Magnitude-only Bode plot of frequency response (2024)

Magnitude-only Bode plot of frequency response

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Syntax

bodemag(sys)

bodemag(sys1,sys2,...,sysN)

bodemag(sys1,LineSpec1,...,sysN,LineSpecN)

bodemag(___,w)

Description

bodemag enables you to generate magnitude-only plots to visualize the magnitude frequency response of a dynamic system.

For a more comprehensive function, see bode. bode provides magnitude and phase information. If you have System Identification™ toolbox, bode also returns the computed values, including statistical estimates.

For more customizable plotting options, see bodeplot.

example

bodemag(sys) creates a Bode magnitude plot of the frequency response of the dynamic system model sys. The plot displays the magnitude (in dB) of the system response as a function of frequency. bodemag automatically determines frequencies to plot based on system dynamics.

If sys is a multi-input, multi-output (MIMO) model, then bodemag produces an array of Bode magnitude plots in which each plot shows the frequency response of one I/O pair.

example

bodemag(sys1,sys2,...,sysN) plots the frequency response of multiple dynamic systems on the same plot. All systems must have the same number of inputs and outputs.

example

bodemag(sys1,LineSpec1,...,sysN,LineSpecN) specifies a color, line style, and marker for each system in the plot.

example

bodemag(___,w) plots system responses for frequencies specified by w.

  • If w is a cell array of the form {wmin,wmax}, then bodemag plots the response at frequencies ranging between wmin and wmax.

  • If w is a vector of frequencies, then bodemag plots the response at each specified frequency.

You can use this syntax with any of the input-argument combinations in previous syntaxes.

Examples

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Bode Magnitude Plot of Dynamic System

Create a Bode magnitude plot of the following continuous-time SISO dynamic system.

H(s)=s2+0.1s+7.5s4+0.12s3+9s2

H = tf([1 0.1 7.5],[1 0.12 9 0 0]);bodemag(H)

Magnitude-only Bode plot of frequency response (1)

bodemag automatically selects the plot range based on the system dynamics.

Bode Magnitude Plot at Specified Frequencies

This example uses:

  • Control System ToolboxControl System Toolbox

Open Live Script

Create a Bode magnitude plot over a specified frequency range. Use this approach when you want to focus on the dynamics in a particular range of frequencies.

H = tf([-0.1,-2.4,-181,-1950],[1,3.3,990,2600]);bodemag(H,{1,100})grid on

Magnitude-only Bode plot of frequency response (2)

The cell array {1,100} specifies the minimum and maximum frequency values in the Bode magnitude plot. When you provide frequency bounds in this way, the function selects intermediate points for frequency response data.

Alternatively, specify a vector of frequency points to use for evaluating and plotting the frequency response.

w = [1 5 10 15 20 23 31 40 44 50 85 100];bodemag(H,w,'.-')grid on

Magnitude-only Bode plot of frequency response (3)

bodemag plots the frequency response at the specified frequencies only.

Compare Bode Magnitude Plots of Several Dynamic Systems

This example uses:

  • Control System ToolboxControl System Toolbox

Open Live Script

Compare the magnitude of the frequency response of a continuous-time system to an equivalent discretized system on the same Bode plot.

Create continuous-time and discrete-time dynamic systems.

H = tf([1 0.1 7.5],[1 0.12 9 0 0]);Hd = c2d(H,0.5,'zoh');

Create a Bode magnitude plot that displays the responses of both systems.

bodemag(H,Hd)

Magnitude-only Bode plot of frequency response (4)

The Bode magnitude plot of a discrete-time system includes a vertical line marking the Nyquist frequency of the system.

Bode Magnitude Plot with Specified Line and Marker Attributes

This example uses:

  • Control System ToolboxControl System Toolbox

Open Live Script

Specify the color, linestyle, or marker for each system in a Bode magnitude plot using the LineSpec input arguments.

H = tf([1 0.1 7.5],[1 0.12 9 0 0]);Hd = c2d(H,0.5,'zoh');bodemag(H,'r',Hd,'b--')

Magnitude-only Bode plot of frequency response (5)

The first LineSpec argument 'r' specifies a solid red line for the response of H. The second LineSpec argument 'b--' specifies a dashed blue line for the response of Hd.

