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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

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

pulse trains vs band limited signals
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
modulation

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

amplitude modulation
Amplitude Modulation

Generate a signal by modulating

the amplitude of the carrier with

the signal wave

Signal

Carrier

Modulation Index

multiplying sine cosine waves

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

am signal spectrum

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
power of an am signal
Power of an AM Signal

Modulation Index

Carrier Power

Total Power

Note: Largest practical modulation index is 1

am detection
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
frequency phase modulation
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

fm pm signal spectrum

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