CHAPTER 2
Amplitude Modulation and Single-Sideband Modulation
1. Modulation is the process of having the information to be communicated modifies a higher frequency signal called a carrier.
2. Amplitude modulation (AM) is the oldest and simplest form of modulation.
3. In AM, the amplitude of the carrier is changed in accordance with the amplitude and frequency or the characteristics of the modulating signal. The carrier frequency remains constant.
4. The amplitude variation of the carrier peaks has the shape of the modulating signal and is referred to as the envelope.
5. A time-domain display shows amplitude versus time variation of AM and other signals.
6. Amplitude modulation is produced by a circuit called a modulator which has two in puts and an output.
7. The modulator performs a mathematical multiplication of the carrier and information signals. The output is their analog product.
8. The ratio of the peak voltage value of the modulating signal Vm to the peak value of the carrier Vc is called the modulation index m (m = Vm l Vc). It is also referred to as the modulation coefficient or factor and the degree of modulation.
9. The ideal value for m is 1, Typically m is less than 1. The condition where m is greater than 1 should be avoided as it introduces serious distortion of the modulating signal. This is called overmodulation.
10. When the modulation index is multiplied by 100, it is called the percentage of modulation.
11. The percentage of modulation can be computed from AM waveforms displayed on an oscilloscope by using the expression
where Vmax is the maximum peak carrier amplitude and
Vmin is the minimum peak carrier amplitude.
12. The new signals generated by the modulation process are called sidebands and occur at frequencies above and below the carrier frequency.
13. The upper fUSB and lower fLSB sideband frequencies are the sum and difference of the carrier frequency fC and the modulating frequency fM and are computed with the expressions
fUSB = fc + fm
fLSB = fc – fm
14. A display of signal amplitudes with respect to frequency is called a frequency-domain display.
15. An AM signal can be viewed as the carrier signal added to the sideband signals produced by AM.
16. The total transmitted power in an AM signal is the sum of the carrier and sideband powers PT =( PC + PUSB + PLSB) and is distributed among the carrier and sidebands. This power distribution varies with the percentage of modulation. The total power is
PT =Pe(1 + m2/2)
The power in each sideband is
Ps = Pc(m2)/4
17. The higher the percentage of modulation, the greater the sideband power and the stronger and more intelligible the transmitted and received signal.
18. Despite its simplicity and effectiveness, AM is a highly inefficient method of modulation.
19. In an AM signal, the carrier contains no information. Any transmitted information lies solely in the sideband. For that reason, the carrier may be suppressed and not transmitted.
20. An AM signal with suppressed carrier is called a double-sideband (DSB) signal.
21. Since the same transmitted information is contained in both upper and lower sidebands, one is redundant. Full information can be transmitted using only one sideband.
22. An AM signal with no carrier and one sideband is called a single-sideband (SSB) signal. The upper and lower sidebands contain the same information, and one is not preferred over the other.
23. The main advantage of an SSB AM signal over an AM or DSB signal is that it occupies one-half the spectrum space.
24. Both DSB and SSB signals are more efficient in terms of power usage. The power wasted in the useless carrier is saved thereby allowing more power to be put into the sidebands.
25. Power in an SSB transmitter is rated in terms of peak envelope power (PEP), the power that is produced on voice peaks. PEP output is computed using the expression
PEP = V2/R
where PEP is in watts and V is the rms voltage across the antenna load impedance R. The PEP input is computed using the expression PEP = Vs x Imax
where Vs is the de supply voltage of the final amplifier stage and Imax is the amplifier current on voice peaks. 26. The average output of an SSB transmitter is one-fourth to one-third of the PEP value.
27. Frequency conversion is a form of AM used to translate signals to higher or lower frequencies for improved processing.
28. Frequency conversion is carried out by a circuit called a mixer or converter.
29. The mixer performs analog multiplication of the input signal and a local oscillator signal.
30. The frequency conversion process faithfully retains any modulation (AM, PM, etc.) on the input. The translated signal contains the same modulation.
31. The output of a mixer consists of the local oscillator signal fo and the sum and difference frequencies of the local oscillator and input frequencies:
fS
fo
fO + fS
fO - fS or fS - fO
32. Either the sum or the difference frequency is selected with a filter, while the others are rejected.
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