37 votes
Accepted

What does a Bode plot represent and what is a pole and zero of a Bode plot?

Bode plot Take a typical 2nd order low pass filter amplitude response bode plot: - 3-D picture introducing pole-zero diagram Here's the bigger picture of that response when combined with a pole ...
  • 396k
30 votes

Gain margin and Phase Margin Physical Meaning

Gain and phase margin are usually applied to systems that are amplifiers of some sort with negative feedback around them. The more negative feedback, the tighter the system is controlled. However, ...
13 votes
Accepted

Why are decibels used for Bode plots?

Bode plots show gain and phase. Gain is measured in V/V, A/A, or W/W, and is thus dimensionless. Decibels are ratios, like percentages or parts per million. The question, then, is why we sometimes use ...
  • 20.7k
11 votes

Why are decibels used for Bode plots?

A first order filter has a roll-off of 20dB per decade. That is, every time the frequency increases/decreases (for high/low pass filter) by a factor of 10, the signal power drops by a factor of 100. ...
  • 936
11 votes

What are some insights from looking at Bode plots

The bode plot is a representation of the bigger picture. That bigger picture is the pole zero diagram: - The top three images (all bode plots) give you different examples of a 2nd order low pass ...
  • 396k
11 votes
Accepted

Why is frequency not measured in db in bode's plot?

Frequency is not a measure of signal amplitude or power hence, talking about "this or that" frequency in decibels makes no sense. If somebody says "increase the frequency by 3 dB"; ...
  • 396k
11 votes

Why is frequency not measured in db in bode's plot?

We could start using dBHz and see how it feels. My guess is it won't make life any simpler or help understanding. NOTE however : logarithmic frequency scale is already in use, and has been for a while ...
  • 66.1k
11 votes
Accepted

How can a Bode plot be like that?

How do I get the slope and intersection point? Start with a very simple example: - $$H(s) = \dfrac{1}{s}$$ When \$s = 1\$, the amplitude of the transfer function is clearly 1. When \$s = 0.1\$, the ...
  • 396k
10 votes
Accepted

What does actually the "unravel branch wrap" option do to the phase plot in LTspice?

It adds or subtracts multiples of \$2\pi\$ (360 degrees) to each point in the phase plot so that the phase is plotted continuously rather than having jumps in it. We know that physically the phase ...
  • 145k
10 votes

Is -3dB bandwidth defined for any type of transfer function?

Is -3dB bandwidth defined for any type of transfer function? Maybe you do not realize what -3dB actually means? -3 dB means that the amplitude of the voltage (current as well) has dropped by a factor ...
  • 79.1k
8 votes
Accepted

What are some insights from looking at Bode plots

One of the main innovations Bode proposed with Bode Stability plots was how the plot asymptotes behave for stable systems. A knowledge of these rules allows compensation just by manipulating the ...
  • 4,099
8 votes
Accepted

Why a low-pass filter applied to a square wave gives a weird wave?

There are many types of low pass filter. The one that gives your 'weird waveform' is the simplest 1st order RC filter. This passes the fundamental signal. However, it not only passes all the higher ...
  • 145k
8 votes

Problem with Bode diagram in LTspice

The AC analysis is a linearized analysis. That means it simulates the small-signal behavior of each element at its operating point. In the case of the switch, that means it is held in either the on or ...
  • 122k
7 votes

Low Pass Filter Wrong Pole Location

It's just a conversion from radians to degrees. The expression for a capacitor's impedance uses units of radians/sec. for frequency. The simulator is reporting frequencies in Hertz. 10 rad/sec is 1....
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7 votes
Accepted

How to sketch the Bode diagram of the output filter of a Buck converter?

