How to get the -3dB Frequency value for a semilog plot ?

Hi,

I try to add a line on my graph on -3dB for my Bode Graph, i use a table for store all of my data like Frequency, Gain, Ouput voltage, Input voltage. I already make the semi-log plot but i don't know how get the value.

First of all i try to make a line and find the intersection but i don't understand how i can't use it.

I think i need to make an linear regression of my table to solve the equation "f(x)=-3", i'm not sure.

Thanks !

Answer 1

[Pavan Guntha]

You may have to first find the index at which the gain is -3 dB (i.e., in Classeurl.g). Then the frequency can be found by calculating the value of Classeurl.f at that index. The following code illustrates the idea:

index = Classeurl.g == -3;  
f_3dB = Classeurl.f(index);
% To plot a vertical line at -3 dB frequency:
semilogx(x,y,'-')  
hold on  
xline(f_3dB)  
hold off

You may refer to the documentation of xline for changing the properties as per your requirement.