cbadc.analog_system.IIRDesign

class cbadc.analog_system.IIRDesign(wp, ws, gpass, gstop, ftype='ellip')

Bases: cbadc.analog_system.AnalogSystem

An analog signal designed using standard IIRDesign tools

This class inherits from cbadc.analog_system.AnalogSystem and is a convenient way of creating IIR filters in an analog system representation.

Specifically, we specify the filter by the differential equations

\(\dot{\mathbf{x}}(t) = \mathbf{A} \mathbf{x}(t) + \mathbf{B} u(t) + \mathbf{\Gamma} \mathbf{s}(t)\)

\(\mathbf{y}(t) = \mathbf{C}^\mathsf{T} \mathbf{x}(t) + \mathbf{D} u(t)\)

\(\tilde{\mathbf{s}}(t) = \tilde{\mathbf{\Gamma}}^\mathsf{T} \mathbf{x}(t)\)

where

internally \(\mathbf{A}\) \(\mathbf{B}\), \(\mathbf{C}^\mathsf{T}\), and \(\mathbf{D}\) are determined using the scipy.signal.iirdesign().

Furthermore, as this system is intended as a pure filter and therefore have no

\(\mathbf{\Gamma}\) and \(\tilde{\mathbf{\Gamma}}^\mathsf{T}\) specified.

Parameters

  • wp (float or array_like, shape=(2,)) –

    Passband and stopband edge frequencies. Possible values are scalars (for lowpass and highpass filters) or ranges (for bandpass and bandstop filters). For digital filters, these are in the same units as fs. By default, fs is 2 half-cycles/sample, so these are normalized from 0 to 1, where 1 is the Nyquist frequency. For example:

    • Lowpass: wp = 0.2, ws = 0.3

    • Highpass: wp = 0.3, ws = 0.2

    • Bandpass: wp = [0.2, 0.5], ws = [0.1, 0.6]

    • Bandstop: wp = [0.1, 0.6], ws = [0.2, 0.5]

    wp and ws are angular frequencies (e.g., rad/s). Note, that for bandpass and bandstop filters passband must lie strictly inside stopband or vice versa.

  • ws (float or array_like, shape=(2,)) –

    Passband and stopband edge frequencies. Possible values are scalars (for lowpass and highpass filters) or ranges (for bandpass and bandstop filters). For digital filters, these are in the same units as fs. By default, fs is 2 half-cycles/sample, so these are normalized from 0 to 1, where 1 is the Nyquist frequency. For example:

    • Lowpass: wp = 0.2, ws = 0.3

    • Highpass: wp = 0.3, ws = 0.2

    • Bandpass: wp = [0.2, 0.5], ws = [0.1, 0.6]

    • Bandstop: wp = [0.1, 0.6], ws = [0.2, 0.5]

    wp and ws are angular frequencies (e.g., rad/s). Note, that for bandpass and bandstop filters passband must lie strictly inside stopband or vice versa.

  • gpass (float) – The maximum loss in the passband (dB).

  • gstop (float) – The minimum attenuation in the stopband (dB).

  • ftype (string, optional) –

    IIR filter type, defaults to ellip. Complete list of choices:

    • Butterworth : ‘butter’

    • Chebyshev I : ‘cheby1’

    • Chebyshev II : ‘cheby2’

    • Cauer/elliptic: ‘ellip’

    • Bessel/Thomson: ‘bessel’

N

state space order \(N\).

Type

int

N_tilde

number of signal observations \(\tilde{N}\).

Type

int

M

number of digital control signals \(M\).

Type

int

M_tilde

number of control signal observations \(\tilde{M}\).

Type

int

L

number of input signals \(L\).

Type

int

A

system matrix \(\mathbf{A}\).

Type

array_like, shape=(N, N)

B

input matrix \(\mathbf{B}\).

Type

array_like, shape=(N, L)

CT

signal observation matrix \(\mathbf{C}^\mathsf{T}\).

Type

array_like, shape=(N_tilde, N)

D

direct matrix

Type

array_like, shape=(N_tilde, L)

Gamma

control input matrix \(\mathbf{\Gamma}\).

Type

None

Gamma_tildeT

control observation matrix \(\tilde{\mathbf{\Gamma}}^\mathsf{T}\).

Type

None

Example

>>> import numpy as np
>>> import matplotlib.pyplot as plt
>>> import matplotlib.ticker
>>> from cbadc.analog_system import IIRDesign
>>> wp = 2 * np.pi * 1e3
>>> ws = 2 * np.pi * 2e3
>>> gpass = 0.1
>>> gstop = 80
>>> filter = IIRDesign(wp, ws, gpass, gstop)
>>> f = np.logspace(1, 5)
>>> w = 2 * np.pi * f
>>> tf = filter.transfer_function_matrix(w)
>>> fig, ax1 = plt.subplots()
>>> ax1.set_title('Analog filter frequency response')
>>> ax1.set_ylabel('Amplitude [dB]', color='b')
>>> ax1.set_xlabel('Frequency [Hz]')
>>> ax1.semilogx(f, 20 * np.log10(np.abs(tf[0, 0, :])))
>>> ax1.grid()
>>> ax2 = ax1.twinx()
>>> angles = np.unwrap(np.angle(tf[0, 0, :]))
>>> ax2.plot(f, angles, 'g')
>>> ax2.set_ylabel('Angle (radians)', color='g')
>>> ax2.grid()
>>> ax2.axis('tight')
>>> nticks = 8
>>> ax1.yaxis.set_major_locator(matplotlib.ticker.LinearLocator(nticks))
>>> ax2.yaxis.set_major_locator(matplotlib.ticker.LinearLocator(nticks))

See also

cbadc.analog_system.ButterWorth, cbadc.analog_system.ChebyshevI, cbadc.analog_system.ChebyshevII, cbadc.analog_system.Cauer

Raises

InvalidAnalogSystemError – For faulty analog system parametrization.

