Aamplitudes by NI-SCOPE Express and NI-SCOPE Example, why they are different?

[rockymountai]

Hello, I am quite new to NI-SCOPE module. Fow now I am doing some FFT analysis to get power spectrum of a voltage signal, which is generated by a Tektronix signal generator. Amplitude of 0(higher level) and -5 volts (low level), frequency is 25 MHz. DAQ card is NI USB 5133.

I used an example by NI-SCOPE called "niScope EX Windowing.vi" and my own program which has just a "NI-SCOPE Express", a "Spectrum Measurement" express VI and a graph indicator.The amplitudes are quite different. I deliberately use 0 volts for high level and -5 for low level. The niScope EX Windowing.vi gives the right amplitude while the NI-SCOPE Express doesn't at all.

Would you guys let me know what is wrong in my program please? I really appreciate that.

[dansch]

Hi Rockymountai,

Without seeing your block diagrams or the configuration of the NI-SCOPE Express VI, it is hard to tell what settings might be different. What voltage ranges are you setting? Keep in mind that these are peak-peak values so a range of '5' actually means that you wish to acquire between +2.5 V and -2.5 V. If the signal goes beyond those values, it will be clipped to the upper or lower threshold. Also, do you have both programs configured for 1M Ohm input impedance, DC coupling, etc.? These settings can all change the way your signal appears on the graph when you sample it. 

For the most part, the signals in each of the programs you showed look different. One signal goes from +5V to -5V and appears as a normal, periodic signal (sine or triangle wave) of 25 MHz. The other signal in the Windowing example appears to be 25 MHz, but does not have the same shape. My opinion is that perhaps that signal is DC coupled and is actually going from 0V to -10V, but the sample that would be at -10V is actually being clipped to the lower threshold, which is just below -5V (assuming you have set a voltage range of '10'). In your Express VI program, the signal appears to be AC coupled or something, so it is centered at 0 instead of -5V, and therefore it may not be clipped. This is just an idea, and hopefully it gives you a place to begin your troubleshooting, but as I mentioned before, it is hard to tell without knowing all the configuration that you have applied in each program. Best of luck, and I hope this helps!

Message Edited by dansch on 06-17-2010 01:59 PM

[rockymountai]

Hello, Daniel 🙂 thank you for your attention.

Probably we can just focus on my own program, the settings of the NI-SCOPE Express are DC coupling, 100MS/s, Recod Length of 500, Range of 20 V, Input Impedance of 1 Mohm (by 5133 specifications), Acquire "N Samples". What I observed are that

(1), the acquired amplitude is always two times of what set on the function generator,

(2), the waveform doesn't respond to offset even though I used the DC coupling,

 Seems like there is only one thing I can do is to check the function generator 😞

[rockymountai]

I checked again those phenomena. Like if I set the peak to peak to 5 V, then waveform will be from -5 to 5 V, which has two times the amplitudes. And no matter how I change the offset, the waveform is always at the center.

[DFGray]

Most function generators have an output impedance of 50Ω.  With your input impedance of 1MΩ, this will give you twice the voltage you expect on your input. Change your input impedance to 50Ω on the scope and you will probably get what you expect.

[rockymountai]

Yes, you are right. I changed the oscilloscope's impediance to 50 Ohm and the waveforms are identical to function generator's.

My own program (which uses NI-SCOPE Express) doesn't show correct at all. Even I click the "Autoset", set "50 Ohm" and click "OK", it still shows "1 M Ohm" agina once I open it. BTW, NI-SCOPE Soft Front Panel doesn't show the correct results as well.

Thank you for reminding me that, I really appreciate that!

[rockymountai]

Yes, you are right. I changed the oscilloscope's impediance to 50 Ohm and the waveforms are identical to function generator's.

My own program (which uses NI-SCOPE Express) doesn't show correct at all. Even I click the "Autoset", set "50 Ohm" and click "OK", it still shows "1 M Ohm" agina once I open it. BTW, NI-SCOPE Soft Front Panel doesn't show the correct results as well.

Thank you for reminding me that, I really appreciate that!

[DFGray]

The SFP also defaults to 1MΩ input impedance. You can change it in the channel properties.

[rockymountai]

Hello, Would you let me know how to change channel property please? I went through the SFP but didn't figure out yet 😞

[DFGray]

Select Edit»Device Configuration or just press Ctrl–D.  Select the Channel tab.  Select your channel of interest and change the impedance.  Note that this setting will persist from use to use of the SFP until you change it.