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’?

First, all block sizes are always a power of 2; 1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384, 32768 and so on.

In DATS the user enters the desired waterfall frequency spacing (called Requested Frequency Spacing).

For example, if the user entered a value of 2Hz as the requested frequency spacing, and assuming the signal has a 20kHz sample rate, then the formula to find frequency spacing is,

Sample Rate / Resolution = FFT Block size

So

20,000Hz / 2Hz = 10,000

Therefore we would use an FFT block size of 10,000. But this is not possible as FFT’s use a block size that is a powers of 2. Therefore, DATS automatically selects the next highest block size. In this case 16,384Hz. So the Frequency Spacing is not actually 2Hz, but better resolution,

Rearranging the formula above gives us

Sample Rate / FFT Block size = Resolution

So

20,000Hz / 16,384 = 1.220703125Hz

Therefore the Actual Resolution is 1.22Hz

Please also note,

The sample rate has to be at least as big as the block size or higher.

 

 

 

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. Contine reading

Is it important to consider the cable resistance when using strain gauges?

The cable resistance can be a significant factor in the accuracy of the overall measurement. It is very important indeed to consider this point.

Prosig software allows the cable resistance to be entered by the user. Once this resistance has been entered, any calculations or calibrations carried out will be based on the known values of the bridge resistors as well as the resistances of all of the cables.

In most cases this will be negligible except if long cables are used.

When should I use the ground connection on my P8000 data acquisition system?

The ground connection should be used if an issue has been observed in measured data, such as significant electricity supply (50Hz or 60Hz) distortion or significant noise.

It must be attached it to a very good earth point (like a steel pipe or foundation)

It should be noted that sometimes connecting the ground cable to earth can actually make matters worse. It depends on the situation.