Vector volmeter question

[happybird]

Hellow!
Anyone here has the experience using Vector voltmeter to measure the phase shift output signal from a resonator,by this means,the nature frequency of resonator can be determined. my question is:can we use osilloscope to substitute the vector voltmeter by making some labview program to get the phase shift? Thanks!

[JohnM 64]

Hi happybird,

I do not have any experience with a Vector voltmeter, so I honestly am not sure what you mean by using an oscilloscope to substitute for it. I do know that you can use many different oscilloscopes with LabVIEW, and utilize all of the functionality of those scopes within LabVIEW. In fact, there are a variety of plug and play LabVIEW drivers for oscilloscopes that can be found on our Instrument Driver Network.

I hope this helps!

john m
Applications Engineer

[DFGray]

VI stands for Virtual Instrument. Of course you can make a vector voltmeter in LabVIEW using an oscilloscope, provided you have two simultaneously sampled inputs. Attached is a simple example using the NI-SCOPE driver. Conversion to VISA drivers should be easy, if you don't have an NI-SCOPE digitizer. It was deliberately written without using the NI-SCOPE measurement features (such as phase difference and signal magnitude) so that such conversion was possible. Let me know if you have problems.

[happybird]

Thanks for your nice example for vector voltmeter,but there's some subvi missing,so i can not run this program,is it possible that you can add all subvi? and have you use this program to do some measuement?  do you use "dual directional coupler" ?

Message Edited by happybird on 07-14-2005 10:29 PM

Message Edited by happybird on 07-14-2005 10:29 PM

[happybird]

Hellow!,I substitue some subvi for the osilloscope i am using,the program can read the phase now,but how can i do to show the reference waveform(the channel 1) and the actual waveform(channel 2) simultaneously?,i only can get one waveform data now.

The other problem is why  the phase information on the vi is not stable? it is shifting.

Thanks a lot! 

Message Edited by happybird on 07-15-2005 11:29 AM

[Dennis_Knutson]

You're replaced the array that has the channel numbers with a scalar that only has one channel. Puts the channel numbers back into an array and input that into the for loop. Make sure you have autoindexing of this array input enabled.

[happybird]

Thanks! i have done it and i can read two channel 's signal,but i see the phase reading is always shifting,can you give me any suggestion about it?

[Dennis_Knutson]

I'm sorry, no I don't think I can help much since I don't have your instrument. How much is the phase reading varying? Since you're doing a new acquisition with each itration, that might account for some small variations. If that's the case, you could try averaging the readings.

[happybird]

yeah,you are right,i take a new measurement each time.So generally speaking,is it a right way to do measurement? because if i take a single measurement,the signal will vary each time when i take measurement.  Thanks!

[DFGray]

My apologies for the slow reply.  I have been on vacation.

The analysis function I used works best with at least ten cycles of the waveform.  If you don't have that much data, it could introduce inaccuracies.  If this is your problem, try increasing your number of data points (if you can).  Averaging is also an option.

Whether you take a measurement each time and display it or average several measurements and display the result depends upon what you are trying to accomplish.  There is no right or wrong general method.  If you don't mind the jitter in your display (which, as you mention, may be real), use single measurements.  If you do mind, average.  If you average, I would strongly suggest that you at least show some statistics (max, min, standard deviation) as well so you don't hide something you really should know.  You can also do both (raw data and average data with statistics).

Have fun.