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scipy.signal.

# dfreqresp

scipy.signal.dfreqresp(system, w=None, n=10000, whole=False)

  Calculate the frequency response of a discrete-time system.

  Parameters:
    system dlti | tuple

      An instance of the LTI class `dlti` or a tuple describing the system. The number of elements in the tuple determine the interpretation. I.e.:

      - `system`: Instance of LTI class `dlti`. Note that derived instances, such as instances of `TransferFunction`, `ZerosPolesGain`, or `StateSpace`, are allowed as well.

      - `(num, den, dt)`: Rational polynomial as described in `TransferFunction`. The coefficients of the polynomials should be specified in descending exponent order, e.g., z² + 3z + 5 would be represented as `[1, 3, 5]`.

      - `(zeros, poles, gain, dt)`: Zeros, poles, gain form as described in `ZerosPolesGain`.

      - `(A, B, C, D, dt)`: State-space form as described in `StateSpace`.

    w array_like, optional

      Array of frequencies (in radians/sample). Magnitude and phase data is calculated for every value in this array. If not given a reasonable set will be calculated.

    n int, optional

      Number of frequency points to compute if w is not given. The n frequencies are logarithmically spaced in an interval chosen to include the influence of the poles and zeros of the system.

    whole bool, optional

      Normally, if ‘w’ is not given, frequencies are computed from 0 to the Nyquist frequency, pi radians/sample (upper-half of unit-circle). If whole is True, compute frequencies from 0 to 2*pi radians/sample.

  Returns:
    w 1D ndarray

      Frequency array [radians/sample]

    H 1D ndarray

      Array of complex magnitude values

  Notes

  If (num, den) is passed in for `system`, coefficients for both the numerator and denominator should be specified in descending exponent order (e.g. `z^2 + 3z + 5` would be represented as `[1, 3, 5]`).

  Added in version 0.18.0.

  Examples

  The following example generates the Nyquist plot of the transfer function \(H(z) = \frac{1}{z^2 + 2z + 3}\) with a sampling time of 0.05 seconds:

      >>> from scipy import signal
      >>> import matplotlib.pyplot as plt
      >>> sys = signal.TransferFunction([1], [1, 2, 3], dt=0.05)  # construct H(z)
      >>> w, H = signal.dfreqresp(sys)
      ...
      >>> fig0, ax0 = plt.subplots()
      >>> ax0.plot(H.real, H.imag, label=r"$H(z=e^{+j\omega})$")
      >>> ax0.plot(H.real, -H.imag, label=r"$H(z=e^{-j\omega})$")
      >>> ax0.set_title(r"Nyquist Plot of $H(z) = 1 / (z^2 + 2z + 3)$")
      >>> ax0.set(xlabel=r"$\text{Re}\{z\}$", ylabel=r"$\text{Im}\{z\}$",
      ...         xlim=(-0.2, 0.65), aspect='equal')
      >>> ax0.plot(H[0].real, H[0].imag, 'k.')  # mark H(exp(1j*w[0]))
      >>> ax0.text(0.2, 0, r"$H(e^{j0})$")
      >>> ax0.grid(True)
      >>> ax0.legend()
      >>> plt.show()