×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

"too many points" error - FFT

"too many points" error - FFT

(OP)
I have a rather large (110K points) set of acceleration data that I must analyze in Mathcad Plus 6.0.  I have managed everything up to plotting the FFT data in frequency domain.  I can plot with a simple n=1..32768 for the x axis, but if I try to use the frequency data (n=.004,.008..128) for the x-axis I get a "too many points" error.

Any thoughts?  Is .004 just too small to be on a log-scale x-axis?

Thanks in advance for the help!

Note: numbers rounded to protect the significant

RE: "too many points" error - FFT

I think that's the graphics engine having a whine. You are only trying to plot 4.5 decades. Try creating a new x axis with log(x) and see if that works.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: "too many points" error - FFT

(OP)
Hope I understood your suggestion correctly:

I set up a new variable d := log(.004), log(.008)..log(128) and attempted to plot with d on the x-axis.  I had no success plotting (on a standard scale); same "too many points" error.

Any other suggestions?

RE: "too many points" error - FFT

Could it be that you have 32768 points and the range n=.004,.008...128 has only 32000 steps?  log(.004),log(.008)...log(128) is only 7 steps.

RE: "too many points" error - FFT

(OP)
I'm showing the rounded versions here, actual numbers have that many data points.  Shouldn't matter either way though, MathCAD will plot the number of matching points that it has.

I'm confused on how "log(.004),log(.008)...log(128) is only 7 steps."  I get -2.398, -2.097, -1.796... 2.107.

RE: "too many points" error - FFT

I agree that it shouldn't matter.
I divided by the wrong number - should be 16 steps.

RE: "too many points" error - FFT

On the graph itself you can divide (or multiply) n to rescale the  x data points.  So if it plots OK with n:=1...32768, and [n] in the x placeholder, it will also plot with [n/256] in the x placeholder, but the xaxis will now be scaled by 256.

Another approach is to just plot two vectors.  I got in the habit of doing that when I used Axum for my plots.  In this case you would use your range variable (n) to make a vector with you FFT amplitude, and one with your frequency (for instance:  frequency[n-1 := n/256 (assuming ORIGIN=0)).  Then you could plot those two vectors (no subscripts).

Peter

RE: "too many points" error - FFT

(OP)
Thank you pstuckey!  A star for you!

I used the n/256 on the x-axis for now, but will look into using vectors in the future.

Now if I could just smooth the plots I'd be all set... but that's a new thread if I can't figure it out.

RE: "too many points" error - FFT

The plot function has a limit of 500,000 points

TTFN

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a partâ€™s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!