Gibbs phenomenon issue in square wave from labview builtin function generator

[BarryGRAEllc]

I am not able to look it up right now, but I have been thru this exact problem before.

Instead I can give some general info as to what is causing it. When you have a gentle roll-off of 6-12dB/oct you will see either a nice square wave or a rounded leading edge square wave depending on square wave Freq and the filter Freq. But if you ever "knock out" completely a frequency component (or several) you will get the results your are seeing.

Pick out a filter an octave below your brickwall filter for a start. You will have to trade slope accuarcy for flat top accuracy.

Barry / GRAE LLC / LLNL

PS: I should have said a low-pass filter in LabView.

[ritch_by_nature]

Sorry about the typos in my previous post. but I have tried many filter so far but nothing has worked yet.

[ritch_by_nature]

I am kinda lost when you mentioned ocatve, please clarify because im not dealing with sound or vibration. I just applied a bessel low pass filter with a 5th order though the ripples on the edges are still there but they have been reduced

[Bob_Schor]

An Octave is a factor of 2 in frequency.

BS

[ritch_by_nature]

I came to find out that the problem is not in the signal at all. I connect the signal coming from the NI 9263 to a scope. And there's no ringing at all. Everything come out perfect. But it's still showing at the acquiring end at the NI 9239. I tried using capacitors across the signal lines (wires) but that didn't make a difference. I tried using a 10k resistor at the + and - wires of the signal at the input of NI 9239, still nothing.

[ben64]

There is filtering in the 9239, I recommend you read the "Understanding NI 9229/9239 Filtering" section of the user manual. I would try to increase the sampling rate (that correspond to the Data Rate in the following picture, also look at the Understanding Data Rates section of the manual) to include more of the signal impair harmonics (the frequency components) in the passband.

Ben64

[Kevin_Price · Accepted solution]

I don't know details about your cDAQ device, but the "pre-ringing" characteristic of the acquired signal is something I've known to be aware of with DSA (dynamic signal acquisition) devices.   That's the key characteristic barry was referring to also.  Here's a knowledgebase article about it.   Perhaps your cDAQ device has a similar internal bandwith-limiting digital filter?

-Kevin P

[Bob_Schor]

Son-of-a-gun, it really is a Gibbs phenomenon!  I didn't know this "hidden gotcha" about Sigma-Delta devices (I've mostly dealt with "ordinary" A/D, D/A, and DIO stuff).

BS

[Bob_Schor]

Ritch,

     I recommend tagging Kevin's response as the "solution", as it contains a reference to a very cogent explanation that a number of us ("experts"?) missed.

Bob Schor