Methods

__init__(wp, ws, gpass, gstop[, ftype])

Create a IIR filter

control_observation(x)

Computes the control observation for a given state vector \(\mathbf{x}(t)\) evaluated at time \(t\).

control_signal_transfer_function_matrix(omega)

Evaluates the transfer functions between control signals and the system output.

derivative(x, t, u, s)

Compute the derivative of the analog system.

signal_observation(x)

Computes the signal observation for a given state vector \(\mathbf{x}(t)\) evaluated at time \(t\).

transfer_function_matrix(omega)

Evaluate the analog signal transfer function at the angular frequencies of the omega array.

zpk([input])

return zero-pole-gain representation of system
control_observation(x: numpy.ndarray) → numpy.ndarray

Computes the control observation for a given state vector \(\mathbf{x}(t)\) evaluated at time \(t\).

Specifically, returns

\(\tilde{\mathbf{s}}(t) = \tilde{\mathbf{\Gamma}}^\mathsf{T} \mathbf{x}(t)\)

Parameters

x (array_like, shape=(N,)) – the state vector.

Returns

the control observation.

Return type

array_like, shape=(M_tilde,)

control_signal_transfer_function_matrix(omega: numpy.ndarray) → numpy.ndarray

Evaluates the transfer functions between control signals and the system output.

Specifically, evaluates

\(\bar{\mathbf{G}}(\omega) = \mathbf{C}^\mathsf{T} \left(\mathbf{A} - i \omega \mathbf{I}_N\right)^{-1} \mathbf{\Gamma} \in \mathbb{R}^{\tilde{N} \times M}\)

for each angular frequency in omega where \(\mathbf{I}_N\) represents a square identity matrix of the same dimensions as \(\mathbf{A}\) and \(i=\sqrt{-1}\).

Parameters

omega (array_like, shape=(K,)) – an array_like object containing the angular frequencies for evaluation.

Returns

the signal transfer function evaluated at K different angular frequencies.

Return type

array_like, shape=(N_tilde, M, K)

derivative(x: numpy.ndarray, t: float, u: numpy.ndarray, s: numpy.ndarray) → numpy.ndarray

Compute the derivative of the analog system.

Specifically, produces the state derivative

\(\dot{\mathbf{x}}(t) = \mathbf{A} \mathbf{x}(t) + \mathbf{B} \mathbf{u}(t) + \mathbf{\Gamma} \mathbf{s}(t)\)

as a function of the state vector \(\mathbf{x}(t)\), the given time \(t\), the input signal value \(\mathbf{u}(t)\), and the control contribution value \(\mathbf{s}(t)\).

Parameters

  • x (array_like, shape=(N,)) – the state vector evaluated at time t.

  • t (float) – the time t.

  • u (array_like, shape=(L,)) – the input signal vector evaluated at time t.

  • s (array_like, shape=(M,)) – the control contribution evaluated at time t.

Returns

the derivative \(\dot{\mathbf{x}}(t)\).

Return type

array_like, shape=(N,)

signal_observation(x: numpy.ndarray) → numpy.ndarray

Computes the signal observation for a given state vector \(\mathbf{x}(t)\) evaluated at time \(t\).

Specifically, returns

\(\mathbf{y}(t)=\mathbf{C}^\mathsf{T} \mathbf{x}(t)\)

Parameters

x (array_like, shape=(N,)) – the state vector.

Returns

the signal observation.

Return type

array_like, shape=(N_tilde,)

transfer_function_matrix(omega: numpy.ndarray) → numpy.ndarray

Evaluate the analog signal transfer function at the angular frequencies of the omega array.

Specifically, evaluates

\(\mathbf{G}(\omega) = \mathbf{C}^\mathsf{T} \left(\mathbf{A} - i \omega \mathbf{I}_N\right)^{-1} \mathbf{B} + \mathbf{D}\)

for each angular frequency in omega where \(\mathbf{I}_N\) represents a square identity matrix of the same dimensions as \(\mathbf{A}\) and \(i=\sqrt{-1}\).

Parameters

omega (array_like, shape=(K,)) – an array_like object containing the angular frequencies for evaluation.

Returns

the signal transfer function evaluated at K different angular frequencies.

Return type

array_like, shape=(N_tilde, L, K)

zpk(input=0)

return zero-pole-gain representation of system

Parameters

  • input – determine for which input (in case of L > 1) to compute zpk, defaults to 0.

  • int – determine for which input (in case of L > 1) to compute zpk, defaults to 0.

  • optional – determine for which input (in case of L > 1) to compute zpk, defaults to 0.

Returns

z,p,k the zeros, poles and gain of the system

Return type

array_like, shape=(?, ?, 1)