# Simple MATLAB code to plot a BiQuad Filter from the coefficients?

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Mike M on 9 May 2020
Answered: Star Strider on 9 May 2020
A BiQuad filter takes the following form:
The manual gives examples of how to use "freqz" for plotting frequency of functions and "phasez" for phase:
But none of these specifically handle a situation where you are trying to define a BiQuad filter by the standard variables g, B1, B2, a1, and a2.
I know we also have for inputting a biquad:
But I'm still not sure how to get the coefficients entered correctly as it says num and den can store 3 variables each, and I just need two, plus it's obviously important the right coefficient goes in the right place.
What is a simple code to enter a filter based on these specific 5 variables and then plot it for freq/phase?
Also they demonstrate there how to convert the y axis between dB and magnitude from 0-1. But they don't show how to convert the x-axis to frequency in Hz. If it's simple, how can you plot with freq in Hz?

Star Strider on 9 May 2020
I don’t have the DSP Toolbox, so I can’t experiment with dsp.BiquadFilter objects.
According to the documentation use this syntax with freqz:

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