Baseband Modulation/Demodulation techniques

Baseband modulation and demodulation techniques are fundamental to communication systems. Let's first understand what is baseband. Baseband is actual frequency band of signal (e.g. voice, video). If we consider the voice signal then voice signal band is approximately 4kHz. That means voice signal contains frequencies ranging from 0-4kHz.

What is Modulation? Modulation is basically increasing signal frequency someway. This means voice base band is 4kHz and uplifting voice signal frequency to lets say, 1900kHz. Now, question is why we need to uplift frequency of actual baseband signal? And here radio transmission basic concept comes into picture.

In very simple way length of antenna used to transmit signal is directly related to signal wavelength. Wavelength is length of single cycle of signal. And then what is frequency? Frequency is number of cycles of signal per second, or, inverse of time taken to complete one cycle. So, what we can deduce here is if frequency of signal is high then wave length of signal will be short and vice versa.

relation between frequency and wavelength is expressed by,

                             c = f * λ

where c = velocity of light which is approximately 3×10^8 m/s , f = frequency of signal, and λ = wavelength of signal.

Thus, light velocity f and wavelength λ are inversely proportional:


Let's get back to antenna again. The length of antenna required to transmit a signal is directly related (i.e. proportional) to signal wavelength, and, therefore, is inversely proportional to signal frequency.

L∝λ∝1/f => λ=L=1/f

Where L is the length of the antenna.

There are many types of antenna used according to requirement. Here, for understanding we will use basic dipole antenna. Length of dipole antenna used to transmit a signal is often λ/4 or λ/2 meter. Now, for example, let's consider the voice signal. The highest frequency component of voice signal is around 4kHz and its respective wavelength is about 75000 m. So, if we want to construct dipole antenna for voice transmission then the length of λ/4 dipole antenna will be more then 18000 m. Such antenna is not practically possible to construct! So, by uplifting signal's frequency, we decrease the length of the required antenna, making it possible to manufacture.