In electronics, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with respect to a modulating signal. This is done in a similar fashion as a musician may modulate a tone (a periodic waveform) from a musical instrument by varying its volume, timing and pitch. The three key parameters of a periodic waveform are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"), all of which can be modified in accordance with a low frequency signal to obtain the modulated signal. Typically a high-frequencysinusoid waveform is used as carrier signal, but a square wave pulse train may also occur.

In telecommunications, modulation is the process of conveying a message signal, for example a digital bit stream or an analog audio signal, inside another signal that can be physically transmitted. Modulation of a sine waveform is used to transform a baseband message signal into a passband signal, for example low-frequency audio signal into a radio-frequency signal (RF signal). In radio communications, cable TV systems or the public switched telephone network for instance, electrical signals can only be transferred over a limited passband frequency spectrum, with specific (non-zero) lower and upper cutoff frequencies. Modulating a sine-wave carrier makes it possible to keep the frequency content of the transferred signal as close as possible to the centre frequency (typically the carrier frequency) of the passband.

A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator (sometimes detector or demod). A device that can do both operations is a modem (modulator–demodulator).

Aim

The aim of digital modulation is to transfer a digital bit stream over an analog bandpasschannel, for example over the public switched telephone network (where a bandpass filter limits the frequency range to between 300 and 3400 Hz), or over a limited radio frequency band.

The aim of analog modulation is to transfer an analogbaseband (or lowpass) signal, for example an audio signal or TV signal, over an analog bandpass channel, for example a limited radio frequency band or a cable TV network channel.

Analog and digital modulation facilitate frequency division multiplexing (FDM), where several low pass information signals are transferred simultaneously over the same shared physical medium, using separate passband channels.

The aim of digital baseband modulation methods, also known as line coding, is to transfer a digital bit stream over a baseband channel, typically a non-filtered copper wire such as a serial bus or a wired local area network.

The aim of pulse modulation methods is to transfer a narrowband analog signal, for example a phone call over a wideband baseband channel or, in some of the schemes, as a bit stream over another digital transmission system.

In music synthesizers, modulation may be used to synthesise waveforms with a desired overtone spectrum. In this case the carrier frequency is typically in the same order or much lower than the modulating waveform. See for example frequency modulation synthesis or ring modulation.

Analog modulation methods

In analog modulation, the modulation is applied continuously in response to the analog information signal.

Common analog modulation techniques are:

• Amplitude modulation (AM) (here the amplitude of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal)
• • Double-sideband modulation (DSB)
  • • Double-sideband modulation with carrier (DSB-WC) (used on the AM radio broadcasting band)
    • Double-sideband suppressed-carrier transmission (DSB-SC)
    • Double-sideband reduced carrier transmission (DSB-RC)
    • Single-sideband modulation (SSB, or SSB-AM),
    • SSB with carrier (SSB-WC)
    • SSB suppressed carrier modulation (SSB-SC)
    • Vestigial sideband modulation (VSB, or VSB-AM)
  • Quadrature amplitude modulation (QAM)
  • Angle modulation

Phase modulation (PM) is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave.

Unlike its more popular counterpart, frequency modulation (FM), PM is not very widely used for radio transmissions. This is because it tends to require more complex receiving hardware and there can be ambiguity problems in determining whether, for example, the signal has changed phase by +180° or -180°. PM is used, however, in digital music synthesizers such as the Yamaha DX7, even though these instruments are usually referred to as "FM" synthesizers (both modulation types sound very similar, but PM is usually easier to implement in this area).

Theory

PM changes the phase angle of the complex envelope in direct proportion to the message signal.

Suppose that the signal to be sent (called the modulating or message signal) is m(t) and the carrier onto which the signal is to be modulated is

c(t) = A_c\sin\left(\omega_\mathrm{c}t + \phi_\mathrm{c}\right).

Annotated:

carrier(time) = (carrier amplitude)*sin(carrier frequency*time + phase shift)

This makes the modulated signal

y(t) = A_c\sin\left(\omega_\mathrm{c}t + m(t) + \phi_\mathrm{c}\right).

This shows how m(t) modulates the phase - the greater m(t) is at a point in time, the greater the phase shift of the modulated signal at that point. It can also be viewed as a change of the frequency of the carrier signal, and phase modulation can thus be considered a special case of FM in which the carrier frequency modulation is given by the time derivative of the phase modulation.

The spectral behaviour of phase modulation is difficult to derive, but the mathematics reveals that there are two regions of particular interest:

• For small amplitude signals, PM is similar to amplitude modulation (AM) and exhibits its unfortunate doubling of basebandbandwidth and poor efficiency.
• For a single large sinusoidal signal, PM is similar to FM, and its bandwidth is approximately

2\left(h + 1\right)f_\mathrm{M},

where f_\mathrm{M} = \omega_\mathrm{m}/2\pi and h is the modulation index defined below. This is also known as Carson's Rule for PM.

Modulation index

As with other modulation indices, this quantity indicates by how much the modulated variable varies around its unmodulated level. It relates to the variations in the phase of the carrier signal:

h\, = \Delta \theta\,,

where \Delta \theta is the peak phase deviation. Compare to the modulation index for frequency modulation.

An optical modulator is a device which is used to modulate a beam of light. The beam may be carried over free space, or propagated through an optical waveguide. Depending on the parameter of a light beam which is manipulated, modulators may be categorized into amplitude modulators, phase modulators, polarization modulators etc. Often the easiest way to obtain modulation of intensity of a light beam, is to modulate the current driving the light source, e.g. a laser diode. This sort of modulation is called direct modulation, as opposed to the external modulation performed by a light modulator. For this reason light modulators are, e.g. in fiber optic communications, called external light modulators.

With laser diodes where narrow linewidth is required, direct modulation is avoided due to a high bandwidth "chirping" effect when applying and removing the current to the laser.

Classification of optical modulators

According to the properties of material that are used to modulate the light beam, modulators are divided into two groups: absorptive modulators and refractive modulators. In absorptive modulators absorption coefficient of the material is changed, in refractive modulators changed is refractive index of the material.

Absorption coefficient of the modulator's material can be manipulated by Franz-Keldysh effect, Quantum-confined Stark effect, excitonic absorption, or changes of free carrier concentration. Usually, if several such effects appear together, the modulator is called electro-absorptive modulator.

Refractive modulators most often make use of electro-optic effect, other modulators are made with acousto-optic effect or magneto-optic effect or take advantage of polarization changes in liquid crystals. The refractive modulators are named by the respective effect: i.e. electrooptic modulators, acousto-optic modulators etc. Immediate effect of refractive modulator operation is change of the phase of a light beam. This can be converted into amplitude modulation by interferometers or directional couplers.

Separate case of modulators are spatial light modulators (SLMs). The role of SLM is modification two dimensional distribution of amplitude and/or phase of an optical wave.

See:

• Electro-optic modulator, exploiting the electro-optic effect
• Acousto-optic modulator
• Magneto-optic modulators, using magnetooptic effects such as Faraday and Cotton-Mouton effects, one can modulate the amplitude and frequency of light up to tens of GHz.

Modulator, optical

A suppressed-carrier amplitude modulation scheme is three times more .... An approach which marries the advantages of low-level modulation with the ...