FFT "extra signals"

[Glenn_United_Pumps]

Hi all

When I run the FFT of a signal I get a spike of approx 0.3-0.5 at 19.53Hz even when there is no input or the input is grounded... The main issue at hand is we will need to display the FFT analysis from about 5Hz to around 2KHz so I cant simply use the FFT filter to "kill off" that section of the spectrum. I have a feeling it might be something to do with the integral block as when I erase that and use the integral function in the FFT block the 19.53Hz signal jumps to approx 2...

Any ideas are appreciated I have attached the worksheet if anyone wants to have a go. It is designed to have an input from an accelerometer and display the overall value and spectrum of the vibration.

Thanks in advance to everyone your assistance is always appreciated

Glenn

[Glenn_United_Pumps]

Hi Everyone,

One more thing with the FFT analysis is on our old fft analyser we got "spikes" at the frequencies where vibration occured and with dasylab I (at this point in time) can only get triangles when set to the same settings (bandwidth and amplitude). I dont know if i am perhaps using the wrong display function or maybe I am missing a step. I am sure Dasylab should be able to do this task due to the fact it does the FFT analysis function.

Any ideas as always are appreciated.

Thanks to you all for your continued assistance

Glenn

ps: please refer to the attachments for a pictorial representation and the layout used if needed 🙂 "oldnew_and_layout.zip"

[Glenn_United_Pumps]

As for trying to get the output "spikey" I have done that easily with a generator within DasyLab supplying the signal but with an external input (ie accelerometer) the output goes to being very triangular again, I have checked the "external" signal and it is clean,  and producing one frequency only.

I compared this to the internal signal from DasyLab and it appears almost identical (so close the difference is negligible) As to why this is occurring I have absolutely no ideas. The problem I had earlier witht eh "ghost signal" at approx 20Hz I fixed by eliminating the integrator from the circuit and this seems to have fixed that but even with the integrator I stil got a triangular output (not spikey)?????

Any Ideas are always appreciated thanks again everyone

Glenn