Friday, November 20, 2015

Aliasing Automatic Gain Control and FFT


A filter chosen in anticipation of a certain sample frequency is called an anti-aliasing filterAliasing on wiki. A real anti-aliasing filter trades off between bandwidth andaliasing
Undersampling, which causes aliasing.
Without an anti-aliasing filter, frequencies higher than the Nyquist frequency will influence the samples in a way that is misinterpreted by the interpolation process. Ref
  • Paper(pdf) from National Instruments on Aliasing and Sampling at Frequencies Above the Nyquist Frequency.
  • Notes:
    R (sampling rate) = 100MS/s
    fs (signal being sampled = 70MHz
    fN (the Nyquist frequency) = 50MHz
    fa (aliased frequency) = 30MHz
    The frequency of the aliased signal can be found from the following simple equation:
    fa = |R*n - fs|
    Although sampling at twice the Nyquist frequency will ensure that you measure the correct frequency of your signal, it will not be sufficient to capture the shape of the waveform. If the shape of the waveform is desired, you should sample at a rate approximately 10 times the Nyquist theory.
If known input frequency range, then sample rate more than 2*InputFreqRange? will remove aliasing. However, real input signal could include signal frequency higher than wanted frequency range, these high frequency signal will cause aliasing at pre-designed smapling rate. (high freq signal and low freq signal end up with same sampling points at lower (than required) sampling rate.) That's the whole reason u need anti-aliasing filter in the front end to filter out unsupported high frequency signals.

Automatic gain control - (AGC)

Finite Impulse Response (FIR) filter


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