Magnitude of MIMO System

This example uses:

  • Control System ToolboxControl System Toolbox

Open Live Script

For this example, create a 2-output, 3-input system.

rng(0,'twister'); % For reproducibilityH = rss(4,2,3);

For this system, bodemag plots the magnitude-only frequency responses of each I/O channel in a separate plot in a single figure.

bodemag(H)

Magnitude-only Bode plot of frequency response (6)

Input Arguments

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sysDynamic system
dynamic system model | model array

Dynamic system, specified as a SISO or MIMO dynamic system model or array of dynamic system models. Dynamic systems that you can use include:

  • Continuous-time or discrete-time numeric LTI models, such as tf (Control System Toolbox), zpk (Control System Toolbox), or ss (Control System Toolbox) models.

  • Generalized or uncertain LTI models such as genss (Control System Toolbox) or uss (Robust Control Toolbox) models. (Using uncertain models requires Robust Control Toolbox™ software.)

    • For tunable control design blocks, the function evaluates the model at its current value for both plotting and returning frequency response data.

    • For uncertain control design blocks, the function plots the nominal value and random samples of the model. When you use output arguments, the function returns frequency response data for the nominal model only.

  • Frequency-response data models such as frd models. For such models, the function plots the response at frequencies defined in the model.

  • Identified LTI models, such as idtf, idss, or idproc models.

If sys is an array of models, the function plots the frequency responses of all models in the array on the same axes.

wFrequencies
{wmin,wmax} | vector

Frequencies at which to compute and plot frequency response, specified as the cell array {wmin,wmax} or as a vector of frequency values.

  • If w is a cell array of the form {wmin,wmax}, then the function computes the index at frequencies ranging between wmin and wmax.

  • If w is a vector of frequencies, then the function computes the index at each specified frequency. For example, use logspace to generate a row vector with logarithmically spaced frequency values.

Specify frequencies in units of rad/TimeUnit, where TimeUnit is the TimeUnit property of the model.

Algorithms

bodemag computes the frequency response as follows:

  1. Compute the zero-pole-gain (zpk (Control System Toolbox)) representation of the dynamic system.

  2. Evaluate the gain and phase of the frequency response based on the zero, pole, and gain data for each input/output channel of the system.

    • For continuous-time systems, bodemag evaluates the frequency response on the imaginary axis s = and considers only positive frequencies.

    • For discrete-time systems, bodemag evaluates the frequency response on the unit circle. To facilitate interpretation, the command parameterizes the upper half of the unit circle as:

      z=ejωTs,0ωωN=πTs,

      where Ts is the sample time and ωN is the Nyquist frequency. The equivalent continuous-time frequency ω is then used as the x-axis variable. Because H(ejωTs) is periodic with period 2ωN, bodemag plots the response only up to the Nyquist frequency ωN. If sys is a discrete-time model with unspecified sample time, bodemag uses Ts = 1.

Version History

Introduced in R2012a

See Also

bode | bodeplot | freqresp | nyquist | spectrum | step

Topics

  • Plot Bode and Nyquist Plots at the Command Line
  • Dynamic System Models

MATLAB Command

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Magnitude-only Bode plot of frequency response (7)

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Magnitude-only Bode plot of frequency response (2024)

FAQs

How do you plot the magnitude of a frequency response? ›

It is customary to plot the magnitude of the frequency response function on the log scale as |G(jω)|dB=20log10|G(jω)|. The magnitude of the loop gain is given in dB as: |KGH(jω)|dB=20logK+∑mi=120log|1+jωzi|−(20n0)logω−∑n1i=120log|1+jωpi|−∑n2i=120log|1−ω2ω2n,i+j2ζiωωn,i|.

How do you find the magnitude of a Bode plot? ›

To find the magnitude of the output, simply multiply the magnitude of the input (A) by the magnitude of the transfer function (M). The phase of the output is sum of the input phase (φ) and the phase of the transfer function (θ).

