Name ………………………………………………………
Advancing Physics AS
Chapter 3.2a Signalling
Student Notes
August 2008
John Mascall
The King’s School, Ely
Section 3.2 Signalling with electromagnetic waves: radio spectrum; polarisation;
spectrum of a signal; bandwidth
Learning outcomes
●
Communication with electromagnetic waves uses frequencies from a few thousand
hertz to infrared frequencies and above, divided into bands used for different
purposes.
●
Electromagnetic waves can be polarised; the orientation of a detector has to take
this into account.
●
A signal can be analysed into the frequencies it consists of – its spectrum.
●
A signal channel has a capacity, the rate at which it can transmit information,
measured in bits per second.
●
The bandwidth of a signal is the range of frequencies in its spectrum. The larger the
bandwidth the greater the rate of transmission of information.
●
Noise limits the rate at which information can be transmitted.
The radio spectrum
It is worth recalling details of the electromagnetic spectrum (page 6 of the student text)
with particular reference to communication wave bands such as radio.
Display Material 80O
OHT 'Signal bands for communication'
Communication wavebands
frequency
wavelength
10 km
30 kHz
LF
low frequency
navigation, radio beacons,
long-distance broadcasting
MF
medium
frequency
national broadcasting,
aeronautical nav igation
HF
high frequency
long-distance broadcasting,
amateur radio, maritime radio
VHF
very high
frequency
FM radio, mobile radio
communications
UHF
ultra high
frequency
telev ision, mobile telephone
networks
SHF
super high
frequency
satellite links, ground
microwave links, radar
1 km
300 kHz
3 MHz
30 MHz
100 m
10 m
300 MHz
3 GHz
30 GHz
300 GHz
1m
100 mm
10 mm
EHF
extremely
high
radar, radio astr onomy
far infrared
infrared astronomy
1 mm
100 m
3 THz
mid infrared
infrared astronomy
near infrared
optical fibre, remote controls,
bar codes, CD player
10 m
30 THz
300 THz
1 m
Communications use wave lengths of between 1m and 10km
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The following paragraph taken from Activity 110H Home Experiment ‘Home experiments
with radio and television signals’ illustrates the meaning of bandwidth:
Do you have a portable FM radio with dial rather than push-button tuning? If so, spin the dial and
notice the frequencies at which stations come up. Typical frequencies are in the range 90–100
MHz or so. The strong signals will not be closer than 0.2 MHz (200 kHz) apart. Notice the range of
frequencies over which you can still hear a strong signal as you tune the radio 'through' its
frequency. It may be about 0.1 MHz either side of the correct frequency. That 'bandwidth' allows
for the variations in frequency produced by the radio waves carrying the audible signal.
Polarisation
It is convenient to start with portable television aerials, noting their polarisation (and
directionality).
We revisit Activity 110H Home Experiments 'Home experiments with radio and television
signals'. You may wish to carry out further work on this at home.
Polarisation with waves on a rope, light, 3 cm microwaves, and 1 GHz UHF radio waves
should all be demonstrated using Activity 120P Presentation 'Polarisation of waves'.
Display Material 90O
OHT 'Polarisation'
Polarisation by scattering should be demonstrated.
When the permitted direction of vibration or polarisation of the filter is parallel
to the direction of the polarisation of the wave, it is transmitted by the filter.
When the permitted direction of vibration or polarisation of the filter is perpendicular
to the direction of the polarisation of the wave, it is absorbed or reflected but
not transmitted.
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The following activities are optional:
Activity 130E Experiment 'Polarisation by scattering'
Activity 140D Demonstration 'Polarisation of reflected light'
Spectra of signals
It is useful to start with the spectra of sounds. Sounds can be synthesised and the sound
spectra analysed. In this part it is important to keep going backwards and forwards
between waveform and spectrum, and considering the relation between them.
You can try Activity 150H Home Experiment ‘Telling frequencies apart’ which shows that
the ear can sort out a sound into the different frequencies that sound is made up of. Your
eyes are unable to do this. If you shine two differently coloured lights onto a screen you
will see one new colour and not two mixed colours.
