2-14
sidebands that must be transmitted to obtain the desired fidelity is related to the deviation (change in
carrier’ frequency) divided by the highest audio frequency to be used. At this point you may want to
review chapter 2 of NEETS, Module 12, Modulation Principles. For example, if the deviation is 40
kilohertz and the highest audio frequency is 10 kilohertz, the modulation index is figured as shown below:
In this example, a modulation index of 4 equates to 14 significant sidebands. Because the audio
frequency is 10 kilohertz and there are 14 side-bands, the bandwidth must accommodate a 140-kilohertz
signal. You can see this is considerably wider than the 10-to-15-kilohertz bandpass used in AM
transmitting.
FREQUENCY CONVERSION.—Frequency conversion is accomplished by using the heterodyne
principle of beating two frequencies together to get an intermediate frequency. So far, you have only
become familiar with single conversion; however, some receivers use double or triple conversion. These
methods are sometimes referred to as double or triple heterodyning. Receivers using double or triple
conversion are very selective and suppress IMAGE SIGNALS to yield sharp signal discrimination.
(Image signals are undesired, modulated carrier signals that differ by twice the intermediate frequency
from the frequency to which the superheterodyne receiver is tuned.) Double and triple conversion
receivers also have better adjacent channel selectivity than can be realized in single conversion sets.
In military communications receivers you may sacrifice fidelity to improve selectivity. This is
permitted because intelligence (voice, teletypewriter) can be carried on a fairly narrow band of
frequencies. Entertainment receivers, on the other hand, must reproduce a wider band of frequencies to
achieve their high-fidelity objective.
Q13. What frequency conversion principle is used to develop the IF?
Q14. What is the function of the detector?
Q15. What is the major disadvantage of an fm signal as compared to an AM signal?
SINGLE-SIDEBAND
You know from studying the single-sideband transmitter material in this chapter you may transmit
only one sideband of an AM signal and retain the information transmitted. Now you will see how a
single-sideband signal is received.
Advantages
Figure 2-11 illustrates the transmitted signal for both AM and ssb. Ssb communications has several
advantages. When you eliminate the carrier and one sideband, all of the transmitted power is concentrated
in the other sideband. Also, an ssb signal occupies a smaller portion of the frequency spectrum in
comparison to the AM signal. This gives us two advantages, narrower receiver bandpass and the ability to
place more signals in a small portion of the frequency spectrum.
