4 Modulation What is modulation? Why modulate? Modulation is to vary a carrying signal’s amplitude, frequency or phase in order to convey information.Why modulate?Avoid interference by efficient use of spectrumPropagation characteristicsAntenna constraints based on wavelength
5 AM - Amplitude Modulation Technique which varies the amplitude of a carrier signal in proportion to the instantaneous amplitude of the modulating signalMostly used in commercial AM band as well as shortwave radio stations, maritime, aviation, military, and amateur operators.The most common modulating signal is the human voice, although can transmit music as well.
22 Principal AM Characteristics The frequency power spectrum has a dominant carrier and two sidebands called the upper and lower sidebands (USB/LSB).USB and LSB components are mirror images of each other and contain the same information.The transmit power varies continuously as a function of the modulation index.The carrier can be suppressed in order to lower transmit power requirement. (DSBSC)SSB is produced when one of the sidebands is filtered (USB or LSB).
23 Frequency and Phase Modulation When the frequency of a carrier is varied in accordance to the instantaneous amplitude of the modulating signal, the result is frequency modulation (FM).When the phase of a carrier is varied in accordance to the instantaneous amplitude of the modulating signal, the result is phase modulation (PM).
24 Frequency and Phase Modulation FM and PM techniques are very similar mathematically.In practice:FM is more common in analog modulation (handheld radios, commercial FM radio)PM is more common in digital modulation (spread spectrum, microwave point-to-point)
25 Key FM Characteristics FM is more immune to noise that AMNoise effects are less on frequency than amplitudeFM requires more bandwidth than AMAllows higher fidelity for voice and musicMathematical description is more complicated than AM
26 FM Signal in Time Domain We can see that having a sine function withinanother sine function is much more complex toevaluate than the additive terms of AM.
27 Modulation indexModulation index m is the ratio of the maximum frequency carrier deviation and the frequency of the modulating signal
35 Bandwidth Requirement for FM Bessel Functions:Carson’s Rule:Bessel Function is slightly more conservative that Carson’s Rule
36 Example of FM Systems Commercial FM Broadcast Narrowband δ is 75kHz Modulation frequencies 100Hz and 15 kHz (much better than AM)Each channel has a 200 kHz widthNarrowbandMostly used in handheld radio applications and first generation cell phones (AMPS)Channel widths could range from 6.25 kHz to 30 kHzLow modulation index
37 FM Highlights FM signal has constant power level Has much higher SNR than AMSignal quality improves with greater modulation indexThe price to pay for this feature is the need for more bandwidth