How to Calculate Alias Frequency

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Traditional analog signals such as audio and video cannot be used directly by computers, smartphones and other digital equipment; they must first be converted into the ones and zeroes of digital data through a process called sampling. Aliasing is an undesired effect in which the sampling frequency is too low to accurately reproduce the original analog content, resulting in signal distortion. Frequency aliasing is a common problem in signal conversion systems whose sampling rate is too slow to read input signals of a much higher frequency.

    Note down the value of sampling rate of your data acquisition system. Call it "Rs" for simplicity. Sampling rate of a data acquisition system is defined as the number of times it can acquire a sample of an input signal per second.

    Divide the sampling rate by two to calculate the Nyquist frequency for your system. For example, if the sampling rate of your system is 10 Ms/s (10,000,000 samples per second), the Nyquist frequency of your system will be 5 MHz. Call it "Ns" for simplicity.

    Note the frequency of the signal that has to be sampled using your data acquisition system. Call it "Fs" for simplicity. Calculate the closest integer multiple of the sampling rate "Rs" to the frequency of the sampled signal. Call it "Rint" for simplicity. For example, if the samling rate is 10 Ms/s and the frequency of the sampled signal is 56 MHz, the closest integer multiple would be 5.

    Calculate the alias frequency (Falias) for your system using the formula: "Falias = Absolute((Rs*Rint) - Fs)."


About the Author

Aalhad Deshmukh started writing technical articles in 2009 for eHow. He has more than five years of experience in research and development in software and electronics engineering using tools like LabVIEW and C++. Deshmukh holds a Doctor of Philosophy in electrical engineering from the University of South Carolina.

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