In the beginning, radio acted solely as a wireless telegraph device, used to send Morse code. It was first employed for point-to-point communication where regular telegraph lines were unreliable. It was not until Canadian-born American physicist Reginald Fessenden came up with the idea for modulating these waves that it became possible for the waves to carry the information necessary for sound or pictures. he received a patent in 1901 for a new type of radio transmitter, which would produce what became known as "continuous waves." This is the first design to employ the principles behind amplitude modulation. It wasn't until 1906 that amplitude modulation was ready for public demonstration. Fessenden created the first radio broadcast in 1906 and took radio from a point-to-point wireless telegraph to a system of audio transmission to multiple people. Although Fessenden's invention was already capable of transmitting audio, the first commercial radio station was not established until 1920. This is a direct result of the cost and complexity of the device. The later invention of the vacuum-tube transmitter (first appearing in 1915) created the opportunity for widespread public broadcasting.

A communication system is made up of many things. The most basic communications system model requires a transmitter, a channel, and a receiver. Every communication system has these parts but other considerations are also made. For example, is the system to communicate to a single receiver or a million? How far do the communications have to travel? What kind of information is being sent? All these questions influence the design of any communication. Our group focused on AM radio transmission but we will discuss other types of systems as well.

Broadcast communication and point-to-point communication are the two major types of transmission in communication systems. Broadcast communication systems are intended to send information to many receivers at once. This is effectively used by television stations, radio stations, teachers in lecture halls, emails, and the internet. All of these are examples of one transmitter broadcasting information to many receivers. Point-to-point communication systems are just the opposite of broadcasting. Each transmitter is sending information to a limited number of receivers, in most cases it is only one. Examples of this are the telephone system, emails, personal conversations, and letters. Depending on the information content and sensitivity, one system is often preferred and is just more effective. It would be very expensive for television stations to implement point-to-point systems. Each user would have a transmitter at the station just for their receiver. Some systems like Citizens Band radio often have point-to-point information being transmitted but it is broadcast for all to hear. The government often implements point-to-point systems because of the highly sensitive material being sent.

The transmitter is a key part of the system. Factors such as what kind of information is to be sent, who should receive it, and how far does it have to travel all influence the transmitter's design. The earliest electronic transmitter was the telegraph. It was capable of sending dots and dashes. This sort of binary system worked for simple communication in the past, but it was obvious that a better system was needed. Communications across seas used to be as fast as the ship carrying it in the old days. Those days are over. Today people want, and often need, to communicate with others instantly and effectively. There are many transmitters in every home these days. Telephones, cell phones, and computers account for a great proportion of personal transmitters. Transmitters can be broken down into two categories: wired and wireless. Satellites are obviously wireless transmitters where as telephones are both wireless or wired. Wireless transmitters sometimes add increased use to users by becoming portable communication devices. The trend in telephones and many other systems suggests that wireless transmitters are the preferred devices. Information leaves the transmitter to make its way across the channel in hopes of reaching the transmitter intact. The channel can be a nasty place. The channel anything separating the transmitter and receiver. Information can be completely lost in the channel if the communications system is not carefully set up. Noise is introduced in the channel to signals and can destroy them in some cases. Examples of channels are the vacuum of space, air, water, optic fiber, and wires. Which type of channel is used in a communications system depends on how far the information needs to be sent and what kind of information is being sent. Many systems are a combination of types of channels.

Once information has made its way across the channel a receiver must pick it up. Most people would consider it silly to transmit without having anything receive those transmissions but some organizations don't, like NASA. Receivers have a key role in a transmission system. The rest of the system has gone to waste if information cannot be received. The receiver has to be able to know where to find information being transmitted to it and how it can get it. In AM radio you just need a capacitor, diode, and resistor to build a receiver. Most systems aren't this simple though. Many devices are required and sometimes computer code as well. Some information is coded at the transmitter and must therefore be decoded by the receiver. This obviously requires that the receiver and transmitter know the code. Receivers these days come in more shapes and sizes than can be imagined. Pagers and other devices have shrunk to the point where one doesn't know another person has them anymore.

AM radio is a broadcast communication system. It transmits audio signals that can be tuned in by anyone. It is also an example of a wireless communications system. It is based on a simple concept of amplitude modulation. A base signal is multiplied by a sinusoid to modulate the signal and place it at a carrier frequency for transmission. It then travels through the air to each receiver where it is demodulated. Listeners then may proceed to enjoy. The details of amplitude modulation are discussed in another section.

AMPLITUDE MODULATION (AM) COMMUNICATION SYSTEMS

There are two devices of basic importance in the type of communications systems we will be looking at – the transmitter and the receiver. Obviously, the transmitter sends out or “broadcasts” signals that the receiver picks up and interprets.

If we choose to modulate a signal by its amplitude, we must hold the frequency of the signal constant. To do this, we simply shift the spectrum of our signal m(t) to the carrier frequency w_c that we have chosen. This alters the spectrum of our signal in the frequency domain from M(w) to ½[M(w+w_c) + M(w- w_c)] (see figure below). Thus, the spectrum of our signal is shifted to the left and right by w_c.

M(w)

When the altered signal is sent from the transmitter to the receiver, the receiver must create a carrier that is in phase and frequency synchronism with the transmitter’s carrier. Or, we could just transmit a carrier along with the signal.

More generally, we have two options:

1) Create a sophisticated receiver (expensive) and a simple transmitter

2) Have the transmitter transmit a carrier A cos w_ct along with the modulated signal m(t) cos w_ct (transmitter needs a great deal of power – also expensive)

As AM communication systems are generally designed to be broadcast from one transmitter to a number of receivers, it makes more sense to take option two above, and build one high-powered transmitter that sends a carrier along with the modulated signal and many inexpensive receivers.

Our transmitted signal is of the form:

s_AM(t) = A cos w_ct + m(t) cos w_ct

m(t)

m(t) cos w_ct

A cos w_ct + m(t) cos w_ct

Thus, A + m(t) is now the modulating signal. The transmitted signal is simply a sinusoid with frequency w_c that fits in an “envelope” defined by A + m(t) and –[ A + m(t)] (see diagram below).

Now we must detect the signal m(t) from the modulated signal we received. To do this, we must first set a condition for envelope detection:

A + m(t) ³ 0 for all t

If we let m_p be the peak amplitude of m(t), then our condition becomes:

A ³ m_p

If this is true, then we can easily detect the envelope with simple circuitry (explained below) and hence determine m(t). Otherwise, we must use synchronous demodulation, which is a much more complex and expensive process than envelope detection.

The envelope detector is a simple circuit and uses few compnents (see diagram below). The AM signal goes through the diode, charging the capacitor until the voltage in the capacitor exceeds the voltage of the signal. At this point, the diode opens and the capacitor discharges slowly through the resistor, until the AM signal has a higher voltage than the capacitor again. In this way, the output voltage picks up the peak values of the sinusoidal AM signal, which define the envelope A + m(t). Knowing A, we now know what the original signal was.

Envelope Detector