Activity 170E Experiment ‘Spectrum analysis: simple signals’ starts with simple
waveforms from a signal generator. Different signals are then added to look at the effect of
having more than one frequency. A spectrum analyser can be used to identify which
frequencies are present in the waveform.
In Activity 160S Software Based 'Filtering sounds' you can make a sound with two
frequencies, hear them both, and then get rid of one of them. This exercise uses the
Audacity software.
In Activity 180S Software Based 'Spectrum analysis: Going further' we use Multimedia
Sound to carry out spectral analysis on more complex waveforms.
The bandwidth of a signal’s spectrum is the range of frequencies it covers.
This idea can be reinforced by using an exercise on listening to sounds with reduced
bandwidth to simulate the problems experienced by the deaf.
Try Activity 190S Software Based 'Hearing impairment: Using a digital filter'.
The healthy human ear is able to hear sounds with frequencies from a few tens of hertz to
between 15 and 20 kHz. This range is greatly reduced for people who have hearing
difficulties and this demonstration will give you some idea of what such a partially deaf
person might hear.
Try Activity 210S Software Based 'Cleaning up a sound'
Being able to see a recorded sound as a complex of frequencies helps to suggest
strategies for identifying and highlighting the sound you intended to record. In this activity
you manipulate a sound file that contains wanted and unwanted signals.
Activity 220S Software Based 'Building up a sound' involves synthesising a complex
waveform from pure tones. Fourier showed that a waveform of any complexity can be
broken down into a mixture of waveforms which are pure tones – that is, waves which can
be described by simple sines and cosines. In this activity you build up a complex waveform
from a series of simple tones. Tone telephones use such mixtures of tones to signal the
different dialling numbers.
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The following sound files from the Advancing Physics CD-ROM may useful.
File 10L
Launchable File 'Samples of music'
File 20L
Launchable File 'Samples of everyday sounds'
File 30L
Launchable File 'Samples of speech'
File 40L
Launchable File 'Whistle over radio'
Much of the work on sounds can be summarised using Display Material 100S
Computer Screen 'Atlas of sound spectra'
Here you can see the waveforms and spectra of a variety of sounds, some natural and
some electronically generated.
The oboe
The oboe produces a complex
sound, with a considerable number
of different frequencies in its
spectrum. The musical character of
the sound is indicated by the
discretely spaced frequencies,
having simple numerical
relationships to one another.
The clarinet
The clarinet here, like the oboe,
produces a range of discrete
frequencies simply related to one
another. The lower frequencies are
rather dominant, but the spectrum
extends over a wide range.
The xylophone
The sound of the xylophone has
been caught just as a note is struck.
Initially the sound contains a highfrequency 'ringing', but this dies
away and the sound becomes the
pure tone being struck.
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The snare drum
A drum does not produce a simple
musical tone consisting of one or
more discrete related frequencies,
but instead produces a spectrum
covering a wide range of
frequencies. The sound has a pitch,
decided by the range of frequencies
over which the spectrum is centred.
A single pure tone
An electronically generated 1000 Hz
pure sinusoidal oscillation has a simple
spectrum: just a single peak at 1000 Hz.
Two tones sounding together
Tones of 1000 Hz and 3000 Hz were
here electronically combined. The
spectrum shows two peaks.
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'White' noise
This noise was electronically generated.
It makes a 'rushing' sound something
like wind in trees or a mountain stream
or waterfall. It is called 'white' noise
because its spectrum is uniformly
spread over the whole audible
frequency range. ('Pink' noise has larger
low-frequency components.)
Single short pulse
This single short pulse of 1000 Hz
tone, lasting only 5 ms (five cycles)
was generated electronically. It is
difficult to obtain the 'spectrum' of
such a sound, but the spectrum
shown does have the important
correct feature of spreading over a
wider range of frequencies than a
continuous pure tone.
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