Modulation

is the process of impressing a low-frequency information signal (baseband signal) onto a higher frequency carrier signal

Modulation

f(t) = Asin(ωt + ϕ)

is a process of mixing a signal with a sinusoid to produce a new signal. This new signal, conceivably, will have certain benefits of an un-modulated signal, especially during transmission. If we look at a general function for a sinusoid:

we can see that this sinusoid has 3 parameters that can be altered, to affect the shape of the graph. The first term, A, is called the magnitude, or amplitude of the sinusoid. The next term, ω is known as the frequency, and the last term, ϕ is known as the phase angle. All 3 parameters can be altered to transmit data.

The sinusoidal signal that is used in the modulation is known as the carrier signal, or simply "the carrier". The signal that is used in modulating the carrier signal(or sinusoidal signal) is known as the "data signal" or the "message signal". It is important to notice that a simple sinusoidal carrier contains no information of its own

In other words we can say that modulation is used because the some data signals are not always suitable for direct transmission, but the modulated signal may be more suitable.

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Types of modulations

Analog modulations Digital modulations

AM PM FM ASK FSK PSK

Remember the radio spectrum:

Amplitude Modulation (AM)

Amplitude modulation is the process of varying the amplitude of a carrier wave in proportion to the amplitude of a baseband signal. The frequency of the carrier remains constant

Frequency Modulation (FM)

Frequency modulation is the process of varying the frequency of a carrier wave in proportion to the amplitude of a baseband signal. The amplitude of the carrier remains constant

Phase Modulation (PM) Phase modulation is the process of varying the Phase of a carrier wave in proportion to the amplitude of a baseband signal. The amplitude of the carrier remains constant

In this type of modulation, Ac, wc are kept constant. The phase angle ϕ of the carrier wave is varied according to the modulating wave. The common name for "phase modulation" and "frequency modulation" is "angle modulation".

The digital transmission of analog signals involves pulse modulation. In analog pulse modulation, time is represented in discrete form. In digital pulse modulation, both time and signal amplitude are represented in discrete form.

Note:-

Note:-• “AM” radio band ~500 to 1600 kHz, • “FM” radio band 88 to 108 MHz

FM and PM closely related

• “Carrying one signal on another” - uses carrier

Why use modulation?

• Modulated carrier transmitted • Problems with transmitting baseband signals – Antennas difficult at low frequencies – Noise and interference at low frequencies – Can’t share with others

• Easier to transmit carrier at higher frequency – Can choose convenient frequency • Antennas can be smaller • May be useful propagation effects – Fractional bandwidth much smaller • Antennas and other components easier to design • Can have many frequency channels

Application:Think about your car radio. There are more than a dozen (or so) channels on the radio at any time, each with a given frequency: 100.1MHz, 102.5MHz etc... Each channel gets a certain range (usually about 0.2MHz), and the entire station gets transmitted over that range. Modulation makes it all possible, because it allows us to send voice and music (which are essential baseband signals) over a bandpass (or "Broadband") channel.

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Amplitude modulation (AM) • Simplest case of AM is where carrier is just turned on or off.

• Continuous amplitude modulation (eg AM radio) • Information contained in the envelope shape

Modulation by a sine wave:

v(t) = AC cos (2π fCt) {1 + m cos (2π fmt)} AC = un modulated peak carrier amplitude fC = carrier frequency fm = modulation frequency m = modulation index (“degree” of modulation) • m must be between 0 and 1 • If m > 1 get over modulation (bad …distortion)

• Modulation produces two new components called sidebands, at frequencies above and below the carrier

AM spectrum

• For a fully modulated carrier (m=1), 2/3 of the power is in the carrier, the rest in the sidebands

• The spacing in frequency between carrier and sidebands is equal to fm (the modulating frequency) • Hence a bandwidth of 2 x fm is required to transmit the modulated carrier • With a more complicated modulating signal (e.g. voice) the sidebands will be extended, so that a bandwidth of twice the highest modulating frequency present is required

Where do the sidebands come from?

The basic components of a phase-locked loop (PLL) are a stable reference oscillator, a phase detector, a frequency divider, a voltage-controlled oscillator (VCO), amplifiers, and filters. The following figure shows a simplified PLL block diagram.

PLLs work by comparing the generated signal phase to the reference oscillator signal phase and then using the difference to adjust the generated signal. To compare the two waves, both signals must be of the same frequency. The frequency divider is used to divide down the generated signal. Because the frequency divider can divide down only an integer number of times, the generated signal must be an integer multiple of the reference oscillator signal. The phase detector returns a voltage depending on the phase difference between the two waves. This voltage is amplified, filtered, and then fed into the VCO. The VCO adjusts the signal it outputs based on this voltage.