Generating and Acquiring Signal in Signal Express

[AshishMaharjan]

Hi,

I am just beginning to use Signal Express. I was trying to do a simple signal generation and acquisition with PXI 6552.

I created a digital signal and generated it in CH0 but I probed CH0 and couldn't see the signal I generated in the oscilloscope.

But when I used Stimulus/Response, I could see the generated signal when I proed the CH0 with oscilloscope.

Then I used oscilloscope to generate a simple square waveform and connected with CH1. Then I used Acquire Signal expecting to see the square waveform but I didn't.

Is there something basic that I am missing here!!

[Coal_Man]

Hello Ashish,

What settings are you using to generate your signal in the HSDIO Generate step?  You may want to double check the signal that you are generating and how you are passing this to the generation block.  Are you generating directly from an HWS file, or are you creating the signal from another SignalExpress block and then passing it in as a shared signal?

As for the second question about not seeing the square waveform, double check the voltage levels you have set for the acquisition and make sure that these match up to the voltage level of the signal you are generating with your external device.  It could be that the 6552 is seeing the signal, but the limits for detecting high and low are out of range.  This only really applies if you are seeing no signal at all.  If you are seeing a signal, but it is not the square wave you are providing to the 6552, it is most likely that the sampling rate and the rate of the square wave are not matched properly.  If you are sampling too fast, you will see multiple high or low samples for a single high or low state.  If you sample too slow, you will most likely not see a proper pattern either.  In both cases, if you are not synchronizing the signals with a shared clock or at the very least, starting the acquisition an appropriate time after the transition of the signal being generated, then you will probably sample at a time that the generated signal is transitioning.  Thus, you will be sampling at a time where the signal is between low and high, giving you an unpredictable result. 

Finally, you will want to check the leads for the oscilloscope to make sure you are matching the impedance properly.  The lines should be 50 ohms.  Without matching the impedance of the lead to the device, you will see the signals reflected back and forth through the line, causing your acquisition to be invalid.  Also, if you are taking measurements with the oscilloscope in the middle of the line, you will not be seeing the signal as the 6552 will be, as there are multiple signals in the line when you work at such high speeds.  There is a lot of information about this in the NI Digital Waveform Generator/Analyzer Help under the Termination section.

I hope this helps.