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F1 Radio Communication
F1.1 Modulation
Modulation is when a high frequency carrier wave is made to vary in accordance with the amplitude
of 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.
Voltage
Carrier
time
Voltage
Information
time
Voltage
Amplitude
Modulated
Carrier
time
©ikes1201
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:
x
 100%
y
m =
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%.
y
x
modulating signal
carrier
y+x
y
y-x
©ikes1201
amplitude modulated signal
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.
(a)
©ikes1201
(b)
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.
Voltage
Carrier
time
Voltage
Information
time
Voltage
Frequency
Modulated
Carrier
time
©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 the
restriction of the number of channels available in a frequency band.
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 fs, is combined with a carrier
wave of frequency fc, a signal, comprising of three frequencies (fcfs), fc and (fc+fs), results. This
is shown in the diagram below.
Carrier
power
f c – fs
Lower side tone
f c + fs
fc
frequency/Hz
©ikes0807
Upper side tone
It can be shown that for 100% modulation the amplitude of each side tone is half of the amplitude
of the carrier wave and so since
power 
V2
R
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.
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.
amplitude
Modulated signal bandwidth
Carrier
Lower sideband
fc
frequency/Hz
Upper sideband ©ikes1201
If the frequency of the modulating signal ranges from 100Hz to 5kHz then the lower sideband will
range from (fc5000)Hz to (fc100)Hz, the upper sideband will range from (fc+100)Hz to
(fc+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.
F1.9 An AM Radio Receiver
The diagram below shows the block diagram of a simple AM radio receiver.
aerial
tuned circuit
rf amplifier
demodulator
af amplifier
©IPK0807
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.