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

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:

https://www.mathworks.com/help/signal/ref/freqz.html

https://www.mathworks.com/help/signal/ref/phasez.html

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:

y = biquad(x,num,den,g)                    

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?

Thanks for any answers.

Answer 1

[Star Strider]

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:

freqz(sysobj)

to analyse your filter.

Also see: Analysis Methods for Filter System Objects