Radio Transmitters and Receivers Transmitters Receivers As use of the radio frequencies increases, more efficient  allocation of bandwidth is imperative. Sending information  via radio waves using the narrowest bandwidth possible is  the focus of engineering moving forward. At the same time,  fully representing all of the desired information or increasing  the amount of information conveyed is also desired. Various  methods are employed to keep bandwidth to a minimum,  many of which restrict the quality or quantity of information  able to be transmitted.  wave. Modulator circuits modify the carrier wave with the  processed information signal. Essentially, this is all there is  to a radio transmitter. NOTE: Modern transmitters are highly refined devices with  extremely precise frequency oscillation and modulation. The  circuitry for controlling, filtering, amplifying, modulating,  and oscillating electronic signals can be complex. In lower frequency ranges, such as those used for ground  wave and some sky wave broadcasts, SSB transmissions  are a narrow bandwidth solution. Each side band represents  the initial information signal in its entirety. Therefore, in  an SSB broadcast, the carrier wave and either the upper  or lower sidebands are filtered out. Only one sideband  with its frequencies is broadcast since it contains all of the  needed information. This cuts the bandwidth required in  half and allows more efficient use of the radio spectrum.  SSB transmissions also use less power to transmit the same  amount of information over an equal distance. Many HF long- distance aviation communications are SSB.  [Figure 11-83] A transmitter prepares and sends signals to an antenna that, in  the process described above, radiates the waves out into the  atmosphere. A transmitter with multiple channel (frequency)  capability contains tuning circuitry that enables the user to  select the frequency upon which to broadcast. This adjusts  the oscillator output to the precise frequency desired. It is  the oscillator frequency that is being tuned.  [Figure 11-84] As shown in  Figure 11-84 , most radio transmitters generate  a stable oscillating frequency and then use a frequency  multiplier to raise the AC to the transmitting frequency. This  allows oscillation to occur at frequencies that are controllable  and within the physical working limits of the crystal in  crystal-controlled oscillators. Radio transmitters and receivers are electronic devices that  manipulate electricity resulting in the transmission of useful  information through the atmosphere or space. A transmitter consists of a precise oscillating circuit or  oscillator that creates an AC carrier wave frequency. This  is combined with amplification circuits or amplifiers. The  distance a carrier wave travels is directly related to the  amplification of the signal sent to the antenna.  Other circuits are used in a transmitter to accept the input  information signal and process it for loading onto the carrier  Antennas are simply conductors of lengths proportional to  the wavelength of the oscillated frequency put out by the  transmitter. An antenna captures the desired carrier wave  as well as many other radio waves that are present in the  atmosphere. A receiver is needed to isolate the desired carrier  wave with its information. The receiver also has circuitry  to separate the information signal from the carrier wave.  It prepares it for output to a device, such as speakers or a  display screen. The output is the information signal originally  introduced into the transmitter.  A common receiver is the super heterodyne receiver. As with  any receiver, it must amplify the desired radio frequency  captured by the antenna since it is weak from traveling  through the atmosphere. An oscillator in the receiver is  used to compare and select the desired frequency out of all  of the frequencies picked up by the antenna. The undesired  frequencies are sent to ground.  11-37 Figure 11-84.  Block diagram of a basic radio transmitter.  Figure 11-83.  The additional frequencies above and below the  carrier wave produced during modulation with the information  signal are known as sidebands. Each sideband contains the unique  information of the information signal and can be transmitted  independent of the carrier wave and the other sideband.  Lower sidebands Upper sidebands are removed SSB bandwidth Carrier is removed Frequency oscillator Audio microphone Audio processing Frequency multiplier Modulator Power amplifier