Envelope Detection - Band specification question

[ebloohm]

Hello,

I'm using the Envelope Detection vi from the Sound and Vibration Meas Suite but I'm having trouble determining values for the Band Specification.

I understand that one value is the center freq and the other specifies the span on either side of this center freq.

The waveform I'm feeding to this function has a resolution of 5000 S/s so I assumed my span can't exceed the max frequency content of my waveform.

If I enter a center freq of greater than 2500Hz I get an error stating that the value is invalid.  This makes sense.

However, with a center freq of 2500Hz my span can only be as high as 1250Hz. If I enter 1251Hz or higher I get the following error:

Error -20003 occurred at SoundVib_Resampling.lvlib:OAT Single Stage Decimation with IC.vi in
SoundVib_Resampling.lvlib:oa_Decimation with CIC and Compensation Filter.vi:13 in
SoundVib_FaultAnalysis.lvlib:oa_Shift and Low Pass Filter (1 Channel).vi:3 in
SoundVib_FaultAnalysis.lvlib:OAT Envelope Detection (Waveform Output, 1 Channel).vi:1 in
VibrationAnalysis_2.vi

Possible reason(s):

Analysis:  The number of samples must be > 0.

Can someone explain to me what these band specification values are for and what their limits are?

Thank you.

[ebloohm]

Nobody out there with experience in the S&V Meas. Suite?

[F._Schubert]

You are aware, that the nyquist theorem states that the highest bandwidth you can measure is 1/2 of the sampling frequency? So the 2.5 kHz limit has nothing to do with the span. And also a span of 1.25 kHz (assuming it is in both directions) is the 2.5 kHz limit.

But I never worked much in this domain, so I'm of no further help.

Felix

[ebloohm]

Thank you.  I didn't see that.

Yet, if I have a center freq at 2500 and the span at 1250, that would seem to give me a total span from 1250 to 3750 (past the 1/2 sampling rate).

This still leaves me wondering what I should be setting these values at.

If anyone can explain (or direct me to a resource) I'd appreciate it.

Thanks.

[F._Schubert]

Because it is the bandwith that is limited, not the frequency (as normally thaught). This is the basis behind the concept of undersampling (where your sampling frequency is below the carrirer signal, but the signal bandwith is inside the 1/2 nyquist criteria). But this is the limit of my knownledge.

Felix

[HeatherBaker]

Center frequency will need to be a simple calculation performed by you and frequency span defines the total amount of the frequency spectrum captured.  The Frequency Control section of this article is a good read as well.