heterodyning process. Describe the basic difference between an AM and an fm receiver. Describe single-sideband suppressed carrier communications. ">
INTRODUCTION TO COMMUNICATIONS THEORY
Upon completion of this chapter you will be able to:
In the previous chapter you learned the fundamentals of U.S. naval telecommunications and communications. Now, let's look at the equipment and systems that are used to communicate in the Navy. The fundamental equipment used to communicate are the transmitter and receiver.
Transmitters and receivers must each perform two basic functions. The transmitter must generate a radio frequency signal of sufficient power at the desired frequency. It must have some means of varying (or modulating) the basic frequency so that it can carry an intelligible signal. The receiver must select the desired frequency you want to receive and reject all unwanted frequencies. In addition, receivers must be able to amplify the weak incoming signal to overcome the losses the signal suffers in its journey through space.
Representative transmitters and their fundamental features are described for you in this module.
Basic communication transmitters include continuous wave (cw), amplitude modulated (AM), frequency modulated (fm), and single sideband (ssb) types. A basic description of each of these transmitters is given in this chapter.
CONTINUOUS WAVE TRANSMITTER
The continuous wave is used principally for radiotelegraphy; that is, for the transmission of short or long pulses of rf energy to form the dots and dashes of the Morse code characters. This type of transmission is sometimes referred to as interrupted continuous wave. Cw transmission was the first type of radio communication used, and it is still used extensively for long-range communications. Two of the advantages of cw transmission are a narrow bandwidth, which requires less output power, and a degree of intelligibility that is high even under severe noise conditions. (For example, when the receiver is in the vicinity of rotating machinery or thunderstorms.)
A cw transmitter requires four essential components. These are a generator, amplifier, keyer, and antenna. We have to generate rf oscillations and have a means of amplifying these oscillations. We also need a method of turning the rf output on and off (keying) in accordance with the intelligence to be transmitted and an antenna to radiate the keyed output of the transmitter.
Let's take a look at the block diagram of a cw transmitter and its power supply in figure 2-1. The oscillator generates the rf carrier at a preset frequency and maintains it within close tolerances. The oscillator may be a self-excited type, such as an electron-coupled oscillator, or a quartz crystal type, which uses a crystal cut to vibrate at a certain frequency when electrically excited. In both types, voltage and current delivered by the oscillator are weak. The oscillator outputs must be amplified many times to be radiated any distance.
Figure 2-1. - Cw transmitter block diagram.
The buffer stage or first intermediate power amplifier stage (referred to as the ipa) is a voltage amplifier that increases the amplitude of the oscillator signal to a level that drives the power amplifier (pa). You will find the signal delivered by the buffer varies with the type of transmitter and may be hundreds or thousands of volts.
The buffer serves two other purposes. One is to isolate the oscillator from the amplifier stages. Without a buffer, changes in the amplifier caused by keying or variations in source voltage would vary the load of the oscillator and cause it to change frequency. It may also be used as a frequency multiplier, which is explained later in this text.
As you can see in the figure, a key is used to turn the buffer on and off. When the key is closed, the rf carrier passes through the buffer stage; when the key is open (buffer is turned off), the rf carrier is prevented from getting through.
The final stage of a transmitter is the power amplifier (referred to as the pa). In chapter 3 of NEETS, Module 1, Introduction to Matter, Energy, and Direct Current, you learned that power is the product of current and voltage (P = IE). In the power amplifier a large amount of rf current and voltage is made available for radiation by the antenna.
The power amplifier of a high-power transmitter may require far more driving power than can be supplied by an oscillator and its buffer stage. One or more low-power intermediate amplifiers are used between the buffer and the final amplifier that feeds the antenna. The main difference between many low- and high-power transmitters is in the number of intermediate power-amplifier stages used.
Figure 2-2 is a block diagram of the input and output powers for each stage of a typical medium-power transmitter. You should be able to see that the power output of a transmitter can be increased by adding amplifier stages capable of delivering the power required. In our example, the .5 watt output of the buffer is amplified in the first intermediate amplifier by a factor of 10, (this is a times 10 [ X 10] amplifier) giving us an input of 5 watts to the second intermediate amplifier. You can see in this example the second intermediate amplifier multiplies the 5 watt input to it by a factor of 5 ( X 5) and gives us a 25 watt input to our power (final) amplifier. The final amplifier multiplies its input by a factor of 20 (X 20) and gives us 500 watts of power out to the antenna.
Figure 2-2. - Intermediate amplifiers increase transmitter power.
In AM transmitters, the instantaneous amplitude of the rf output signal is varied in proportion to the modulating signal. The modulating signal may consist of many frequencies of various amplitudes and phases, such as the signals making up your own speech pattern.
Figure 2-3 gives you an idea of what the block diagram of a simple AM transmitter looks like. The oscillator, buffer amplifier, and power amplifier serve the same purpose as those in the cw transmitter. The microphone converts the audio frequency (af) input (a person's voice) into corresponding electrical energy. The driver amplifies the audio, and the modulator further amplifies the audio signal to the amplitude necessary to fully modulate the carrier. The output of the modulator is applied to the power amplifier. The pa combines the rf carrier and the modulating signal in the power amplifier to produce the amplitude-modulated signal output for transmission. In the absence of a modulating signal, a continuous rf carrier is radiated by the antenna.
Figure 2-3. - AM radiotelephone transmitter block diagram.
In frequency modulation (fm) the modulating signal combines with the carrier to cause the frequency of the resultant wave to vary with the instantaneous amplitude of the modulating signal.
Figure 2-4 shows you the block diagram of a frequency-modulated transmitter. The modulating signal applied to a varicap causes the reactance to vary. The varicap is connected across the tank circuit of the oscillator. With no modulation, the oscillator generates a steady center frequency. With modulation applied, the varicap causes the frequency of the oscillator to vary around the center frequency in accordance with the modulating signal. The oscillator output is then fed to a frequency multiplier to increase the frequency and then to a power amplifier to increase the amplitude to the desired level for transmission.
Figure 2-4. - Fm transmitter block diagram.
True harmonics are always exact multiples of the basic or fundamental frequency generated by an oscillator and are created in amplifiers and their associated circuits. Even harmonics are 2, 4, 6, and so on, times the fundamental; odd harmonics are 3, 5, 7, and so on, times the fundamental. If an oscillator has a fundamental frequency of 2,500 kilohertz, the harmonically related frequencies are
You should note that the basic frequency and the first harmonic are one and the same.
The series ascends indefinitely until the intensity is too weak to be detected. In general, the energy in frequencies above the third harmonic is too weak to be significant.
In some electronics books, and later in this chapter, you will find the term SUBHARMONIC used. It refers to a sine wave quantity (for example, an oscillator output) that has a frequency that is a submultiple of the frequency of some other sine wave quantity it helped make. For example, a wave that is half the fundamental frequency of another wave is called the second subharmonic of that wave; one with a third of the fundamental frequency is called a third subharmonic; and so forth.
Q.4 What purpose does a microphone perform in an AM transmitter?
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