By Dr Colin Mercer, Technical Director, Prosig
Accurate measurement of a signal depends on the dynamic range and the overall level of the data acquisition system. The overall level setting may be thought of as determining the largest signal that can be measured. This clearly depends on the present gain setting. That is the overall level is related to the gain. Clearly if the overall level is too small (gain too high) then the signal will be clipped and we will have poor quality data. The dynamic range then tells us that for the given overall level what is the smallest signal we can measure accurately whilst simultaneously measuring the large signal.
In a very simple sense suppose we have an artificial signal which consists of a sinewave at a large amplitude A for the first half and that this is followed by a sinewave with a small amplitude a for the second half. We will set the gain (the overall level) to allow the best measurement of the A sinewave. The dynamic range tells us how small a may be so we can also measure that without changing settings.
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By Dr Colin Mercer, Technical Director, Prosig
It is sometimes necessary to pass a signal through a high pass filter to eliminate low frequency signals. These may arise for instance from whole body vibrations when perhaps our interest is in higher frequency components from a substructure such as an engine or gearbox mounting. The vibration levels are speed sensitive and the usual scheme is to record a once per revolution ‘tacho’ signal with the vibration data. The tacho signal, which ideally is a nice regular pulse train, is processed to find rotational speed and hence to select which part of the vibration signal is to be frequency analysed. The most common form of analysis is a waterfall type such as shown below. read »»»
By Dr Colin Mercer, Technical Director, Prosig
In many real-world applications it is impossible to avoid “spikes” or “dropouts” in data that we record. Many people assume that these only cause problems with their data if they become obvious. This is not always the case. Consider the following two time histories. read »»»
By Dr Colin Mercer, Technical Director, Prosig
Not all systems vary linearly. One very well known case is, of course, thermocouples. International standard curves are available for these so they present little difficulty. The issue discussed here is determining a calibration curve and if appropriate reducing to a polynomial. read »»»
By Dr Colin Mercer, Technical Director, Prosig
Differentiation and integration are often used to compute between accelerations, velocities and displacements. These relatively straightforward mathematical operations may cause surprises when carried out numerically. The available frequency range over which reliable results are obtained is surprisingly small. This brief note illustrates the problems which may occur and indicates techniques to resolve the problems. read »»»
By Dr Colin Mercer, Technical Director, Prosig
Some devices, particularly digital tape recorders, apply A-weighting to all their data in order to achieve acceptable data compression. This is fine unless you want to analyse the unweighted data or apply a different weighting factor. Using DATS it is a simple task to instruct the WEIGHT module to either simply unweight the data or remove one weighting factor and apply another. read »»»
By Dr Colin Mercer, Technical Director, Prosig
To illustrate the use of the cross correlation function, a source location example is shown below. For this it is assumed there is a noise source at some unknown position between 2 microphones. A cross correlation technique and a transfer function like approach were used to determine the location. read »»»
By Dr Colin Mercer, Technical Director, Prosig
Sometimes we have digitised data at a much higher rate than we need. How can we reduce the sampling rate? If I wanted to say halve the sample rate can I just throw away every other data point?
The answer is NO, except in pathological conditions where you know that there is no frequency content above the new Nyquist frequency. read »»»
By Dr Colin Mercer, Technical Director, Prosig
The PROB module in DATS for Windows provides, amongst other options, a probability density analysis. Also, the signal generation suite has a module, GENPRB, which generates a classical Gaussian probability density curve (and others). How then may these be used to compare the probability density of our measured signal with that of a true Gaussian one. The method is quite straight forward and is a matter of scaling. read »»»
By Dr Colin Mercer, Technical Director, Prosig
In many instances we need to filter a signal to remove unwanted frequencies. If we use classical filters such as Butterworth, Chebyshev or even Bessel then a phase delay is introduced. This phase delay is itself a function of frequency so that the signal content at one frequency is delayed a different amount to that at another frequency. Why does this matter? read »»»
By Dr Colin Mercer, Technical Director, Prosig
For various reasons data captured in the real world often contains spikes that will give erroneous results when analysed. DATS for Windows provides various ways of editing and removing these anomalies. Let us consider a real life case history. read »»»
