The Freq. Response tab displays the filter frequency response (magnitude only).
The frequency response of the currently selected coefficient set is plotted against normalized frequency. Where the Quantization option is set to Integer Coefficients, there is only a single plot based on the specified coefficient values. Where the Quantization option has been set to Quantize Only, an ideal plot is displayed based on the provided values alongside a Quantized plot based on a set of coefficient values quantized according to the specified coefficient bit width. Where the Quantization option is set to Maximize Dynamic Range, the coefficients are first scaled to take full advantage of the available dynamic range, then quantized according to the specified coefficient bit width. The quantized coefficients are summed to determine the resulting gain factor over the provided real coefficient set, and the resulting scale factor is used to correct the filter response of the quantized coefficients such that the gain is factored out. The scale factor is reported in the legend text of the frequency response plot and on the Summary page. See Coefficient Quantization for more details.
The filter gain displayed is for a single rate implementation and does not take into account the zero insertion between output samples in the up-sampling processes in a interpolating filter. Therefore, following the zero insertion the average filter gain is reduced by the up-sampling rate.
• Set to Display : This selects which of multiple coefficient sets (if applicable) is displayed in the Frequency Response Graph.
• Passband Range : Two fields are available to specify the passband range, the left-most being the minimum value and the right-most the maximum value. The values are specified in the same units as on the graph x-axis (for example, normalized to pi radians per second). For the specified range the passband maximum, minimum and ripple values are calculated and displayed (in dB).
• Stopband Range : Two fields are available to specify the stopband range, the left-most being the minimum value and the right-most the maximum value. The values are specified in the same units as on the graph x-axis (for example, normalized to pi radians per second). For the specified range the stopband maximum value is calculated and displayed (in dB).
You can specify any range for the passband or stopband, allowing closer analysis of any region of the response. For example, examination of the transition region can be done to more accurately examine the filter roll-off.