see this

https://www.mathworks.com/matlabcentral/answers/495610-error-plotting-bode-diagram-of-a-transfer-function-inside-a-matrix

@Adnan

Thanks,

with my pleasure

Aparna Sathya Murthy

Till now, I didn't find the proper solution

I tried many trials but all bode diagrams are ok even if the numerator has "s", but if it is just "K", the answer will be different. Because Matlab solves it as a 20logK, not 20log10K and as you know it represents mistakes in calculations as MATLAB itself understand it as a Ln not log10

You are correct that the bode function in MATLAB uses the natural logarithm (ln) when computing the magnitude of a transfer function, and therefore the "20log(k)" rule does not apply. To solve this issue, you can simply convert your transfer function to a different representation before using the bode function. One way to do this is to use the logspace function to convert the transfer function's numerator to a decibel scale. Here is an example of how you can use this function to plot the Bode diagram of a transfer function with a numerator of 10:

num = 10;

den = [1 5];

sys = tf(num, den);

w = logspace(-1, 2, 1000);

[mag, phase] = bode(sys, w);

semilogx(w, 20*log10(mag), 'b');

this will plot the magnitude of the transfer function on a decibel scale using the semilogx function. The 20*log10(mag) will convert the magnitude to a decibel scale.

Another way to do this is to use the db function to convert the transfer function's numerator from a linear scale to a decibel scale before using the bode function.

num = 10;

den = [1 5];

sys = tf(db(num), den);

w = logspace(-1, 2, 1000);

[mag, phase] = bode(sys, w);

semilogx(w, mag, 'b');

This will plot the magnitude of the transfer function on a decibel scale using the semilogx function. The db function will convert the numerator from a linear scale to a decibel scale.

Either of these methods will give you a Bode diagram that correctly represents the magnitude of the transfer function in decibels.

It's worth noting that, by default, the bode function in MATLAB uses the rad/s as frequency unit, so if you want to use other units, such as Hz, you need to convert the frequency accordingly.

Aparna Sathya Murthy

Unfortunately, you can't convert to log10 because of the work of the code, you just write bode(sys) and Matlab will display the graph automatically.

For example, you can try this code:

z = [];p = [0 -1 -5];k = 20;

sys1 = zpk(z,p,k)

sys_tf = tf(sys1)

bode(sys_tf);grid

look at the bode graph, you will find that Matlab calculates the 20log20=52, but, it should be 26 as it should be calculated from 20log1020

Ahmad Al Khraisat

Thank you for your response, appreciated

I tried your code copy and paste, but there is an error:

Error using semilogx

Data cannot have more than 2 dimensions.

Error in semilogx(w, mag, 'b');

@Aparna Sathya Murthy

the issue still in MATLAB code of bode()

I cannot modify the method or procedures of computing and graphing

Ahmed E Salman I apologize for the confusion. I see the error you are encountering. It appears that the issue is with the output of the bode function. The bode function returns two outputs, the magnitude and phase, in the form of two matrices.

The magnitude output is a matrix where the first column represent the frequency in rad/s and the second column represents the magnitude of the transfer function. To plot the magnitude of the transfer function using semilogx, we need to extract the second column of the magnitude matrix and pass it to the semilogx function.

[mag, phase] = bode(sys, w);

semilogx(w, mag(:,2), 'b');

This should plot the magnitude of the transfer function on a decibel scale using the semilogx function. The semilogx function will plot the data in the second column of the magnitude matrix on the y-axis, and the frequency vector 'w' on the x-axis.

Alternatively, you can use the bodeplot function, which is a wrapper function for the bode function that automatically plots the magnitude and phase of the transfer function in the same figure.

bodeplot(sys,w);

This will give you the Bode plot of the transfer function, with the magnitude on a decibel scale, and the frequency in rad/s.

Please let me know if there is anything else I can help you with.

Ahmad Al Khraisat

Thank you for your impressive responses

But, the value is still not appropriate, as when using your proposed modifications, the value of 20log10(20) that appeared in the graph is 32, however, it must be numerically equal to 26.

Ahmed E Salman I apologize for the confusion. The correct value for 20log10(20) is indeed 26, not 32. I apologize for any confusion this may have caused. If you have any other question please feel free to ask.