# second order differential equations & Laplace

Discussion in 'Mathematics and Physics' started by derick007, Feb 5, 2013.

1. ### derick007Member

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I have been inputting values into my ss (quite a few by now) and plotting the results on a graph. I update the graph every time I add a term (whether plus or minus, charging or discharging) and have notice that when a add the charging term, the exponentially decaying oscillating sinusoid, centres it self around the amplitude of the square input i.e 23.5v meaning the output averages out at 23.5 v !!!

When I add the discharging term the graph centres itself around zero ????

Kind regards, Derek

2. ### MrAlWell-Known MemberMost Helpful Member

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Hello again,

Well, something somewhere must be wrong

Would it help if i posted a bigger example of 2nd order?

3. ### derick007Member

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I have found it very slow inputting values as I am trying to work out an output value every 10us.

I am upto term f22 and think I may have started to see a change in the output which may give me the result I am after.

Although I would like to plot values upto 1500us, I am only upto 250us and present and slowly increasing this and adding additional terms, f23, f24, f25 ....

I think every term f23 etc only adds 10us, therefore to get to 1500us, I will need to add something like f150 terms and I am only at f22 !!!!

Kind Regards, Derek

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5. ### MrAlWell-Known MemberMost Helpful Member

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Hi again,

Well if you are working at around 6us and 20us period then you need to evaluate every 6us and 20us until you get to the last set, then you have to evaluate at smaller periods. But for 100 sets each 6us and 20us you should be able to do just those and then start going slower. So for 1500us you should be able to do up to around 1400us using just 6us and 20us (for example) and then start using smaller increments to get a finer output once you get within range of the target time slot.

6. ### derick007Member

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Hi Mr Al

Hope you are keeping well. You have given me a lot of help in the past and I was wondering if you could do so again.

I have posted a new thread in the mathematics forum entitled Fourier Transform of a QAM signal. Any help would be greatly appreciated.

Many thanks, derick007

7. ### MrAlWell-Known MemberMost Helpful Member

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Hello again derick007,

I will help as much as i can as i dont remember much about QAM and since i use Laplace Transforms almost exclusively now i hardly ever use Fourier Transforms much at all. They are very interesting though and i'm in the process of remembering some of the theory right now
In the mean time, SteveB is nice enough to lend some tips in the new thread. I'll reply there too if i have any information.

8. ### derick007Member

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MrAl

Many thanks for getting back.

Regards derick007