COMMUNICATION PHYSICS

INTRODUCTION

Heart attacks are still one of the biggest killers in the world. The first hour after the initial attack is crucial. If the correct treatment and drugs can be given in this time most sufferers will survive to lead full lives. This has been known for many years and has been one of the reasons why survival rates are much poorer heart attack victims who live in more remote areas.

^Pixabay

The paramedics of nowadays are now able to use the latest modern communications technology to save more lives. They use portable cardiac sensing and monitoring equipment to measure the patient’s vital signs. These are transmitted instantly, sometimes using the mobile phone network, sometimes by satellite link, to a heart specialist at the hospital. The specialist can diagnose as accurately as if the patient were in the hospital, then give appropriate instructions to the paramedics. All this can be done while the patient is on the way to the hospital so that drugs can be administered immediately on arrival, without wasting more time in doing further tests.

The ideas in this article

The example above is just part of a communications revolution that we are living through. The growth of the internet and the mobile phone network has been logarithmic in the last decade.

As the demand for communication capacity increases, the technology must strive to provide it. The radio spectrum is highly congested, but the use of light in optical fibres is making broadband (high capacity) systems possible. And with the emergence of 4G and 5G, the future is very exciting.

In this article, I will be answering the question: ‘How is it possible to transmit so much information so easily? But first, we need to understand some important aspects of radio and telephone communication.

RADIO COMMUNICATION

Sound offers a flexible means of communication between people. We come equipped with our own transmitter and receiver – our voice and our ears. It does have some disadvantages, though: sound does not carry far, and if several people ‘transmit’ at the same time, the ‘receiver’ finds it very difficult to sort the messages out.

^{Original: Penubag Vector: Victor Blacus, CC BY-SA 3.0}

Radio waves offer one answer because they ‘carry’ the sound (or any other information we send) much further – as electromagnetic waves, travelling at a speed of 3 × 10⁸ metres per second in a vacuum. The diagram above shows where radio waves fit into the electromagnetic spectrum, and some of the features of the radio spectrum.

Radio waves have frequencies ranging from tens of kilohertz to thousands of megahertz. The sound waves we hear on the radio and telephone are within the range of human hearing: that is, from about 20 Hz to 20 000 Hz. Generally, radio waves have frequencies far higher than the frequency of the sound information that they ‘carry’.

THE TRANSMITTED SIGNAL

Communication systems using radio waves can transmit a variety of information which can be received at its destination as sound, video pictures or data of various types. Both sound and radio travel as waves. When a microphone converts sound into a changing electrical voltage, we refer to it as a signal: the information carried by the sound wave is transferred to the signal. Similarly, in a radio, the received radio wave is converted to a signal that is then processed by the radio: the signal is a fluctuating voltage which then causes a fluctuating current to flow along a wire. But before we look at the signal that is received, let’s see what a transmitted signal consists of.

There are two signals that go to make the signal being transmitted. First, there is a radio signal of constant amplitude and frequency, which is often called the carrier wave. This carries the information from the second signal, the information signal, which is combined with the carrier. The information signal fluctuates (varies), as the information changes. It can also be intermittent, meaning it may stop and start. The two signals – the carrier signal and the information signal – are combined in a process called modulation.

^{Signal processing system. Brews ohare - Own work, CC BY-SA 3.0}

THE MODULATION PROCESS

There are several different ways of achieving modulation. In the simplest type of modulation used to transmit simple codes such as Morse code. The radio wave is merely switched on and off.

More complex methods of modulation are needed to transmit sound information such as speech or music. The two methods, namely amplitude modulation (AM) and frequency modulation (FM), which are used by broadcasting stations are described below.

AMPLITUDE MODULATION (AM)

When the information signal is combined with the carrier wave, the amplitude of the carrier signal is altered. Below is an outline for a simple amplitude modulated radio transmitter designed to transmit sound signals. It shows how the signal is built up.

Audio amplifier: a microphone converts the sound into an electronic signal, which is usually very small, so an amplifier makes it larger.
Radio-frequency oscillator: an oscillator is a circuit that generates an electronic ‘carrier’ signal, in this case, a radio frequency signal, of constant amplitude and frequency.
Mixer or modulator: the circut in which the information signal, in this case, the signal produced from the sound, and the carrier are combined.
Radio-frequency (RF) amplifier: the signal produced by the modulator is a more complex radio frequency signal. A special amplifier makes this signal stronger before it is fed into the aerial which ‘radiates’ the signal.

THE MODULATOR SIGNAL

The signal from the modulator is quite complex. It is a mixture of signals fed into the modulator. It is easier to understand if we think about a transmitter sending a simple sound (audio) signal such as a pure tone – a note of single frequency. Let’s call it f_o. The radio frequency carrier signal has a frequency of f_c. f_c would be a much higher frequency than f_o.