What is the frequency response of a Bode plot? ›

Bode plots show the frequency response, that is, the changes in magnitude and phase as a function of frequency. This is done on two semi-log scale plots. The top plot is typically magnitude or “gain” in dB. The bottom plot is phase, most commonly in degrees.

What is Bode in Matlab? ›

bode( sys ) creates a Bode plot of the frequency response of a dynamic system model sys . The plot displays the magnitude (in dB) and phase (in degrees) of the system response as a function of frequency. bode automatically determines frequencies to plot based on system dynamics.

What is magnitude in frequency response? ›

The magnitude describes the strength of each frequency in the signal. The phase describes the sine/cosine phase of each frequency. The phase can also be thought of as the relative proportion of sines and cosines in the signal (i.e., a phase of zero contains only cosines and a phase of 90 degrees contains only sines).

How to draw a Bode plot frequency response? ›

Creating a Bode Plot
  1. Rewrite the given transfer function into first and second-order terms.
  2. Plot each element approximation for Gain, s terms, poles, zeros, complex poles, complex zeros, etc.
  3. Round off corners. ...
  4. Add the plots together, if Magnitude is measured in dB, or multiply Magnitude plot if on logarithmic scale.

What is the absolute magnitude Bode plot? ›

The Bode magnitude plot is a graph of the absolute value of the gain of a circuit, as a function of frequency. The gain is plotted in decibels, while frequency is shown on a logarithmic scale. It is therefore a log–log plot.

What is magnitude of a plot? ›

the magnitude plot is a straight line with a slope of 20 d B per decade, passing through the abscissa axis at ω = 1 rad/s, and the phase plot is a constant equal to 90 ∘ (Fig. 5.9): Figure 5.9. Bode plots of the monomial term j ω . | j ω | d B = 20 log 10 ω , 1 j ω = − 90 ∘ .

What should frequency response be? ›

Ideal speaker frequency response would cover the full spectrum of human hearing, i.e., 20 Hz to 20,000 Hz, with a smooth response across this range. However, perfect sound reproduction is difficult due to several factors like speaker design, room acoustics, and human hearing limitations.

What is the formula for the Bode plot? ›

The standard transfer function of a Bode magnitude plot is: T F = K ( 1 + s ω 1 ) ( 1 + s ω 2 ) … s n ( 1 + s ω 3 ) ( 1 + s ω 4 ) … Here, ω1, ω2, ω3, ω4, … are the corner frequencies. n is the number poles at the origin.

What should a frequency response graph look like? ›

It shows your eyes how your ears might receive a speaker's bass, midrange, and treble sound production. The X axis of such a graph plots frequency, usually from 20 to 20kHz, and the Y axis shows amplitude measured in dB SPL. It's common for the Y axis to span 50 or 60 dB.

Why is it called Bode plot? ›

Among his several important contributions to circuit theory and control theory, engineer Hendrik Wade Bode, while working at Bell Labs in the 1930s, devised a simple but accurate method for graphing gain and phase-shift plots. These bear his name, Bode gain plot and Bode phase plot.

How to determine the bandwidth of a Bode plot? ›

Bandwidth is defined as the frequency at which the closed-loop magnitude response is equal to -3 dB and/or the phase is shifted by -45o.

How is frequency response plotted? ›

Frequency response plots show the complex values of a transfer function as a function of frequency. In this case, the frequency function G(iω) is the transfer function evaluated on the imaginary axis s=iω. In this case, the frequency function G e i ω T is the transfer function G(z) evaluated on the unit circle.

What is the magnitude of response? ›

The magnitude response refers to the plot of the gain in magnitude versus frequency when a pure complex exponential signal is input to an LTI system. AI generated definition based on: Nonlinear Digital Filters, 2007.

How to find the magnitude of a signal? ›

The magnitude is the square root of the sum of the squares of the real and imaginary parts. The phase is relative to the start of the time record or relative to a single-cycle cosine wave starting at the beginning of the time record. Single-channel phase measurements are stable only if the input signal is triggered.

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