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Convert transfer function filter parameters to zero-pole-gain form



[z,p,k] = tf2zpk(b,a) finds the matrix of zeros z, the vector of poles p, and the associated vector of gains k from the transfer function parameters b and a. The function converts a polynomial transfer-function representation


of a single-input/multi-output (SIMO) discrete-time system to a factored transfer function form



Use tf2zpk when working with transfer functions expressed in inverse powers (1 + z–1 + z–2). A similar function, tf2zp, is more useful for working with positive powers (s2 + s + 1), such as in continuous-time transfer functions.


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Design a 3rd-order Butterworth filter with normalized cutoff frequency 0.4π rad/sample. Find the poles, zeros, and gain of the filter.

[b,a] = butter(3,0.4);
[z,p,k] = tf2zpk(b,a)
z = 3×1 complex

  -1.0000 + 0.0000i
  -1.0000 - 0.0000i
  -1.0000 + 0.0000i

p = 3×1 complex

   0.2094 + 0.5582i
   0.2094 - 0.5582i
   0.1584 + 0.0000i

k = 0.0985

Plot the poles and zeros to verify that they are where expected.


Figure contains an axes object. The axes object with title Pole-Zero Plot contains 10 objects of type line, text.

Input Arguments

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Transfer function numerator coefficients, specified as a vector or matrix. If b is a matrix, then each row of b corresponds to an output of the system. b contains the coefficients in descending powers of z. The number of columns of b must be equal to the length of a. If the numbers differ, make them equal by padding zeros. You can use the function eqtflength to accomplish this.

Data Types: single | double

Transfer function denominator coefficients, specified as a vector. a contains the coefficients in descending powers of z.

Data Types: single | double

Output Arguments

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Zeros of the system, returned as a matrix. z contains the numerator zeros in its columns. z has as many columns as there are outputs.

Poles of the system, returned as a column vector. p contains the pole locations of the denominator coefficients of the transfer function

Gains of the system, returned as a column vector. k contains the gains for each numerator transfer function.

Extended Capabilities

Version History

Introduced before R2006a

See Also

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