Advanced Radioand Radar

Part 4Receivers

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

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

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

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.

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.

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

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.

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

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

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 !

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

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

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

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.

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

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.

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

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

FM Receivers

Receiver DemodulatorAudio

Amplifier

Y

LoudSpeaker

AM ReceiverCarrier Input

RecoveredSignal

FM Receiver

Amplifier for Output

LoudSpeaker

RecoveredSignal

Reference SourceDiscriminator

Ratio DetectorCarrier

Input

YAe

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

Check of UnderstandingWhat do FM receivers use

to demodulate signals?

Mixer

Modulator

Amplifier

Discriminator

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.

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

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

This diagram shows a discriminator(ratio detector)

what does the item ‘P’ represent?

Reference Source

Carrier Input

Output Amplifier

Check of Understanding

Recovered Signal

This diagram shows a discriminator system,what does the item ‘W’ represent?

Carrier Input

Receiver

Output Amplifier

Check of Understanding

Ratio Detector

This diagram shows a discriminator(ratio detector)

what does the item ‘X’ represent?

Carrier Input

Output Amplifier

Check of Understanding

Recovered Signal

Reference Source

This diagram shows a discriminator system,what does the item ‘Y’ represent?

Carrier Input

Receiver

Output Amplifier

Check of Understanding

Ratio Detector

This diagram shows a discriminator system,what does the item ‘Z’ represent?

Carrier Input

Receiver

Output Amplifier

Check of Understanding

Ratio Detector

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?

In this diagram,what does the block marked ‘R’ represent?

Carrier Input

AF Amplifier

RF Amplifier

Check of Understanding

IF Amplifier

In this diagram,what does the block marked ‘S’ represent?

Amplifier

Demodulator

Check of Understanding

Local Oscillator

Mixer

In this diagram,what does the block marked ‘T’ represent?

Mixer

Demodulator

RF Amplifier

Check of Understanding

IF Amplifier

Demodulator

Local Oscillator

IF Amplifier

Mixer

Check of UnderstandingIn this diagram,

what does the block marked ‘U’ represent?

In this diagram,what does the block marked ‘V’ represent?

Amplifier

RF Amplifier

Demodulator

Check of Understanding

Local Oscillator

In this diagram,what does the block marked ‘W’ represent?

IF Amplifier

Oscillator

RF Amplifier

Check of Understanding

AF Amplifier

What type of circuit is used in a receiverto recover FM signals?

Modulator

Discriminator

Local Oscillator

Demodulator

Check of Understanding

Advanced Radioand Radar

End of Presentation