3.0 Continuous-Wave Analog Modulation

Continuous-wave modulation represents the foundational approach to analog signal transmission. This family of techniques involves modifying one of the three key parameters—amplitude, frequency, or phase—of a high-frequency sine wave, known as the carrier wave. The carrier itself contains no information but acts as a vehicle to transport the lower-frequency message signal over long distances.

Amplitude Modulation (AM) and its Variants

Amplitude Modulation (AM) is a technique where the amplitude of the carrier signal is varied in accordance with the instantaneous amplitude of the modulating (message) signal. The frequency and phase of the carrier remain constant.

An imaginary line connecting the positive and negative peaks of the modulated carrier wave is called the Envelope, which recreates the shape of the original message signal. The standard equation for an AM wave is: S(t) = Ac[1 + Ka*m(t)]cos(2πfct)

A critical parameter in AM is the Modulation Index (µ), which describes the level of modulation a carrier wave undergoes. It determines the quality of the transmitted signal:

• Under-modulation (µ < 1): The modulation is weak, but the signal is transmitted without distortion.
• Perfect modulation (µ = 1): This is the ideal state for maximum modulation without distortion.
• Over-modulation (µ > 1): The carrier experiences a phase reversal, causing distortion and interference that cannot be easily eliminated.

The bandwidth required for AM transmission is twice the frequency of the modulating signal (BW = 2fm).

The basic form of AM, known as Double Sideband Full Carrier (DSB-FC), is inefficient because the carrier itself carries no information. This led to the development of more efficient variants:

• Double Sideband Suppressed Carrier (DSB-SC): The power-wasting carrier is suppressed, and the saved power is redistributed to the two sidebands, which contain the information.
• Single Sideband Suppressed Carrier (SSB-SC): This technique goes a step further by suppressing both the carrier and one of the sidebands. Since both sidebands contain the same information, SSB is highly efficient, as it concentrates the transmitter’s entire power into a single information-carrying sideband, maximizing the power-per-hertz of the signal.
• Vestigial Sideband (VSB) Modulation: VSB serves as a compromise between DSB-SC and SSB. It transmits one full sideband and a part (a “vestige”) of the other. This approach is primarily used for the transmission of television signals, where it offers a balance of bandwidth efficiency and simpler filter design compared to pure SSB.

Angle Modulation

In Angle Modulation, the angle of the carrier wave—either its frequency or its phase—is varied according to the message signal, while the amplitude remains constant.

Frequency Modulation (FM)

In Frequency Modulation (FM), the carrier’s frequency is varied in accordance with the instantaneous amplitude of the modulating signal. The amount of frequency change is described by two key terms:

• Frequency Deviation (Δf): The difference between the modulated frequency and the normal carrier frequency.
• Carrier Swing: The total variation in frequency from the lowest to the highest point, equal to 2 x Δf.

FM is classified into two types:

• Narrowband FM: Characterized by a small bandwidth and a small modulation index. It is commonly used in mobile communications like police wireless and taxicabs.
• Wideband FM: Has an infinite theoretical bandwidth and a large modulation index. It is used in entertainment broadcasting applications like FM radio.

Phase Modulation (PM)

In Phase Modulation (PM), the carrier’s phase is varied in accordance with the instantaneous amplitude of the modulating signal. PM and FM are closely related, as a change in phase inherently causes a change in frequency, and vice versa. Crucially, phase modulation is an indirect method of producing an FM signal.

While continuous-wave techniques form the bedrock of analog broadcasting, the demand for higher fidelity and data integrity necessitated a shift toward discrete signal processing, beginning with the fundamental principles of pulse modulation.