Modes

The term mode has varying meanings, according to the context, but the most common are permitted modes in amateur licensing.

Waves have three characteristics that can be changed, Amplitude, Frequency and Phase. A mode is the way of changing electromagnetic waves, modulating them so that transmission of information is possible. Modulating signals can be either analogue, for example sound or digital, for example simple binary on-off.

= Analogue Modulation methods =

There are two main analogue modes, or methods of modulation: Amplitude Modulation (AM), in which the phasor amplitude changes, and Angle Modulation, in which the phasor angle changes.

Double Sideband (DSB), Single Sideband (SSB) and Vestigal Sideband (VSB) are all forms of AM. Frequency Modulation (FM) and Phase Modulation (PM) are all forms of Angle modulation.

Amplitude Modulation (AM)
The transceiver produces a carrier wave at the frequency of transmission. Voice is superimposed on the carrier wave, and alters its shape by changing the Amplitude or height of the wave. Hence the frequency and wavelength of the carrier do not change with this form of modulation.

See Amplitude Modulation for more information.

Double-Sideband Modulation (DSB)
Double Sideband is what's usually meant when people talk about AM. In DSB transmissions, the message signal is transmitted in two sidebands, one being the mirror image of the other. The carrier may be either transmitted at full power (DSB-FC), at reduced power (DSB-RC), or completely eliminated (DSB-SC).

Conceptually, the power level at the carrier frequency, equates to the DC bias in the input signal. Mathematically, it looks something like this:

$$x(t) = (A + m(t))\cos( 2 \pi f_c t + \phi )$$

Single-Sideband Modulation (SSB)
Single sideband is what you get if you take a DSB-SC signal, and pass it through a sharp high-pass or low-pass filter to reject the offending sideband. It may be generated through high-pass/low-pass filter, or it may be done using a Harley Modulator, which cancels out the unwanted sideband through the use of a Hilbert Transform.

Frequency Modulation (FM)
The transceiver produces a carrier wave, in the same way as for Amplitude Modulation. In this case however, voice is added to the carrier so that is frequency changes. This in turn affects the wavelength of the carrier, but the amplitude remains constant.

See Wikipedia's Frequency Modulation article for more information.

$$x(t) = \cos( 2 \pi ( f_c + \Delta f m(t) t + \phi ) )$$

Phase Modulation (PM)
This mode is seldom used in amateur radio. It's very similar to FM, but rather than the frequency changing, it's the phase of the signal that changes according to the modulating signal.

$$x(t) = \cos( 2 \pi ( f_c t + \Delta \phi m(t) ) )$$

The well-known PSK31 digital mode is a form of phase modulation.

Lesser known modes

 * Quadrature amplitude modulation (QAM). In this mode, two carrier waves, 90° out of phase with each other are produced. QAM is a variant of AM, in which both carriers are modulated by an audio signal.

= Digital modulation =

Technically, whenever a signal is turned on and off to enable transmission of information, it can be considered to be a digital mode. Under this definition, CW is certainly a digital mode. This section refers to methods of transmitting and receiving (rather than modulating) that are digital, or that require digital processing in part of the transmission or receiving process.

Continuous Wave (CW)
A continuous wave is an electromagnetic wave of constant amplitude and frequency, a pure carrier, and information is carried by turning the wave on and off, and measuring the interval. Morse code is often transmitted using CW.

See Wikipedia Continuous Wave for more information.

Amplitude Shift keying (ASK)
The amplitude of the carrier is varied according to a digital signal.

See Wikipedia Amplitude Shift Keying for more information.

Frequency Shift Keying (FSK)
The frequency of the carrier is varied according to a digital signal.

See Wikipedia Frequency Shift Keying for more information.

Phase Shift Keying (PSK)
The phase of the carrier is modulated by a digital signal. In its simplest terms, this could mean for example that the phase of the carrier is turned through 180° with each change in the digital signal. In practical terms, PSK allows long distance communication even when noise level are high.

See Wikipedia Phase Shift Keying

Digital modes in practice
The licensing regime defines digital modes as those modulation techniques that require digital data processing. In Australia refer to the ACMA LCD ( Licence Conditions Determination) for exact details. You will need to scroll down the page to find the link.

What do digital modes sound like? Click here to find out.

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