Question

During the process of digital sampling, data can be lost due to aliasing. Analyse this problem and provide at least two possible solutions. Also, determine the rate at which a signal should be sampled, if the highest frequency is 60MHz.

Answer 1

Aliasing is an effect that occurs when a signal is sampled at a sampling rate less than twice the maximum frequency present in it, this causes different signals to become indistinguishable when sampled. As a consequence, a signal reconstructed from such samples is different from the original continuous signal thus introducing distortion in the system.

The following diagrams show the same original signal (yellow) sampled firstly at 1.1 x Fmax & then secondly at 2 x Fmax (here Fmax is the highest frequency in the signal)

The above sampled signal is the same as a signal = 0.1 x Fmax i.e. it is an alias of a different signal thus incorrectly representing our original signal.

Two possible solutions to minimize aliasing problem are:

1. Increasing the sampling rate: If the sampling rate can be practically increased, then increasing the sampling rate to at least twice the maximum frequency present in the signal would help avoid the problem of aliasing.

2. Anti-aliasing Filters: These filters, which are usually low pass or bandpass filters provide pre-filtering the signal to suppress high-frequency components, thus limiting the bandwidth of the

As per the Nyquist-Shannon Theorem, the sampling rate must be equal to or more than double the highest frequency:

SR = Fmax * 2

So the given signal should be sampled at frequency = 2*60 MHz = 120 MHz

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