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I tried to find the stopband edge frequency.I know that the signal amplitude falls to -3 dB below the passband edge frequency.I examine a lot of DSP books so i don't any information about how to calculate stopband edge frequency.

Low Pass Filter

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In general there is no standard way to calculate the stop band edge, other than specifying a desired stop band attenuation and checking at which frequency this attenuation is first attained (for a low pass filter).

For filters with an equiripple behavior in the stop band (e.g. FIR filters designed by the Remez exchange algorithm, or Chebyshev II and elliptic IIR filters), there is a clear stop band edge, which is the lowest frequency (for low pass filters) at which the attenuation equals the minimum stop band attenuation (attained at the local maxima of the filter's magnitude response in the stop band).

However, your filter looks like it has a monotonic magnitude response, so the definition of the stop band edge is arbitrary.

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edit: This is for passband edge frequency.
One Way the cutoff frequency can be calculated as: consider H(jw) normalized filter transfer function $$ H(jw)=\frac{V_0(jw)}{V_i(jw)} $$ Here the 3dB frequency is when H(jw) is 1/sqrt(2) time the max value.Then it is finiding the values omega which satisfies the below equation $$ H(jw)=\frac{1}{\sqrt2} $$ This we can see when we take logarithm of 1/sqrt(2) we get approximately -3dB

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  • $\begingroup$ Thanks Vinith.But this is not answer my question.Your answer all true for passband like my picture above( I already calculated this value which is passband edge frequency ).However if ı apply your equations like above on stopband,the amplitute below 0.As you can see in this picture,i don't have a under 0 values( y axis ) for stopband then how can i calculate or find ? $\endgroup$ – yigit May 24 '15 at 16:07
  • $\begingroup$ Amplitude below 0 is considered in log scale.You have to take the log of the above function $\endgroup$ – Vinith May 24 '15 at 16:26
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    $\begingroup$ Your answer pertains to the pass band edge, not to the stop band edge, as required by the OP. $\endgroup$ – Matt L. May 24 '15 at 16:45

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