Increase sample frequency of the Amplitude and Level measurements vi

[kimbo mu]

I used Amplitude and Level measurements vi to measure the Vpp ( peak to peak value) of an AC acquired from a DAQ system. The Vpp changed at a certain frequency. When the change frequency is very high, the Amplitude and Level measurements vi can not measure the changing Vpp and provide a changeless Vpp.  The attached figure is a result at low frequency. The lower graph is the original 16 kHz AC and the upper chart is the Vpp measured from the Amplitude and Level measurements vi.

Is there anybody know how to increase sample frequency of the Amplitude and Level measurements vi to solve this problem ? Or if you have another way solve it, please kindly reply me. Thank you very much in advance !!

Message Edited by kimbo mu on 10-17-2008 10:18 PM

Message Edited by kimbo mu on 10-17-2008 10:20 PM

[Dennis_Knutson]

I don't see anyplace on that front panel where the amplitude and level measurement is being displayed. All that you have is a small segment of a waveform. Since the Amplitude and Level function does not return a waveform, I have no idea what the top chart is supposed to be or where you got it.

[kimbo mu]

I used a waveform chart(top chart) to display the Vpp obtained from the Amplitude and Level function. Since the Vpp changes at certain frequency,  it look like a square signal. Attached is the block diagram which you can see the Amplitude and Level function.

[Ipshita_C]

Hi Kimbo,

Please specify what you mean by "the Vpp changed at a certain frequency". Are you measuring an AC voltage that varies in frequency over time? If yes, then does it vary at fixed intervals? The Amplitude and Level Measurements VI that you are using is an Express VI whiich means that it is like a built-in sub-VI in LabVIEW. You can access the code for it by right-clicking on it and selecting Open Front Panel and then choosing Convert when the dialog box appears. Howver, unless we know what 'the changing frequency' means we cannot know which value to change in the code. 

Ipshita C.

[kimbo mu]

Hi Ipshita C.

Yes, the AC voltage varies in frequency over time. Attached is the chart of original signal obtained from DAQ system(top chart) and the peak to peak value measured from Amplitude and Level Measurements VI( lower chart).  If you still do not understand what I mean, please feel free to tell me. Thanks.

[Kevin_S]

Hi kimbo mu,

I see that you're using the Amplitude and Level Measurements Express VI.  That's great for immediate results.  It's not so great when the default options selected are not what you want.

I would recommend that you use the Amplitude and Levels.vi.  This is the non-Express VI version.  Note that the State Settings input allows you to specify the histogram size (the number of bins in the histogram that LabVIEW uses to determine the high and low state levels of the waveform).  See the LabVIEW Help for more information on this.

Enjoy,

Kevin S.

Applications Engineer

National Intruments