Phase Difference Measurement by means of Digital Sampling

[lucien85]

A software is to be developed with Lab-View to record and compare two voltage signals simultaneously. The software shall be of the function to determine the time difference at the zero-crosses of the two signals with confirmed errors.
The analyzed results will be interpreted as the phase differences to do phase calibrations.

I am using 'Clark-Hess Model 5500-2 Digital phase standard' which generates 2 analog sine waves of the same frequency, connected to a PC with Labview via GPIB. http://www.clarke-hess.com/5500.html

I am new to Labview and thus may i know:

1)How do i generate the waveform of the 2 sine waves concurrently on Labview from the hardwave mentioned above? The instrument driver is not available and thus the driver samples are not available to me. I had tried the DAQ assistant(through acquire signals>sine wave>voltage) but it fails to detect any instrument.

2)What sampling rate & resolution of the analog to digital converter hardware do i need? (I read somewhere and it vaguely says that it more practical to have 10x sampling rate of the signal measured, instead of 2x by theory.) Or, are there better/cheaper ways to digitize the 2 sine waves? Model 5500-2 has got a frequency range of 1hz-200khz.

3)Using Labview, how do i perform linear regressions at the zero crossings of the digital signals to measure phase difference? Do i need to incorperate 3rd party programs like Matlab to do this?

4)I read about using FFT for phase measurement, thus what others methods can i use in labview to measure phase difference of the 2 digital signals?

[lucien85]

Hi, anyone can help please 😄 thanks !

[johnsold]

lucien85,

This sounds like a school assignment.  If it is, please tell us.  We will not do your homework for you, although we are glad to help you learn about LV.

1. Do you need to control the generator from software?  The data sheet indicates that it has a GPIB interface.  Presumably the manuals which come with the instrument specify the commands.  You need VISA, not the DAQ Assistant to communicate with the GPIB instrument.  You also need a GPIB device of the computer.  Modules are available which plug into PCI slots, coneect via SUB or Ethernet. Do you have one?

1.a. The requirements seem to indicate that you need to measure the zero crossings of the two sine waves.  You will not get that from the GPIB bus.  What kind of data acquisition device do you have?  Since you are looking at sine waves, you will need an analog input device with at least two channels.

2. The Nyquist criterion says that you can completely reconstruct a signal if it is sampled at a rate higher than twice the highest frequency component in the signal. So the minimum sampling frequency is just more than twice the signal frequency.  However, to get phase information you may need a higher sampling rate.  What is the minimum phase difference you need to measure? Does it need to work over the entire fequency range of the generator or only on a subset?  You can calculate the sampling rate based on how many samples you need in each cycle of the sine wave to resolve the phase difference. 

3. LV has linear regression functions.  Search the palettes and look at the help files.  You should be able to meet the requirements with just LV and the appropriate drivers for your instrument and DAQ device.

4. Direct measurement of the phase of signal two with respect to signal one should be rather straightforward.  Also, look at trigonometry: What is the product of two sines of equal frequency and different phase?  Implement the necessary algotrithm in LV.

Lynn

[lucien85]

Thanks for yr help Lynn.

I would like to displayed the 2 digitally sampled voltage waveforms simultaneously, by which are generated from my clarke-hess

model 5500-2 phase standard.

My objective is: To measure phase difference through means of digital sampling. A software is to be developed with Lab-View to record and compare two voltage signals simultaneously. The software shall be of the function to determine the time difference at the zero-crosses of the two signals with confirmed errors. The analyzed results will be interpreted as the phase differences to do phase calibrations.

1)May i know how do i display the 2 digitally sampled voltage waveforms on Labview? i have read about the available NI high-speed digitizers and thus i would like to know the physical setup of the the digitizers and whether if there are better alternatives to achieve my objective. The 2 voltage signals have got to be acquired at the same time so that there wouldnt be any phase errors between the 2 signals.

2)After acquiring the 2 voltage waveforms on Labview, i will apply various methods to measure the phase difference between the 2 voltage waveform through FFT,linear regressions at zero-crossing, sine wave fit methods etc. Thus i would like to know whether if labview is able to perform such methods and measure the phase difference from the waveforms acquired, or do i need 3rd party programs for more complex algorithms?

[johnsold]

lucien85,

1). To display the data just wire the waveform or array to a Graph indicator.  Askign a question like this suggests that you need to start by working through the on-line tutorials for LV.  They will help you get through basic issues like this.

1)a. You will need to read the specifications of the digitzers and any other devices you consider to see if they meet your requirements.  If you want me to do it all for you, you can hire me.

Two types of multichannel digitizers/analog to digitial converters are commonly available.  The most common type, which is almost exlcusively used in less expensive devices, is the multiplexing type.  These have just one A/D converter and swicth the input channels to the converters.  Simultaneous sampling devices usually have an A/D converter per channel, although other desings are possible. It is easier to get precise phase measurements from a simultaneous sampling device.  It is possible to correct for the sample timing skew in multiplexing devices by analyzing the timng logic and calibration.  Which is more appropriate for your application depends on many details.

2) LabVIEW can perform all the analyses you mention.  It is also possible to implement highly complex and custom algorithms by building on the available built in functions.

Lynn

[lucien85]

Yes, i would prefer the ADC per channel architecture than the multiplexed. My objective is to capture the 2 waveforms simultaneously from 2 analog outputs and display on Labview for precise phase measurements through digital sampling means.

However, prior to that, i would like to double check with you guys what are all of the hardwares/connections that i need before i make any purchases.

Other than a digitizer capable of simultaneous sampling, what other hardwares do i need? (I am aware of the resolution,sampling rate, bandwith etc that i need, thus these specs are not an issue.)

Does Simultaneous Sampling Multifunction DAQ devices serve the same function as digitizers except that they are capable of taking in more than 2 channels? If so, are high speed digitizers/oscilloscope sufficient to fullfill my objective?

If not, can anyone recommend any models/categories?

[johnsold]

The only thing which really matters for the zero-crossing methods is the sample rate. You also mentioned other techniques, such as Fourier transform methods.  The amplitude resolution required for these techniques will depend on the algorithms as well as the sampling rate.

You need to decide what you want to do before you can specify the sampling device.

Lynn

[lucien85]

My 1st priority is using the zero crossing method, others are not my main priority.

[johnsold]

What phase resolution do you need?

What will the highest signal frequency be?

Lynn

[lucien85]

0.001 degrees & 100khz