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Convert digital filter to transfer function


[num,den] = tf(d)



[num,den] = tf(d) converts a digital filter, d, to numerator and denominator vectors.


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Design a 6th-order highpass FIR filter with a passband frequency of 75 kHz and a passband ripple of 0.2 dB. Specify a sample rate of 200 kHz. Compute the coefficients of the equivalent transfer function.

hpFilt = designfilt('highpassiir','FilterOrder',6, ...
         'PassbandFrequency',75e3,'PassbandRipple',0.2, ...
[b,a] = tf(hpFilt)
b = 1×7

    0.0003   -0.0019    0.0048   -0.0064    0.0048   -0.0019    0.0003

a = 1×7

    1.0000    4.0580    7.5656    8.1243    5.2561    1.9348    0.3164

Input Arguments

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Digital filter, specified as a digitalFilter object. Use designfilt to generate a digital filter based on frequency-response specifications.

Example: d = designfilt('lowpassiir','FilterOrder',3,'HalfPowerFrequency',0.5) specifies a third-order Butterworth filter with normalized 3 dB frequency 0.5π rad/sample.

Output Arguments

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Numerator coefficients, returned as a row vector.

Data Types: double

Denominator coefficients, returned as a row vector.

Data Types: double

Introduced in R2014a