Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • 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.

Students Click Here

Autopower Crosspower nomnclature

Autopower Crosspower nomnclature

Autopower Crosspower nomnclature

So we are all happy with the terms Gxx, Gyy, Gxy in our signal processing world. But does anyone know where the letter G came from? I was trying to explain these things to a colleague today, but all I could point to were definitions of Gxx,Gxy,Gyy. Not why we use the letter G


RE: Autopower Crosspower nomnclature

You're dealing with matices, the various subscripts refer to Fourier transforms of cross correlations of different signals.

RE: Autopower Crosspower nomnclature

As with many things, there's history and tradition. Oft times, it's simply the nomenclature used by a seminal paper or book, and rather than introducing your own nomenclature and then have to explain that zzXX is the same as Gxx in "the" text, it's easier to use the same notation.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Autopower Crosspower nomnclature

I notice some people use C for cross power (what do they use for coherence?) and others use S. I've definitely seen A for autospectrum. Didn't there used to be an amazing web based maths website? Did it get eaten by Wolfram?


Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Autopower Crosspower nomnclature

Never give up, your just starting.

The HP doc refers only to single input point monitoring of simple systems.

When you have complex systems with multiple sensors, you'll likely need to identify the various interactions even with single point excitation.

With multi-point excitations, it gets more interesting.

The Gij's are matrix elements, or in Einstin notation

Yi = Gij Fj for i={x,y,z}, j={x,y,z}, where Yi is the response, Fj excitations

meaning Yx = Gxx Fx + Gxy Fy + Gzz Fz and so on

RE: Autopower Crosspower nomnclature

Yes hacksaw. All of what you say is true and completely familiar (apart from the bit about the HP doc only dealing with single inputs, which is not true). I only asked if anyone knew why G was the letter universally used in the notation.

I wondered if there was a mathematician being honoured or some Greek influence. Like this nice explanation of why we always use S for entropy, but never question it.


RE: Autopower Crosspower nomnclature

I've only ever done signal analysis at the application level, mostly using the B&K green book as a reference. I did one back derive coherence to see what was going on as I has a boss who seemed to regard it as the be all and end all of QC for transfer functions measurements... and then some years later found exactly the same explanation in one of the books, probably B&K.

So I haven't got any feel for the history of it.


Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Autopower Crosspower nomnclature


I see some online resources stating that H1(f)=Gxy(f)/Gxx(f) and others that H1(f)=Gyx(f)/Gxx(f). The former being more common. The latter (IMO) giving the phase relationship I'd expect (y, a pure delayed version of x causes a decreasing phase ramp in H1).


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.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


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
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’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:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close