Background Starting with your result and proceeding: $$\begin{align*} H\left(s\right)&=\frac{R}{R\,L\,C\,s^2+L\,s+R}\\\\ &=\frac{1}{L\,C\,s^2+\frac LR\,s+1}\\\\ &=\frac{\frac 1 {L\,C}}{s^2+...
  • 72.7k
7 votes

Pole zero plot of band reject filter

Perhaps this is where the confusion starts: 's' is complex (re + j*im, or sigma + jw), not just imaginary. Those two terms are often accidentally used interchangeably, and they shouldn't be. G is ...
  • 2,965
6 votes
Accepted

Interpretation of mathematicial and corresponding physical meaning of gain and phase margin

There are two key frequencies you can quickly identify from the bode plot of the closed-loop system. The unity gain frequency is where the gain is 0dB i.e. neither ...
  • 13.5k
6 votes

Why are decibels used for Bode plots?

Do you really think that a "terribly meaningless unit" would be used with enormous success over many decades? Are you not a bit curious if this "meaningless" unit has perhaps some advantages? here are ...
  • 23k
6 votes

What are some insights from looking at Bode plots

From your Bode plot (or 'frequency response' is probably a more descriptive term), just by cursory inspection it can be seen that: the system is 2nd order (since the high frequency roll-off is 40dB/...
  • 7,467
6 votes
Accepted

What is an "origin pole"?

The "origin pole" is indeed the \$1/s\$ term in the transfer function \$H(s)\$. In the bode plot it results in a first order transfer that does NOT flatten out for low frequencies. Your Bode plot is ...
  • 79.1k
6 votes
Accepted

Interpreting bode plot of type 3 compensation network for buck regulator

Calculated values for the poles and zeros look about right, except for the missing integrator pole at the origin. Maybe it would be best to break the loop up into pieces. Write the transfer function ...
  • 6,963
6 votes

What exactly is Pole Frequency in a Filter and how does it affect the frequency response?

1) Why does the magnitude Bode Plot of the response of a filter NOT approach Infinity at a pole? Try looking at this picture and recognize that poles may exist as infinities in the bode plot but ...
  • 396k
6 votes

Transfer function of electrical network

A quick look at the fast analytical techniques or FACTs gives you the transfer function of this guy in the blink of an eye. First, turn the stimulus off or reduce \$V_{in}\$ to 0 V: replace the source ...
  • 15.7k
6 votes

How are the -3dB point and the pole of an op-amp related?

Because a typical op-amp is dominant pole compensated, it has a single pole in the open-loop transfer function at low frequency, and a -20 dB/decade roll off. This leads to the concept of "gain-...
  • 20.6k
5 votes

Gain margin and Phase Margin Physical Meaning

People tend to make this way too complicated and difficult to understand. Stability margins are only defined for an ideal, linear transfer function model - a model expressed in terms of rational ...
5 votes

Gain margin and Phase Margin Physical Meaning

May I add a 4th answer in short? 1.) A circuit with feedback is unstable in case the loop gain has a phase shift of 360deg at a frequency where the loop gain magnitude is still larger the 0 dB. Note ...
  • 23k
5 votes
Accepted

What is the transfer function for a first order active high-pass filter

Your expression for \$H(s)\$ is correct: $$H(s)=\frac{RCs}{1+RCs}$$ where \$\tau=RC\$ is the time constant of the high pass filter. The most important features of this transfer function are the ...
  • 3,613
5 votes
Accepted

Low Pass Filter Wrong Pole Location

Complex Number Diversion Complex numbers have both a Cartesian notation as well as two equivalent polar notations. The Cartesian notation, \$a+b\:i\$, is very hard to use in electronics but it is very ...
  • 72.7k
5 votes
Accepted

Are phase angles in filter transfer functions primarily for stability?

Different applications are sensitive to phase variation with frequency for different reasons. It can impact stability margin if the filter is in a closed-loop system. The total error between a source ...
  • 38.8k
5 votes

Zeros and poles in the following bode plot

How is it possible to make transfer function to become zero or infinity. You will only get zeros and poles in the bode plot if those zeros and poles lie on the \$j\omega\$ axis. The bode plot is ...
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