Navigation:  Tutorials > Measurement tutorials > Strain measurement > DEWESoft 7.0  data acquisition, processing, analyzing and storage software Quarter bridge measurements

Now let's do some more measurements. Let's take a look at recorder. If a static force is applied on the tuning fork, we can see the changing offset of the signal. We might also try hitting the tuning fork so it makes a sound to reflect the natural frequency of the tuning fork (the conventional way it is used). We can see this as a high frequency vibration with falling amplitude because of air friction and friction in the fork.

 

When looking at the FFT screen (let's change it to the logarithmic scale that can be used to see all the amplitudes in a nice way, we can see that there is an obvious peak at approximately 440 Hz. We can also place a cursor at this point by simply clicking on the peak in the FFT. The frequency shown is 438.8 Hz . It is not exactly 440 because the tuning fork used here is surely isn’t is what it should be after many years of use, but also because the FFT has a certain line resolution.

This line resolution depends on the sampling rate and the number of lines chosen for the FFT. If we want to have a faster response on the FFT, we would choose fewer lines, but we would have a lower frequency resolution. If the user wants to see the exact frequency, it is necessary to set a higher line resolution. This is well described in reference guide, but a simple rule of thumb is: if it takes 1 second to acquire the data from which the FFT is calculated, the resulting FFT will have 1 Hz line resolution. If we acquire data for 2 seconds, line resolution will be 0.5 Hz.

 

 

This is also a perfect example to take a look how to use the filters in DEWESoft. Clearly, there is one part of the signal in the form of the offset (static load) and one part in a form of dynamic ringing with a 440 Hz frequency.

If we want to extract those two components from the original waveform, we need to set two filters - one low pass and the other high pass. So we add two filters in the Math section.

 

 

 

 

If we show those two filters on the recorder, we can see that the signal is nicely decomposed to the static load and dynamic ringing. The user can use this technology to cut off unwanted parts of the signal or to extract wanted frequency components of the certain signal.

 

At this point it might be worth noting that IIR filters are used where we want higher calculation speeds and cutoff rates are needed. We can also use FIR filters if we don't want to have any phase shifts. More details can be found in the Filter comparison section of FIR filter in the user's manual.