f1_radio_r2

8/3/2019 F1_Radio_R2

http://slidepdf.com/reader/full/f1radior2 1/6

F1 Radio Communication

F1.1 Modulation

Modulation is when a high frequency carrier wave is made to vary in accordance with the amplitudeof a signal wave. While there are many ways in which this can be achieved, each method

essentially changes either the amplitude or the frequency of the carrier wave.

F1.2 Carrier and signal waves

The signal wave is the information which is to be sent from one place to another. It can be speech,

music, video or computer information.

The carrier wave is the high frequency electromagnetic wave which will transport the information

as radio waves. It is important that the pure carrier wave is at a constant frequency so that its

amplitude, frequency or phase can be moulded, or modulated with the signal wave form.

F1.3 AM and FM

Amplitude Modulation

The diagram below represents an Amplitude Modulated wave. In amplitude modulation, the

magnitude of the carrier is varied in accordance with the amplitude of the information signal. This

modulation system was widely used in radio transmission but is now being replaced by Frequency

Modulation and the various methods of digital modulation.

AmplitudeModulatedCarrier

time

Carrier

time

Information

time

Voltage

Voltage

Voltage

©ikes1201

8/3/2019 F1_Radio_R2


With an amplitude modulated wave the amplitude of the radio frequency carrier is varied in

proportion to the audio frequency or information signal, as in the diagram above. The amount, or

depth, of modulation depends upon the ratio of the amplitude of the information signal to the

amplitude of the carrier wave signal.

Depth of modulation

Consider the diagram below: x is the modulating signal amplitude and y is the carrier wave

amplitude.

The modulation depth, m, of the resulting amplitude modulated signal is defined by:

m =x

y100%×

The modulation depth of the waveform in the diagram below is approximately 65% and can be

verified by measuring the amplitudes of the waves in the diagram. The peak amplitude of the

modulated wave is the sum of the modulating and carrier waves. If x is equal to y then the carrier is

100% modulated. If x is increased further then over modulation occurs and the region represented

by y-x in the diagram becomes zero. However, if the modulation depth is too small then the

received signal will be of poor quality because the signal-to-noise ratio will be reduced. The usual

depth of modulation for good quality reception is approximately 80%.

carrier modulating signal

amplitude modulated signal

xy

yy+x

y-x

©ikes1201

Diagram (a) below show an amplitude modulated wave at 100% and diagram (b) shows an

amplitude modulated wave that is being over modulated (i.e. greater than 100%). Both diagrams

assume the same carrier amplitude as for the diagram above.

8/3/2019 F1_Radio_R2


(a) (b) ©ikes1201

Frequency modulation

As the name suggests, the frequency of the carrier wave is varied in accordance with the variations

in the information signal. This is shown in the diagram below.

time

Carrier

time

Information

FrequencyModulatedCarrier

time

Voltage

Voltage

Voltage

©ikes1201

Each graph shows the variation of voltage against time. The first graph shows the high frequency

carrier signal, the second the low frequency information signal and the third the effect of frequency

modulation of the carrier by the information signal. It can be seen that as the information signal

increases and becomes positive, the frequency of the carrier increases and as the information signal

decreases and becomes negative, the carrier frequency is reduced. FM is used in high quality radio

transmission and a typical FM carrier frequency (for radio transmission) is 100MHz and the

maximum frequency deviation is limited, by international agreement, to ± 75kHz. There has to be a

compromise between the improved quality that accompanies increased bandwidth and therestriction of the number of channels available in a frequency band.

8/3/2019 F1_Radio_R2


F1.4 Modulation and frequency problems.

Please see the Student Work Book.

F1.5 Power spectrum of an AM signal.

The process of modulation results in the production of frequencies other than those of the carrier

and the modulating signal. When a modulating signal of frequency f s, is combined with a carrier

wave of frequency f c, a signal, comprising of three frequencies (f c−f s), f c and (f c+f s), results. This is

shown in the diagram below.

frequency/HzLower side tone Upper side tone

Carrier power

f c

f s+f c – f sf c

©ikes0807

It can be shown that for 100% modulation the amplitude of each side tone is half of the amplitudeof the carrier wave and so since

R

Vpower

2

=

each side tone carries only a quarter of the power of the carrier. Since there is no information in the

carrier wave signal and each side tone carries the same information, only one side tone is needed to

recover the transmitted information. This means that 83.3% of the transmitted power is wasted in a

standard AM transmission.

8/3/2019 F1_Radio_R2


F1.6 Sidebands and bandwidth

Since the modulating signal usually consists of a band of frequencies, the resulting frequency

spectrum of the modulated signal is shown in the diagram below.

frequency/Hz

Lower sideband Upper sideband

Carrier

Modulated signal bandwidth

amplitude

f c©ikes1201

If the frequency of the modulating signal ranges from 100Hz to 5kHz then the lower sideband will

range from (f c−5000)Hz to (f c−100)Hz, the upper sideband will range from (f c+100)Hz to

(f c+5000)Hz, and the bandwidth of the transmitted radio signal will be 10kHz or twice the

bandwidth of the modulating signal.

It is possible to suppress one of the sidebands and the carrier if there is a constraint on channel

bandwidth. Since it is only the information in one sideband that is needed on reception, the power

that would put into transmitting the carrier and the other sideband, in a normal AM signal, can be

concentrated into just the one sideband, resulting in a much more potent signal.

F1.7 Sideband and bandwidth problems

Please see the Student Work Book

F1.8 Merits of AM and FM

Advantages and disadvantages of AM and FM transmissions.

AM and FM are both used for radio transmission and each have their benefits.AM is used for low cost transmitters and receivers because the circuitry used is simpler in

comparison with that used in FM.

AM transmissions are more susceptible to noise and distortion which often shows itself as hisses

and crackles. FM transmissions are largely immune from this so long as the radio signal is strong.

Both AM and FM radio signals will suffer from fading; when the strength of the radio signal is

reduced owing to buildings, tunnels, reflections etc. However, AM radio signals are affected much

more than FM radio signals because they are normally transmitted at a much lower frequency.

FM transmissions usually require a much larger bandwidth than AM transmissions. Most

commercial FM stations use the VHF waveband since it enables the larger bandwidth to be

accommodated, but such high frequencies limit transmissions to a short range (‘line of sight’).

This means that more transmitters are needed for nation-wide coverage, although it does permit

stations which are geographically separate (> 100 kilometres) to operate on adjacent parts of the

band without causing interference. This makes VHF FM useful for local radio stations.

8/3/2019 F1_Radio_R2


F1.9 An AM Radio Receiver

The diagram below shows the block diagram of a simple AM radio receiver.

tuned circuit rf amplifier demodulator af amplifier

©IPK0807

aerial

speaker

The aerial receives the radio signals (electromagnetic waves) and converts them into a varying

electric current.

The tuned circuit selects the required frequency signals from all of the signals that are received by

the aerial.

The rf amplifier increases the voltage of the selected signals to a level where they can be

demodulated.

The demodulator often consists of a signal diode which simply blocks the negative going half of

the AM signal. The rf component of the demodulated signal is removed by a capacitor, which

shunts the rf component to earth, leaving the information (af) signal.

The af amplifier provides both voltage and power amplification of the af signal so that it is able to

drive a loudspeaker.

The loudspeaker converts the varying electric current of the af signal into sound waves.

f1_radio_r2

Documents