Digital_Communications_Part_A

Introduction
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I finish the "The Need for Modulation" chapter stating that we need to increase the frequency of our signals, otherwise the antenna size can become impractical to build.

Modulation is going to be the mechanism to solve that issue and it is the scope of this chapter.

Important
  • Remember that the information or signals that we are trying to send are analog, continuous in time, no abrupt changes.

Diagram_PartA_analog_comms_modulation.png

What is Modulation?
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Info

Modulation is defined as the process of changing one or more characteristics of the high-frequency carrier signal in proportion with the instantaneous value of the analog information (or digital data) signal.

  • The low-frequency information signal is superimposed over a hight-frequency carrier signal in the process of modulation
  • Modulation translates a low-frequency signal to the pass band of communication channel
  • The analog information signal is also called baseband signal

Ok, there is some information to digest here. Let's plug some visuals.

If we have a high frequency signal, called carrier signal - , we can change 3 parameters in that carrier signal, the amplitude, the frequency and the phase.

Those changes are dictated by an analog information signal or baseband signal.

math_pure_carrier.png
Later during the math section will also be expressed in the following way:
For simplicity:

  • and will be omitted

Types of Analog Modulation Map
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types_modulation_analog.png

Simulation
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design_icon.png

Info

For all the simulations below, the baseband signal is me using the mouse and controlling the respective parameter in an analog way (or continuous way), no abrupt changes.

Changing the amplitude of the carrier -> Amplitude Modulation
carrier_demo_amplitude_change.gif

Changing the frequency of the carrier -> Frequency Modulation
carrier_demo_frequency_change.gif

Changing the phase of the carrier -> Phase Modulation
carrier_demo_phase_change.gif

Note

Some observations:

  • The amplitude and frequency changes on the carrier are obvious has I change the baseband signal manually.
  • The phase changes are not that obvious and hard to track.
  • That suggests that changing the phase will not be that useful when the baseband signal is an analog signal.

Lab Bench
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lab_icon.png
Connecting a function generator to an oscilloscope, we can observe the same simulated results.
In this case I am connection the Digilent Analog Discovery 2 function generator with a coaxial cable directly to the oscilloscope channel 1.

carrier_demo_discovery_connections.png

On the Waveforms software, I can see the amplitude and frequency changing as I move the sliders accordingly. For the phase, the changes are not visible because of the oscilloscope trigger that it will always sync to the trigger voltage level threshold.

carrier_demos_digilent_test.gif

Theory
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theory_icon.png

So what is the math for all of this? 🤔

Note
  • Consider our analog information or baseband signal or message as
  • frequency will be much smaller than the carrier's frequency
  • Remember it is the carrier frequency with the baseband signal that we send to the antenna.

Amplitude Modulation
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The amplitude of the carrier signal is varied in proportion to the instantaneous value of the baseband signal .

  • Baseband:
  • Carrier:

The simplest form of amplitude modulation (DSBSC) will have the following :

Info

In the next section we will explore variations of this amplitude modulation.

Frequency Modulation
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The frequency of the carrier signal is varied in proportion to the instantaneous value of the baseband signal .

  • Analog information signal or message:
  • Carrier:

For frequency modulation will be the following:
where is called frequency sensitivity factor.

Info

A decidedly unpleasant looking equation 🤯, but we will cover it in detail later.

Phase Modulation
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The phase angle of the carrier signal is varied in proportion to the instantaneous value of the baseband signal .

  • Analog information signal or message:
  • Carrier:

For analog phase modulation will be the following:
where is called phase sensitivity factor.

Info

is barely used as an analog signal but this equation will be useful and important when the baseband signal becomes digital.

References
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Table Of Contents

Introduction

What is Modulation?

Types of Analog Modulation Map

Simulation

Lab Bench

Theory

Amplitude Modulation

Frequency Modulation

Phase Modulation

References