The mixer combines these two signals to produce two new frequencies as well as the two signals we started with. The two new signals are the sum and the difference of f_o and f_c:

(f_c+ f_o) and (f_c - f_o)

The output of the mixer contains all four frequencies:

f_o,f_c, (f_c+ f_o) and (f_c + f_o) and (f_c - f_o)

Three of the frequencies at the output of the modulator are roughly the same, they are all radio frequency signals. The odd one out is f_o. As an audio frequency signal, its frequency is much lower than the other three. The last stage in our simple transmitter is an amplifier. It is a radio frequency amplifier which only amplifies the three radio frequency signals, f_o is effectively filtered out; it doesn’t reach the aerial.

BANDWIDTH

In most real radio transmissions, the sound signal will be more complex than the single tone mentioned earlier. Instead of a single pure frequency, it will be a range of audio frequencies representing, say, a person’s voice or the music of a group. For telephone communication and most radio, the sound is transmitted in a band of audio frequencies ranging from 300 Hz to 4 kHz. We would say that this signal has a signal bandwidth of slightly less than 4 kHz. When this band of frequencies is used to amplitude modulate the carrier, the resulting output is the carrier, f_c, and two sidebands.

In amplitude modulation, the signal transmitted includes all the frequencies shown above. The sidebands are the important part because they carry the information. (It would be pointless to just send the carrier signal.)

The signal transmitted covers a range of frequencies, and we say that the transmitted signal has a bandwidth. The bandwidth of the signal is shown to be 8 kHz. That means that this radio station occupies 8 kHz of frequency space. The space the station occupies is often described as a channel and in this case the channel bandwidth is 8 kHz. Notice also that:

channel bandwidth = 2 × maximum audio frequency

Radio transmissions are organized into bands by international agreement. They are given names relating to their wavelengths, for example, the ‘medium wave band’ and the ‘long wave band’. Each band itself covers a particular range of frequencies. The medium wave band covers a range from 500 kHz to about 1.6 Mhz.

If radio stations on a band wanted to use amplitude modulation to transmit signals over a particular range, they could each be allowed 10 kHz of frequency space This would avoid any overlap of adjacent stations and they could operate without interfering with each other.

Depth of modulation

When two signals are added together as in amplitude modulation. The relative amplitude of the two signals is also important. If the carrier amplitude is much larger than the signal. Then the modulation will be weak and would be difficult to use effectively. On the other hand, if the signal amplitude is too large, then the result is ‘over-modulation’ which results in a distorted signal being received. The amount or ‘depth’ of modulation is usually expressed as a percentage.

In an example of amplitude-modulated carriers; at 50% depth of modulation, the minimum amplitude of the modulated carrier is half of the maximum amplitude. At 100% the minimum is just zero. If the depth of modulation increases any more, then the signal received would be distorted.

Power distribution in amplitude modulation

Amplitude modulation is quite inefficient. The information to be transmitted is contained in the sidebands, yet most of the energy transmitted is contained in the carrier signal. Although this is a disadvantage, receivers for amplitude modulation signals are easy and cheap to make.

As the depth of modulation increases, more energy is carried by the sidebands and less by the carrier but, as explained in the “Depth of modulation”, there is a limit at 100% modulation depth before the signal would be distorted. At this point, the efficiency reaches a maximum of 33%.

It should also be noticed that the information contained in each sideband is the same. It should only be necessary to actually transmit and receive one sideband to access the information. This leads to a far more efficient type of amplitude modulation called ‘single sideband’ or SSB. SSB is more efficient in terms of the energy transmitted and also uses a smaller portion of the frequency spectrum; it has a narrower bandwidth. This means that more SSB transmissions can be allowed in the same band space as ordinary amplitude modulated signals. However, SSB receivers are more complex and expensive.

FREQUENCY MODULATION (FM)

Frequency modulation is another very common form of modulation. FM gives a far better quality signal than AM because AM is very easily affected by ‘noise’ and FM is not.

Noise means the random signals that are present in all circuits, and the atmospheric noise in the signal picked up by the aerial in a radio. Electrical noise can also be produced by electrical machinery, such as drills or vacuum cleaners.

Whatever the source, the noise is a signal which affects and interferes with the amplitude of the information signal being received, and so, in a radio, you will hear a signal distorted by crackles and hiss. Since FM is a frequency variation, changes in amplitude do not affect it nearly as much. The mathematics of FM is more complicated and beyond the scope of this article, but the results are important.

In FM the audio signal which carries the information is used to modulate the frequency of the carrier signal (rather than the amplitude, as in AM). The spectrum of an FM signal is also more complicated than the AM signal spectrum. There are many more sidebands and the channel bandwidth is greater.

FM is used for stereo sound transmissions where high-quality reproduction is important. For stereo quality, the signal bandwidth is 20 kHz, which is sufficient to cover the range of human hearing (often quoted as 20 Hz to 20 kHz). Because FM requires a very big bandwidth, FM stations in Britain are allowed a channel bandwidth between 150 kHz and 200 kHz. This means that the signals need to be transmitted on a frequency band which is much higher than for AM. The FM radio stations in Britain fit into a band from 88 MHz to 108 MHz.

I’ll be stopping here for now. In my next post, I’ll be talking about the methods of modulation and digital communications. Till then, I remain my humble self Hive account@emperorhassy.