IF Amplifier
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In this diagram,what does the block marked ‘S’ represent?
Amplifier
Demodulator
Check of Understanding
Local Oscillator
Mixer
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In this diagram,what does the block marked ‘T’ represent?
Mixer
Demodulator
RF Amplifier
Check of Understanding
IF Amplifier
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Demodulator
Local Oscillator
IF Amplifier
Mixer
Check of UnderstandingIn this diagram,
what does the block marked ‘U’ represent?
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In this diagram,what does the block marked ‘V’ represent?
Amplifier
RF Amplifier
Demodulator
Check of Understanding
Local Oscillator
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In this diagram,what does the block marked ‘W’ represent?
IF Amplifier
Oscillator
RF Amplifier
Check of Understanding
AF Amplifier
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What type of circuit is used in a receiverto recover FM signals?
Modulator
Discriminator
Local Oscillator
Demodulator
Check of Understanding
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Advanced Radioand Radar
End of Presentation