In this section I will introduce you to the fundamental equipment used for communications. These 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 series of tutorials.
Basic communications transmitters include continuous wave (cw), amplitude modulated (AM), frequency modulated (fm), and single sideband (ssb) types. A basic description of each of these transmitters used in communications will be covered in subsequent tutorials.
CONTINUOUS WAVE TRANSMITTER IN COMMUNICATIONS
The continuous wave is used principally for radiotelegraphy communications; that is, for the transmission of short or long pulses of rf energy to form the dots and dashes of the Morse code communications 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:
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 the illustration below. 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.
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 picture, 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). 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.
The picture below 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 the 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 [´ 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 (´ 5) and gives us a 25 watt input to our power (final) amplifier. The final amplifier multiplies its input by a factor of 20 and gives us 500 watts of power out to the antenna.
Intermediate amplifiers increase transmitter power.
Amplitude and frequency modulation in a transmitter
Frequency multiplication and the single sideband transmitter
SSB (Single-sideband) Applications
Receiver fundamentals and characteristics
Carrier Reinsertion and the introduction to receiver circuits
Manual and Automatic gain control (MVC/AGC)
Circuitry of the AGC system
AGC (Forward, reverse, and delayed automatic gain control)
Beat frequency oscillator and squelch
Crystal filters and automatic frequency control (afc)
Frequency synthesis and audio reproduction devices