Advanced Radio and Radar Part 4 Receivers

Introduction Y Y We 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. and a Reciever. Tx Y Rx Y Information Transmitter Receiver

Introduction MODULATION 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

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 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

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.

Aerials Tuned Circuits. The 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

Let’s examine a basic receiver The Receiver Basic Diagram 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 strength for output to the speaker. Let’s examine a basic receiver and what it consists of. Y Ae Loud Speaker Receiver Demodulator Audio Amplifier

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) Receiver Demodulator Audio Amplifier Y Ae Loud Speaker

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

The Receiver When 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 Receiver f1 + f2 Sum Frequency Let’s take two frequency waves & f2 The upper wave, f1 has a lower frequency than f2 A composite wave would be – This resultant wave is the Sum Frequency f1 + f2

The Receiver f2 - f1 Difference Frequency Let’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 Receiver f2 - f1 Difference Frequency Let’s take two frequency waves f1 & f2 If we join up the peaks and troughs If f1 = 248 kHz and f2 = 252 kHz The resultant wave is the Difference Frequency f2 - f1 then this new wave gives a beat of 4 kHz

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 more effectively than the higher radio frequencies.

The Superhet Receiver Basic Diagram Y Ae output 1 2 4 5 6 3 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.

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.

FM Receivers Y Y 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. Receiver Y Ae Audio Amplifier Loud Speaker Demodulator Carrier Input Y Ae Reference Source Discriminator Ratio Detector

FM Receivers Y 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. The FM circuitry is similar to the AM system but uses a discriminator (also called a ratio detector) in place of a demodulator. Carrier Input Y Ae Reference Source Discriminator Ratio Detector Recovered Signal Loud Speaker Amplifier for Output

FM Receivers Y Y AM Receiver FM Receiver Ae Recovered Carrier Signal Demodulator Audio Amplifier Y Loud Speaker AM Receiver Carrier Input Recovered Signal FM Receiver Amplifier for Output Loud Speaker Recovered Signal Reference Source Discriminator Ratio Detector Carrier Input Y Ae

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.

Check of Understanding What do FM receivers use to demodulate signals? Modulator Discriminator Amplifier Mixer

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.

Check of Understanding The superheterodyme receiver is used . . . To operate a lounspeaker without audio frequency amplification For the reception of lower frequency signals than is possible with the tuned circuit receiver When intermediate frequency amplification is not require For improved sensitivity and selectivity

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Advanced Radio and Radar End of Presentation