Advanced Radio and Radar Aerials Transmitters Receivers Lecture

Advanced Radio and Radar Aerials, Transmitters & Receivers Lecture Two

Aerials We know that The length of the aerial dictates the frequency at which it will transmit and receive. But the length of an aerial is also representative of the frequency of electromagnetic (em) radiation that the aerial will receive.

Aerials Remember – The shorter the length an aerial becomes, The higher the optimum frequency it will transmit and receive. The longer the length an aerial becomes, The lower the optimum frequency it will transmit and receive.

Aerials An aerial can vary from a length of wire to a complex array. But whatever its shape, its purpose is to detect the electromagnetic waves (‘em’) and convert them into tiny voltages. So receivers have to be extremely sensitive, and isolate the wanted signal from all the unwanted ones. This is achieved by using Tuned Circuits. which filters out all the unwanted signals.

Introduction Transmitters come in all shapes and sizes. Man-made satellites, Your mobile phone, The car alarm remote, Wi. Fi home hub. Such devices can have a very small power output of only about ½ Watt (not enough to light a single Xmas tree light) up to hundreds of Watts for a satellite. but a Medium Frequency (MF) radio transmitter will have a power rating of up to 500, 000 Watts ie ½ Megawatt.

AM Transmitter Y Aerial Master Oscillator Buffer Amplifier Microphone Amplifier Power Amplifier Let’s examine a basic transmitter and what it consists of. Master Oscillator Gives a continuous high-frequency (RF) current, that produces the em carrier wave. Buffer Amplifier Isolates the oscillator from the power amplifier, and prevents instability occurring. Power Amplifier Used to increase the power of the signal to the required level before radiation to the aerial. Amplifier Amplifies the microphone signal to the desired level for output.

AM Transmitter Y Aerial Master Oscillator Buffer Amplifier Microphone Amplifier Power Amplifier Modulation The oscillator is often crystal controlled, this ensures good frequency stability. Modulation takes place in the power amplifier stage. The modulation adapts the amplitude or frequency of the carrier wave, duplicating the modulating signal.

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

The Receiver Basic Diagram 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) Y Ae Receiver Demodulator Audio Amplifier Loud Speaker

The Receiver To overcome these receiver problems the Super-Heterodyne (superhet) receiver was developed, for improved sensitivity and selectivity by receiving lower frequencies. This lower frequency can be processed more effectively than the higher radio frequencies.

The Superhet Receiver Y Ae Basic Diagram 1 2 4 5 6 output 3 1 RF Amplifier Amplifies and stabilises the signal being received. 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. (gives most of gain). 5 Demodulator Extracts the intelligence from RF to AF signal. 6 AF Amplifier Increases signal to required levels of output. Amplifies the mixer 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 Ae Y Remember the AM receiver Receiver Demodulator Audio Amplifier Loud Speaker The FM circuitry is similar to the AM system but it uses a Discriminator to demodulate the signal. Y Ae Carrier Input Reference Source Discriminator

FM Receivers The discriminator circuit has been designed The FM circuitry is similar to the AM system to detect small differences in frequencies. but it uses These differences are to the a voltage a Discriminator to converted demodulate signal. output that represents the AF component input. Y Ae Carrier Input Reference Source Discriminator Recovered Signal Loud Speaker Amplifier for Output

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

Check of Understanding The shorter the length of an aerial becomes: The lower the optimum frequency that it will transmit and receive. The more efficient it is. The higher the optimum frequency that it will transmit and receive. The less efficient it is.

Check of Understanding The length of an aerial is representative of: The distance from which the aerial will receive em radiation The strength of em radiation that the aerial will receive The amplitude of em radiation that the aerial will receive The frequency of em radiation that the aerial will receive

Check of Understanding In a radio or radar transmitter, the amplifier: Amplifies the modulating signal to the desired level for output Amplifies the loudspeaker signal to the desired level for output Amplifies the microphone signal to the desired level for output Amplifies the headphone signal to the desired level for output

Check of Understanding In a radio or radar transmitter, the part that is used to increase the power of the signal to the required level before radiation from the aerial is called: The buffet amplifier The power amplifier The audio amplifier The main amplifier

Check of Understanding In a radio or radar transmitter, the part that isolates the oscillator from the power amplifying stage, and prevents instability occurring is called: The buffer amplifier The audio amplifier The isolating amplifier The demodulator

Check of Understanding A buffer amplifier is used in a transmitter to provide isolation from the power amplifier stage. The buffer amplifier also helps prevent: Amplifying the input from the microphone Instability occurring The need for a quartz crystal to keep the transmitter frequency The signal being modulated

Check of Understanding In a radio or radar transmitter, an oscillator produces a continuous high-frequency (RF) current. This fixed-frequency alternating current produces: The em “radio wave” The em “continuous wave” The em “short wave” The em “carrier wave”

Check of Understanding In a radio or radar transmitter, the modulator. . . Amplifies the carrier wave. Amplitude or frequency modulates the carrier wave, duplicating the modulating signal. Amplifies the radio wave. Amplitude or frequency modulates the radio wave, duplicating the modulating signal.

Check of Understanding In a radio or radar transmitter, the oscillator is often crystal-controlled to: Ensure good frequency stability Ensure good amplitude stability Ensure good frequency and amplitude stability Ensure several different frequencies can be used

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

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 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 The purpose of the radio frequency amplifier in a receiver is to: Amplify the signal after demodulation Convert the signal to a lower frequency Amplify the signal being recieved Demodulate the signal

Advanced Radio and Radar End of Presentation