are step response and frequency response tranfser functions the same?

[tracidfish]

Hello

I have been given a task where I need to work out the transfer function for a system firstly using the step response method then secondly using the frequency responce method. The system has a transfer function that is a 1st order lag.

I have done both and have plotted the bode plots for the frequency response method.

My question is should the transfer function for the step response method and the frequency responce method be roughly the same? As it currently stands my gain is the same and yet my time contast is different.

The break point on amplitude ratio plot corresponds nicley my -45 degree point on my phase shift plots, so at least theese agree.


Thanks

 

[MrAl]

Hello there,


There are different ways of doing this so perhaps it would be better if you showed some of your work so we can see exactly how you are trying to do this and exactly how you are comparing the two.

The step response is usually a time domain solution and the frequency response is usually a frequency domain solution so it's a little unclear what you are comparing. It could be a course specific problem so it would be good to see what you have done so far first.

 

[tracidfish]

hhhhn h

 

[ronsimpson]

I might have done the same thing using spice.
See the two pictures.
The real circuit was close to the spice.

 

[tracidfish]

Thanks any way guys.

i have modelled it in multisim and have answeard my own question.

i can now see that the frequency response and step response should be the same.

I am happy with out data and our calulations.

It would appear that the weay we conducted the experiment may be causing the error.
I think we now just need to conclude as to why the error may exist.

Thanks

 

[MrAl]

Hello,


Just a guess, but are you sure you are measuring angular frequency and not just plain frequency in Hertz?
I mention this because 1.9 times 3 is somewhat close to the required angular frequency, and mixing the two is a common mistake.

If not you'll have to PM me your frequency response graph.

 

[tracidfish]

,koiup

 

[MrAl]

Hi again,

Ok, so you are saying that when you draw that graph you are using a frequency generator and set the frequency to 1.90/(2*pi) in Hertz right? That means you set the frequency generator to about 0.3 Hertz. Is this correct?

Can you show your actual circuit? That would clear this up much quicker i think
Remember to show component values.