# Bode plot - Phase response - how do find w?

Discussion in 'Homework Help' started by Kai Chiang, Aug 30, 2016.

1. ### Kai Chiang Thread Starter New Member

Oct 27, 2015
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0

The one circle in red.

Wp is found to be 100 Hz and what about w? How do i go about to find w?

2. ### DGElder Member

Apr 3, 2016
351
87
You do not. It is an input variable for the transfer function.

3. ### Kai Chiang Thread Starter New Member

Oct 27, 2015
12
0
But if I dont have the value for w , how do I go about to find the phase response? which is -arctan(w/100) ?

4. ### MrAl AAC Fanatic!

Jun 17, 2014
5,188
1,120
Hi,

You keep changing w to get the entire phase response.
It looks like they went from 1Hz to 100MHz in that picture you posted. That means that they use w=2*pi*f and vary f from 1 to 100e6 and plot the phase for several of those in between frequencies, like 1Hz, 2Hz, ...10Hz, ...100Hz, ...etc., up to 100MHz.

So it is not just ONE value for w, it is a complete range and that is what gives you the plot of the phase.

It is also interesting that over the last few decades the meaning of a "Bode" plot has changed. The guy who invented the technique intended to use straight line approximations to a frequency response, not an actual plot of say magnitude vs frequency which usually comes out a little curved, so that we could come up with quick estimates of the frequency response without actually having to calculate a whole bunch of points. It was an interesting achievement. People started to call the actual true frequency response the "Bode" plot too though over the years because we got software to do all the more detailed work, so the meaning got a little mixed up. I hope Bode (d 1982) is not turning over in his grave over this . I, for one, remember and admire his achievement and always keep it in mind when doing these problems.

Last edited: Aug 31, 2016
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5. ### joeyd999 AAC Fanatic!

Jun 6, 2011
3,963
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I'm not sure history is even taught anymore -- regardless of subject, including History.

6. ### MrAl AAC Fanatic!

Jun 17, 2014
5,188
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Hi,

Ha ha, yeah, sounds typical of today's classroom
In fact, do they teach anything anymore or just bark out facts

Note on blackboard in room 231:
"Students: History has been preempted by Social Studies. Go to room 122".

Note on blackboard in room 122:
"Students: Social Studies has been preempted by History. Go to room 231".

Note on wall of school gym:
"Students: Gym class has been dropped from the syllabus. Go to either room 231 or room 122".