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Advanced Radio and Radar Part 4 Receivers. Introduction We know a basic radio system consists of - a Transmitter a Transmitter The Tx converts information.

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Presentation on theme: "Advanced Radio and Radar Part 4 Receivers. Introduction We know a basic radio system consists of - a Transmitter a Transmitter The Tx converts information."— Presentation transcript:

1 Advanced Radio and Radar Part 4 Receivers

2 Introduction We know a basic radio system consists of - a Transmitter 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. TransmitterReceiver Tx Y Rx Y and a Reciever. Information

3 Introduction We 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

4 Aerials The 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 = λ F

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 wire to a complex array selecting 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 alternate with the frequency of the ‘em’ radiation, and are passed to the receiver circuitry for processing.

7 Aerials The signal strength the aerial inputs to the receiver is very tiny – to the receiver is very tiny – ( 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

8 The Receiver The Receiver- Receives the signal from the aerial (Ae) the aerial (Ae) Demodulator- Extracts the signal from radio frequency RF radio frequency RF to audio frequency AF. to audio frequency AF. Audio Amplifier- increases the signal strength for output to the speaker. ReceiverDemodulator Audio Amplifier YAe Basic Diagram Loud Speaker Let’s examine a basic receiver and what it consists of.

9 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 ReceiverDemodulator Audio Amplifier YAe Loud Speaker

10 The Receiver The superheterodyne principle offers a way to overcome some of these problems. This involves the effect that one ‘sine wave’ has over 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 of different frequencies

11 The Receiver When two notes of near equal frequency sound together - a periodic rise and fall in intensity can be heard - a beat. 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 !

12 The Receiver Let’s take two frequency waves f1f1f1f1 f2f2f2f2 & The upper wave, f 1 has a lower frequency than f 2 A composite wave would be – f 1 + f 2 This resultant wave is the Sum Frequency Sum Frequency

13 The Receiver Let’s take two frequency waves f1f1f1f1 f2f2f2f2 & If we join up the peaks and troughs The resultant wave is the Difference Frequency f 2 - f 1

14 The Receiver Let’s take two frequency waves f1f1f1f1 f2f2f2f2 & If we join up the peaks and troughs The resultant wave is the Difference Frequency f 2 - f 1 If f 1 = 248 kHz and f 2 = 252 kHz then 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 processed This lower frequency can be processed more effectively than the higher radio frequencies.

16 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). (gives most of gain). 5 Demodulator Extracts the intelligence from RF to AF signal. 6 AF AmplifierIncreases signal to required levels of output. 1 Y Basic Diagram Ae 2 output

17 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.

18 Remember the AM receiver The FM circuitry is similar to the AM system but uses a discriminator (also called a ratio detector) in place of a demodulator. FM Receivers Receiver YAe Audio Amplifier Loud Speaker Demodulator Reference Source Discriminator Ratio Detector CarrierInput Y Ae

19 FM Receivers The FM circuitry is similar to the AM system but 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 Loud SpeakerRecoveredSignal Reference Source Discriminator Ratio Detector CarrierInput Y Ae

20 FM Receivers ReceiverDemodulator Audio Amplifier Y Loud Speaker AM Receiver CarrierInput RecoveredSignal FM Receiver Amplifier for Output Loud Speaker RecoveredSignal Reference Source Discriminator Ratio Detector CarrierInput Y Ae

21 A tuned circuit is used to... Amplify all unwanted signals. Amplify all unwanted signals. Filter out all unwanted signals. Attenuate all unwanted signals. Select only unwanted signals. Check of Understanding

22 What do FM receivers use to demodulate signals? Mixer Modulator Amplifier Discriminator

23 Check of Understanding What 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. into tiny voltages.

24 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

25 In a radio receiver, what is the process of converting the radio signal frequency into audio frequency known as? Demodulation Superhetrodyning Local Oscillation Re-amplification Check of Understanding

26 This diagram shows a discriminator (ratio detector) what does the item ‘P’ represent? Reference Source Carrier Input Output Amplifier Check of Understanding Recovered Signal

27 This diagram shows a discriminator system, what does the item ‘W’ represent? Carrier Input Receiver Output Amplifier Check of Understanding Ratio Detector

28 This diagram shows a discriminator (ratio detector) what does the item ‘X’ represent? Carrier Input Output Amplifier Check of Understanding Recovered Signal Reference Source

29 This diagram shows a discriminator system, what does the item ‘Y’ represent? Carrier Input Receiver Output Amplifier Check of Understanding Ratio Detector

30 This diagram shows a discriminator system, what does the item ‘Z’ represent? Carrier Input Receiver Output Amplifier Check of Understanding Ratio Detector

31 Demodulate the signal Amplify the signal after demodulation Amplify and stabilise the signal Convert the signal to a lower frequency Check of Understanding In a receiver, what is the purpose of a radio frequency amplifier?

32 In this diagram, what does the block marked ‘R’ represent? Carrier Input AF Amplifier RF Amplifier Check of Understanding IF Amplifier

33 In this diagram, what does the block marked ‘S’ represent? Amplifier Demodulator Check of Understanding Local Oscillator Mixer

34 In this diagram, what does the block marked ‘T’ represent? Mixer Demodulator RF Amplifier Check of Understanding IF Amplifier

35 Demodulator Local Oscillator IF Amplifier Mixer Check of Understanding In this diagram, what does the block marked ‘U’ represent?

36 In this diagram, what does the block marked ‘V’ represent? Amplifier RF Amplifier Demodulator Check of Understanding Local Oscillator

37 In this diagram, what does the block marked ‘W’ represent? IF Amplifier Oscillator RF Amplifier Check of Understanding AF Amplifier

38 What type of circuit is used in a receiver to recover FM signals? Modulator Modulator Discriminator Local Oscillator Demodulator Check of Understanding

39 Advanced Radio and Radar End of Presentation


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