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Band Limited Signals: Sending Analog Data with Band Limited Signals

Band Limited Signals: Sending Analog Data with Band Limited Signals. Based on Chapter 5 of William Stallings, Data and Computer Communication, 8 th Ed. Kevin Bolding Electrical Engineering Seattle Pacific University. Pulse Trains vs. Band-Limited Signals.

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Band Limited Signals: Sending Analog Data with Band Limited Signals

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  1. Band Limited Signals:Sending Analog Data with Band Limited Signals Based on Chapter 5 of William Stallings, Data and Computer Communication, 8th Ed. Kevin BoldingElectrical EngineeringSeattle Pacific University

  2. Pulse Trains vs. Band-Limited Signals • Band-Limited signals are waves (compositions of sine waves) and constrained to a specified band of a medium’s bandwidth • Pulse Trains consume the entire bandwidth of the transmission medium • Band-limited signals may be used to send analog or digital data • Line coding uses pulse trains to send digital data

  3. Modulation allows the engineer to move the base frequency of a band Allows the use of higher-frequency media Makes frequency-division multiplexing possible Modulation Modulation is the process ofcombining a signal wave and a carrier wave Carrier wave has frequency fc, which must be greater than the modulating wave’s frequency, fm

  4. Amplitude Modulation Generate a signal by modulating the amplitude of the carrier with the signal wave Signal Carrier Modulation Index

  5. pwr 0 fc- fx fc fc+ fx f Multiplying Sine/Cosine Waves If the signal x(t) is a single cosinewave, we get (with a modulationindex of 1): Sum Carrier Difference SpectrumAnalyzer view

  6. M(f) Signal Carrier 0 B Fc f Lower sideband Upper sideband S(f) S(f) Single Sideband 0 Fc-B Fc Fc+B f 0 Fc-B Fc Fc+B f AM Signal Spectrum BandpassFilter (Fc to Fc+B) SignalBW: B • Modulating signal covers a band from 0Hz to B • Carrier signal is concentrated at Fc CarrierFc • Combined signal has bandwidth of 2B, centered at Fc • Includes a large amount of power at Fc • Sending just one sideband reduces the bandwidth to B • Filtering out the carrier reduces the power required

  7. Power of an AM Signal Modulation Index Carrier Power Total Power Note: Largest practical modulation index is 1

  8. AM Detection Vin Vout • To Detect (Demodulate) an AM Signal: • Rectify it to remove the negative portion of the waveform • Low pass filter using a simple RC filter

  9. Frequency/Phase Modulation Generate a signal by modulating the frequency/phase of the carrier with the signal wave Carrier Carrier Angle Offset For phase modulation: Signal For frequency modulation: Signal

  10. Some practical limits • Maximum frequency deviation DF is proportional to maximum amplitude in the modulating signal • Carson’s rule for FM bandwidth requirement: works well FM/PM Signal Spectrum • FM/PM frequencies vary according to an analog signal • Analog signal --> Unlimited precision • Unlimited bandwidth needed

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