5.2 Modulation Modes: FM, SSB & CW

5.2 Modulation Modes: FM, SSB & CW

Radio communication involves superimposing information—such as your voice or data—onto a radio frequency (RF) carrier wave. This process is called modulation. The unmodulated RF wave itself carries no information; it simply serves as the vehicle. At the receiving end, a process called demodulation (or detection) extracts the information from the carrier. In amateur radio, we use several different methods of modulation, each with its own unique characteristics, advantages, and applications. The most common modes you will encounter are Continuous Wave (CW), Amplitude Modulation (AM), Single Sideband (SSB), and Frequency Modulation (FM).

Continuous Wave (CW)

Continuous Wave, or CW, is the oldest and simplest form of radio modulation. It is effectively a digital mode, even though it predates computers by decades. In CW, the RF carrier is simply turned on and off in a pattern to represent characters, typically using Morse code. Because there is no voice information being added, CW requires very little bandwidth—typically only 150 to 500 Hz. This narrow bandwidth makes CW highly efficient; all of the transmitter's power is concentrated into a very small slice of the radio spectrum. As a result, CW is exceptional for weak-signal communication and can often be heard when voice signals are completely lost in the noise.

Amplitude Modulation (AM)

Amplitude Modulation (AM) was the first method used to transmit voice over radio. In AM, the amplitude (the strength or peak voltage) of the RF carrier is varied in direct proportion to the audio signal from the microphone. When you speak loudly, the amplitude of the carrier increases; when you speak softly, it decreases.

An AM signal consists of three distinct parts: the central carrier wave, a lower sideband (LSB), and an upper sideband (USB). The sidebands contain the actual audio information and are mirror images of each other. Because it transmits the carrier and both sidebands, a standard AM signal is relatively wide, occupying about 6 kHz of bandwidth (assuming a 3 kHz audio frequency range). The carrier itself contains no information but consumes about two-thirds of the transmitter's power, making AM an inefficient mode for long-distance communication. Today, standard AM is rarely used in amateur radio, except by enthusiasts of vintage equipment.

Single Sideband (SSB)

Single Sideband (SSB) is a highly efficient derivative of Amplitude Modulation and is the most popular mode for long-distance (DX) voice communication on the HF bands. Engineers realized that since the upper and lower sidebands in an AM signal contain identical information, transmitting both is redundant. Furthermore, since the carrier contains no audio information, it is a waste of power.

In an SSB transmitter, the carrier is suppressed (removed), and one of the sidebands is filtered out before transmission. This leaves only a single sideband containing all the voice information. This process concentrates all of the transmitter's power into a much narrower bandwidth—typically about 2 to 3 kHz. Because it uses less than half the bandwidth of AM and wastes no power on a carrier, SSB is incredibly efficient and provides much greater range for a given power level.

By convention, amateur radio operators use Lower Sideband (LSB) on the 160, 80, and 40-meter bands, and Upper Sideband (USB) on the 20, 17, 15, 12, and 10-meter bands, as well as on VHF and UHF frequencies.

Frequency Modulation (FM)

Frequency Modulation (FM) is the mode most commonly used for local VHF and UHF communication, particularly through repeaters and on handheld transceivers. Unlike AM or SSB, where the amplitude of the signal changes, the amplitude of an FM signal remains constant. Instead, the frequency of the carrier wave is varied (deviated) in proportion to the audio signal. The louder the audio, the further the frequency shifts away from the center carrier frequency. This shift is known as deviation.

The ratio of the maximum frequency deviation to the highest modulating audio frequency is called the modulation index. In amateur radio, standard FM voice typically uses a maximum deviation of 5 kHz. The total bandwidth of an FM signal is significantly wider than SSB, usually occupying about 10 to 15 kHz. Because the amplitude of the signal does not change, FM receivers can use limiter circuits to strip away amplitude-based static and noise (such as lightning crashes or engine ignition noise). This provides the clear, static-free audio that FM is famous for.

FM receivers use a circuit called a discriminator (or sometimes a ratio detector) to recover the audio from the frequency variations. FM transceivers also heavily rely on a squelch circuit to mute the receiver when no carrier is present, as an unsquelched FM receiver produces a loud, rushing noise.

Over-modulation and Distortion

Proper modulation is critical. If you speak too loudly into the microphone or set the microphone gain too high, you can cause over-modulation. In AM and SSB, over-modulation causes the signal to distort and generate spurious emissions known as splatter, which can cause severe interference to stations on adjacent frequencies. In FM, over-modulation causes excessive frequency deviation (over-deviation), resulting in a signal that is too wide for the receiving station's filters, leading to chopped or distorted audio.

Bandwidth Comparisons

Understanding the relative bandwidth of different modes is essential for proper band planning and avoiding interference.

By selecting the appropriate modulation mode for your intended communication, you can optimize your station's efficiency and ensure clear, reliable contacts.