Monthly Archives: November 2009

Wave Modulation – analog

Modulating a wave means changing one or more of its fundamental characteristics to encode information.

The general function for a sinusoid is

f(t) = A \sin(\omega t + \phi)

or

f(t) = A sin(omega t + phi)

we can see that this has 3 parameters that can be altered .

A – called the magnitude, or amplitude of the sinusoid.

omega – known as the frequency

phi – known as the phase angle.

Therefore there are three basic ways to modulate a carrier wave ( which is the modulated unaltered wave carrying some information) :

1. Amplitude Modulation

The amplitude of the carrier signal is modulated (changed) in proportion to the message signal while the frequency and phase are kept constant.

Amfm3-en-de

2. Frequency Modulation

The frequency of the carrier signal is modulated (changed) in proportion to the message signal while the amplitude and phase are kept constant.

Amfm3-en-de (1)

3. Phase Modulation

This encodes information as variations in the instantaneous phase of a carrier wave.It changes the phase angle of wave in direct proportion to the message signal.

fig_2010_07_08

The above discussed techniques are only applicable for analog modulation . For digital modulation got the next blog : Wave Modulation – digital.

Need for Modulation

1. Transmitting medium is acts like a bandpass filter

The transmitting channel ( air / wire o/ fibre ) is like a bandpass filter ie both the lowest frequency components and the highest frequency components are cut off beyond a range with transmission only being practical over some intermediate frequency range . In such a case our orignal wave is unusuable unless we shift our frequency up wthout changing the information contained . Thus modulation is the only solution for transmission.

2. Limits the size of Antenna from huge to small

A baseband (low frequency) signal has a long wavelength thus an anteena of the size of about 1/10 of wavelength is required to listen to it .This results in really large antenna sizes. However since modulation shifts the baseband signal up to a much higher frequency, which has much smaller wavelengths it allows the use of a much smaller antenna.


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