Synchronous multi-channel acquisition USB-6259 (phase measurement)

[Daniel@RWTH]

Hi!

I want to create a multi-frequency-signal (1k to 20kHz) in SignalExpress, generate it with NI USB-6259 BNC and measure it with the same device after the signal has passed a DUT whose phase response I want to determinate.

At the moment I am trying this: I have connected the analog output via a Y-coax (length 1 metre) to TWO analog input channels.

Since the input channels were grouped with the Add Channel button, the data acquisition should take place nearly synchronously.

However, sometimes the phase response is zero (as expected cause the two signals should be equal), but sometimes it "jumps" (especially when I execute the project anew) and decreases or increases linearly over the frequency (so there is a time difference between the two measured signals).

I don't think that the settling time is the problem here, because referring to the manual it's just about a few µseconds and I don't even change the input voltage range between the input-channels. By the way the magnitude-response is fine.

Yet I didn't accomplish to sync the input channels with the output channels either, but first of all I would appreciate a solution to the input-input-synchronization (I don't mind if it's realized in LabView). 

Thanks in anticipation, Daniel

[Marco_Brauner · Accepted solution]

Hello Daniel,

the M-Series DAQ -Devices are utilizing a switch in order to sample multiple channels. So you got to take the switching time into consideration when 

you are doing measurements like Phase-difference of two signals. 

I took your Signal Express Project as well as created a LabVIEW VI to double check, and you can see exactly the same offset between the two 

sampled signals. If you want to measure true phase differences, you got to use an simultaneous sampling device like the S-Series or DSA devices (there are a few more others).

regards

MArco Brauner NIG

[Daniel@RWTH]

Hi Marco!

Thank you very much for your reply, you have saved me much time!

However it's kind of weird that in the M-series manual I could only find a specification for the settling time of the "NI-PGIA" that apparently isn't the problem here (input ranges are equal), and not for the time it takes for the pure switch from one channel to another.

Many Greetings, Daniel