How Do I Upsample and Downsample My Data?
This post covers how to upsample and downsample data and the possible pitfalls of this process. Before we cover the technical details let us first explain what we mean by upsample…
RMS From Time History And FFT Spectrum
The RMS of a time history is a measure of the signal's overall energy and is often used when extracting features from a signal for prognosis and trending of vibration…
What is Operating Deflection Shape (ODS) Analysis?
Operating Deflection Shape (ODS) analysis is a method used for visualisation of the vibration patterns of a machine or structure caused by unknown operating forces. This is different from the study of the…
What is waterfall frequency spacing? And how does the DATS parameter ‘Requested Frequency Spacing’ work?
Let us try to understand what waterfall frequency spacing is. Waterfall frequency spacing is the gap between spectral lines in an FFT plot.
For example, if you had an analysis frequency of 0Hz to 100Hz and 100 spectral lines, then Frequency Spacing is 1Hz.
So why is there a ‘Requested Frequency Spacing’ and an ‘Actual Frequency Spacing’? (more…)
What is “waterfall smearing”?
When analysing a waterfall or performing order analysis it is important to consider the frequency resolution or the frequency spacing.
There is often a desire to increase the resolution to finer and finer detail. But that is a process of diminishing returns, and actually fraught with danger. And that danger is waterfall smearing. (more…)
A Different Way Of Averaging – Time Synchronous Averaging
Assuming one wants a frequency spectrum from an acquired time measurement, it is generally accepted that averaging of a signal in the time domain is not very useful due to…
What Is A Hammer Test Or Hammer Impact Test?
This is often also known as Modal Testing. It is a method of testing that allows us to calculate the natural frequencies (modes), modal masses, modal damping ratios and mode shapes…
What is Resonance? (Part 2)
This article is a follow on from What Is Resonance? (Part 1) and answers some of the issues not covered in that post. How do you find the resonant frequency…
Wide Band Integrators – What Are They?
For some time now it has been conventional ‘wisdom’ that using time based digital integration may cause amplitude errors in the result and that these get worse as the frequency increases. As a result of this, integration using Omega arithmetic has been prevalent by using Fourier Transforms of the signal. This, of course, remains a valid approach and is particularly useful if the data is already in the frequency domain, which was its prime purpose.
Where Does The Noise In A Signal Come From?
When one thinks of noise in a signal, it is generally associated with having been added in some way to the amplitude of a signal. This is not always the…
Understanding Filter Characteristics
Recently when discussing with an engineering student the characteristics of filters, it became clear that some confusion exists around this subject area. This note attempts to explain the differences between types of filter and the effects of the parameters of those filters. (more…)
Figure 5: H1(f)
A Simple Frequency Response Function
The following article will attempt to explain the basic theory of the frequency response function (FRF). This basic theory will then be used to calculate the frequency response function between two points on a structure using an accelerometer to measure the response and a force gauge hammer to measure the excitation.
Fundamentally a FRF is a mathematical representation of the relationship between the input and the output of a system.
(more…)
