Article: Measuring Frequency Response

Hi Tim,

Yummy, thanks.

Yes, your 'stupid mistake' was....

I don't know what the f*ck you think you're on about.

Now, is that my problem?

DNA

in article snipped-for-privacy@corp.supernews.com, Tim Wescott at snipped-for-privacy@wescottnospamdesign.com wrote on 03/25/2005 16:08:

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.

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.

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.

holy shit! have we caught another troll?

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

in article g201e.8099$ snipped-for-privacy@newsfe1-gui.ntli.net, Genome at ilike snipped-for-privacy@yahoo.co.uk wrote on 03/25/2005 17:13:

Hi,

you may want to consider adding a brief discussion about interpreting 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.

Mark

I think you may have a typo in your discussion of eq. 7. You refer to a ­"Pi/2 term" which I don't see.

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.

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

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.

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.

actually

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

DNA

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

Tim Wescott wrote:

Not top-posting in this reply...

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.

Great to share this stuff, Tim, but a link to printable format like pdf 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.

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.

[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 tried (and failed) to make an easily printable version for download.

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

You missed this thread a couple of weeks back about virtual print drivers (3 options):

tried to

haven't

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

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

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

Then, go to.....

and download a nice bit of the soft.

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

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