3.0 Continuous-Wave Modulation Techniques

In continuous-wave modulation, a high-frequency sine wave is used as the carrier. This category is primarily divided into Amplitude Modulation and Angle Modulation.

3.1 Amplitude Modulation (AM)

In Amplitude Modulation (AM), the amplitude of the carrier signal is varied in accordance with the instantaneous amplitude of the modulating signal, while the frequency and phase of the carrier remain constant. An imaginary line connecting the positive and negative peaks of the modulated carrier wave, called the Envelope, recreates the shape of the original message signal.

The standard mathematical expression for an AM wave is: S(t) = A_c[1 + K_a * m(t)]cos(2πf_ct) where μ = K_a * A_m is the modulation index.

Modulation Index and Bandwidth

The Modulation Index (μ), or modulation depth, quantifies the extent of modulation a carrier wave undergoes. It is calculated as: μ = (A_max – A_min) / (A_max + A_min)

• Under-modulation (μ < 1): The modulation is incomplete.
• Perfect Modulation (μ = 1): The ideal level of modulation, often expressed as 100%.
• Over-modulation (μ > 1): The carrier experiences a 180° phase reversal, causing distortion and interference.

The bandwidth required for an AM wave is twice the frequency of the modulating signal: BW = 2 * f_m.

3.2 Sideband Modulation Techniques

An AM signal consists of the carrier frequency and two sidebands (upper and lower), both containing the same information. This is known as Double Sideband Full Carrier (DSB-FC) and is inefficient, as two-thirds of the power is wasted on the information-less carrier.

• Double Sideband Suppressed Carrier (DSB-SC): The carrier is suppressed, and the saved power is distributed to the two sidebands, improving efficiency.
• Single Sideband Suppressed Carrier (SSB-SC or SSB): Both the carrier and one sideband are suppressed. This technique has high power efficiency and requires less bandwidth. It is used in land, air, and maritime mobile communications, as well as military communications.
• Vestigial Sideband (VSB) Modulation: A compromise between DSB-SC and SSB, where one full sideband and a “vestige” (a small part) of the other sideband are transmitted. This technique avoids information loss that can occur with imperfect filters in SSB and is the standard for television signal transmission.

3.3 Angle Modulation (FM and PM)

In Angle Modulation, the angle (frequency or phase) of the carrier signal is varied according to the message signal.

Frequency Modulation (FM)

The frequency of the carrier signal varies in accordance with the instantaneous amplitude of the modulating signal, while the carrier’s amplitude and phase remain constant. The difference between the modulated frequency and the normal carrier frequency is the Frequency Deviation (Δf).

• Narrowband FM (NBFM): Has a small bandwidth and a small modulation index. Used in mobile communications like police wireless and taxicabs.
• Wideband FM (WBFM): Has a large modulation index (>1) and an infinite spectrum of sidebands. Used in entertainment broadcasting such as FM radio.

Phase Modulation (PM)

The phase of the carrier signal varies in accordance with the instantaneous amplitude of the modulating signal, while the carrier’s amplitude and frequency remain constant. PM is an indirect method of producing FM, as a change in phase also changes the wave’s frequency.