# How to plot Bode plot in LT spice...??

#### RRITESH KAKKAR

Joined Jun 29, 2010
2,829
Hi again,

I want to make bode plot in lt spice for low pass filter....
but how??

#### RRITESH KAKKAR

Joined Jun 29, 2010
2,829

#### RRITESH KAKKAR

Joined Jun 29, 2010
2,829
Can any one tell me the use of db here and what is this no. of decade or octave??

#### debjit625

Joined Apr 17, 2010
790
Normally we plot graph in time domain but Bode plots, plot a graph in frequency domain i.e.. it will show you change of some thing with respect to frequency. It plots
amplitude of signal on one axis (on Y axis) and frequency on another axis (on X axis).

In LTSpice you can see how your low pass filter responds to different frequencies using Bode Plots, by default the magnitude i.e.. amplitude of the signal is plotted on Y axis using db ,as the unit is mostly used in engineering but you don't have to go with db you can view the amplitude of the signal in other units also just left click the area where all the db values are written i.e.. on the left side of the bode plot and a window will come up select "Linear" from their.
what is this no. of decade or octave??
Its the way or type of sweep for your simulation, most of the time I use Linear(lin) the spice command is like this "ac oct 30 10 300" the first word "ac" says this is an ac analysis ,"oct" means the type of sweep ,first number "30" is the number of points for sweep, second number "10" is the starting frequency and the last number is "300" the end frequency where the simulation ends. You can set these parameters using Edit Simulation Command window just right click over the spice command.

Here I have used a the Linear option to plot the graph so the voltage across the capacitor is plotted on the Y axis, I have used V1 as "ac 10" so the max amplitude of the signal is 10 volts on Y axis. You could see in the picture that its a low pass filter i.e.. on 10Hz the signal strength is max i.e.. 10V but as we increase the frequency to 300Hz the signal strength decrease to 4.5V, if you simulate for more higher frequencies the signal will almost ends up to 0V. I have added the LTSpice file too.

Good Luck

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• RRITESH KAKKAR

#### RRITESH KAKKAR

Joined Jun 29, 2010
2,829
"oct" means the type of sweep ,first number "30" is the number of points for sweep

Here I have used a the Linear option to plot the graph so the voltage across the capacitor is plotted on the Y axis,
what the diff.in linear than other ...

Last edited:

#### debjit625

Joined Apr 17, 2010
790
Suppose you need to plot voltage across a charging capacitor, now how will you plot it?
I would plot it with respect to time, for example every 1mS I will note the voltage across the capacitor and after the capacitor charges I will plot the data. Now above you could see we need some sort of steps for plotting the graph i.e.. step of 1mS,this value of step is also needed by Spice to plot the graph and described as sweep. For example "ac lin 100 10 300" means their will be 100 even steps between 10Hz and 300Hz to measure the signal.

what the diff.in linear than other ...
In linear option the graph will be plotted using voltage or current in the circuit linearly.

Their are many resource to learn LTSpice google it.

Good Luck

• RRITESH KAKKAR

#### RRITESH KAKKAR

Joined Jun 29, 2010
2,829
OK, just tell why there is two lines not ones??

#### debjit625

Joined Apr 17, 2010
790
The green dashed or dotted line is representing the phase shift of the signal from 10Hz to 300Hz, on right side the phase shift is measured using degrees. You can click over it on right side and a window will come up their click the button "Don't plot phase" the green dashed line will disappear.

Good luck