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Article: Measuring Frequency Response

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

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  1. Tim Wescott

    Tim Wescott Guest

  2. Chris Carlen

    Chris Carlen Guest

    Hi Tim,

    Yummy, thanks.

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

    NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.
  3. Genome

    Genome Guest

  4. 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. 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.
  6. Tim Wescott

    Tim Wescott Guest

    Note to self: make sure that it's clear that I'm doing swept-sine
    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. holy shit! have we caught another troll?

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

    Mark Guest


    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.

  9. Ken Smith

    Ken Smith Guest

    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.
  10. Jerry Avins

    Jerry Avins Guest

    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?

  11. Tim Wescott

    Tim Wescott Guest

    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.
  12. Tim Wescott

    Tim Wescott Guest

    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.
  13. Genome

    Genome Guest

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

  14. Guest


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

    Atmel ShangHai
  15. xray

    xray Guest

    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.
  16. xray

    xray Guest

    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.
  17. xray

    xray Guest

    [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.
  18. JeffM

    JeffM Guest

  19. Genome

    Genome Guest


    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.


    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.

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