# Article: Measuring Frequency Response

Discussion in 'Electronic Design' started by Tim Wescott, Mar 25, 2005.

2. ### Chris CarlenGuest

Hi Tim,

Yummy, thanks.

--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA

4. ### robert bristow-johnsonGuest

just a first impression, i can't tell from the body of the article, but it
appears in the code that you are doing swept frequency measurements. are
you, or am i reading the code wrong ("startF", "stopF").

if you are doing *linearly* swept frequency measurements and your sweep rate
is not slow enough, you might want to review:

it's just to show what the apparent frequency response is and then how to
correct for any deterministic error.

5. ### Eric JacobsenGuest

Tim,

I just did a quick read and it looks like you've covered the topic
pretty well. Not much I'd add, actually.

Well organized, stated pretty simply, well explained. I think you
could expect a red or blue ribbon at the science fair.

Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org

6. ### Tim WescottGuest

Note to self: make sure that it's clear that I'm doing swept-sine
measurements.
The sweep is exponential, and some extremely vague handwaving in there
about making it slow enough -- I should think about how to clarify that.

Generally the method as I use it, with the exponential sweep, moves
things slowly enough that the transient response doesn't cause a great
deal of difficulty -- particularly if you have the system operating in
closed-loop, which generally causes the transient to settle out much
faster than the initial sine wave.

7. ### robert bristow-johnsonGuest

holy shit! have we caught another troll?

(geez i hope not. my spray can of Troll-Away is almost empty.)

8. ### MarkGuest

Hi,

the results. For example the amount and location of peaking in the
closed loop response can give a rough idea of the loop bandwidth and
the stability margin ( From the closed loop peaking, can you tell the
gain and phase margin separately or not??). Also I'm sure you are
familiar with Venabale which uses some similar techniques.

http://www.venable.biz/

Mark

10. ### Ken SmithGuest

Figure 5 etc would be better if the Right hand scale showed the 45, 90 and
180 degree points directly.

The font on the equations is a bit small.

11. ### Jerry AvinsGuest

A nice discussion, Tim. You put it in terms of z-domain transfer
function, but surely s-domain would be at least as appropriate for most
of the paper. Is there a z-domain Bode plot?

Jerry

12. ### Tim WescottGuest

My bad, for crossposting to sci.electronics.offtopic. Genome actually
participates in that group with positive content, but he tends to
average it out with this kind of stuff.

13. ### Tim WescottGuest

It works for any stability problem that can be made into ratios of
polynomials with contiguous regions of stability -- so it works for z,
s, w = (z-1)/(z+1), etc.

I have a mission to make software engineers comfortable with control
theory, so I cast it entirely in terms of the z transform -- I should
probably note that it'll work in the s domain, but then I'd have to
explain how to do sampled-time measurements and convert them to
continuous-time conclusions.

When I'm doing design for discrete-time control I _always_ do the
analysis in the z domain, and if frequency response measurements are
appropriate I nearly always do them in the context of the controller,
rather than trying to make continuous-time measurements and translate.

Well, _I_ call it a Bode plot when I do it in the z domain. You can't
construct it with a pencil and a ruler like you can in the s domain, but
you interpret it exactly the same way so it seems appropriate.

14. ### GenomeGuest

I have a partially full suit with underware, you are naked.

DNA

15. ### Guest

Hi,Tim

Your article makes clear many things for me. I think it very good
except its long line which is bad for printing.

Thomas
Atmel ShangHai

16. ### xrayGuest

What kind of browser are you using? When I do a print preview with
Firefox, the page fills nicely but getting page breaks not to split
images is another issue.

or doc would be nice.

I know... looking the gift horse, etc. But if you could find a way to
present in a printable format that you like, even better.

Thanks for sharing either way.

17. ### xrayGuest

For what it's worth, I converted your pages into a doc file and tried to
use that with GhostView to create a pdf. Apparently my old veraion of
Word is inadequate. Seems the pictures were not imbedded and I haven't
found a setting to change that. Dang!

I tried.

18. ### xrayGuest

[snip stuff that doesn't matter]
Thanks, but this is a web page with figures interspersed. The web page
is fine but hard to print nicely. A formatted version with imbedded
images would be the solution. I tried to convert it but I was reporting
on my failure to generate one with free or available tools.

Maybe someone else has better success. Otherwise, the web page has all
the information, and I found a way to locally print the content. I just

Thanks, again, to Tim for writing and sharing the web page.

20. ### GenomeGuest

Burp!!!!!!!!!!!!!

If you are using MicroWank Intersnuff Expunger then go to....

http://www.wescottdesign.com/articles/FreqMeas/freq_meas.html

Click File, Save As, Web Page Complete and save it somewhere. I think
you might have done so.....

Then, go to.....

http://www.openoffice.org/

You might also need the Java runtime engine.....

http://java.com/en/

Having installed your balls then you will be able to do a File Open in
Open Office thing from the bit that Tim wrote and you saved as a
complete webfile.

Spukely.

Then you click on the PDF icon in the Open Office toolbar and it saves
it as a PDF file.......

And I have to say that the translation is Mother Beautiful!

If you don't want to go through such huge downloads..... I dunno, you
got ghostscript..... go the whole hog, then it's posted in ABSE....

Err, if it turns up.

DNA