Introduction

In telecommunication, the term modulation is a process that varies a modulating signal (intelligent) on to a higher frequency signal (carrier).

Waveform Comparison

Reasons for Modulation

• It is impractical to transmit signal directly as the longer the waveforms the longer the antenna length (λ /4).
• It reduces interference
• It allows two or more signals to be transmitted simultaneously over different frequency band.

Types of Modulation

There two general types of modulation namely analogue and digital. Different parameters are been varied to achieve different resultant waveforms.

 Analogue

There are 3 major classes of Analogue modulation techniques used for Analogue transmission. They are namely:

• Amplitude Modulation (AM)
• Frequency Modulation (FM)
• Phase Modulation (PM)
  

AM works by varying the amplitude of the transmitted signal before Transmission. Transmitted waveforms produced are

• Double-Sidebands (DSB)
• Double-Sidebands-Suppressed-Carrier (DSB-SC)
• Single-Sideband (SSB)

FM works by varying linearly of the instantaneous carrier’s frequency with the intelligent signal before transmission.
PM works by varying linearly of the instantaneous carrier’s phase by 0° or 180 with the intelligent signal before transmission.

Analogue - AM

DSB uses full power transmission contributed by both sidebands and carrier. When the signal is generated, three spikes will appear in the spectrum domain. They are namely

• Lower-Sideband (LSB)
• Carrier (Fc)
• Upper-Sideband (USB).

DSB-SC suppresses the carrier of the waveforms which helps reduced transmission power, as the carrier (Fc) does not carry any information. This reduction will:

• Cut down power wastage
• Give a 50% efficiency rate

SSBfurther reduces one of its sidebands (LSB) and ultimately contribute to even more efficiency usage of both the electrical power and bandwidth (BW).

Digital

There are 3 major classes of digital modulation techniques used for digital transmission. They are namely:

• Amplitude-Shift Keying (ASK)
• Frequency-Shift Keying (FSK)
• Phase-Shift Keying (PSK).

ASK is a form of modulation that represents digital data as variations in the amplitude of a carrier wave.
FSK is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave.
PSK is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave).

Filters

Filters act like a frequency-selective components which are commonly found in communication circuits. By passing or attenuating different frequencies, different types of filters can be achieved. There are 4 general examples of filter namely;

• Low-Pass Filter (LPF) –  Allow flows of low frequencies but blocks high frequencies
• High-Pass Filter (HPF) – Allow flows of high frequencies but blocks low frequencies
• Band-Pass Filter (BPF) – Offer easy access of a specified range of frequencies but blocks all frequencies outside the range
• Band-Stop Filter (BSF) – Blocks a specified range of frequencies but offer easy access all frequencies outside the range