Page 326 - Physics Coursebook 2015 (A level)

Advantages of FM
Advantages of AM
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Cambridge International A Level Physics
frequencies, it was still an advantage. The greater range
of frequencies available means that each station can use a higher bandwidth (about 200 kHz, compared to 9 kHz for AM). FM signals typically contain frequencies of 15 kHz or higher and the quality of sound produced is much higher when using FM transmission.
However, AM transmission has a number of advantages.
■■ The bandwidth needed for each AM transmission is less than for FM transmission. This means that more stations can be included in any given frequency range of the electromagnetic spectrum.
■■ The actual receiver and transmitter used for AM are less complicated and cheaper than for FM transmission.
■■ AM transmissions use lower frequencies and have longer wavelengths than FM. This means that these radio waves can diffract some way around the Earth, whereas FM is line-of-sight only. Thus AM can cover a larger area than FM transmissions, for the same power output.
The relative advantages of FM and AM are summarised in Table 20.1.
QUESTIONS
6 Figure 20.7 shows the frequency spectrum of the signal from a radio transmitter. A carrier and two sideband frequencies are present.
35 40 45 Frequency/kHz
Figure 20.7 For Question 6.
a What is the name of the type of modulation
that produces two sideband frequencies?
b What is the carrier frequency?
c What is the frequency of the signal used to modulate the carrier wave?
d What is the bandwidth of the transmitted signal?
7 a Calculate the number of separate AM radio stations of bandwidth 9 kHz that are possible in the frequency spectrum available for AM between 530 and 1700 kHz.
b Suggest why FM stations of bandwidth 200 kHz are not used for this range of frequencies.
8 Is the greater bandwidth available on FM an advantage or a disadvantage?
9 FM is used largely in towns and AM in rural settings. Suggest why.
below about 0.3 V as a 0 and any voltage above about 1.5 V as a 1. Small fluctuations in voltage will not be noticed.
Figure 20.8 shows an analogue and a digital signal. The digital signal is the number 0101001101, which is actually a pulse of 0 V followed by a pulse of 3 V and so on.
3
0 010100110 1 Time
Figure 20.8 Analogue and digital signals.
less electrical interference and noise
greater bandwidth produces a better quality of sound
greater area covered by one transmitter
smaller bandwidth means more stations available in any frequency range
cheaper radio sets
Table 20.1 The relative advantages of FM and AM broadcasting. Analogue and digital signals
An analogue quantity is one that can have any value, for example the height of a person. A digital quantity has only a few values, usually just two; for example a person is either male or female.
So far, the signals we have dealt with in this book have been analogue signals. For example, the voltage signal generated by a microphone is an analogue signal; the output voltage from the microphone can have any value, within limits, and is an exact representation of the pressure variation in a sound wave.
A digital signal, on the other hand, looks completely different and consists of a series of zeroes (0) and ones (1). A 1 in a digital signal is just the presence of a voltage pulse, usually a voltage value of a few volts. A 0 in a digital signal is the absence of a pulse and is a voltage close to 0 V. A typical digital electronic system will interpret any voltage
Voltage / V