05-15-2012 09:25 AM
I want to use the PXI-5404 to generate a frequency sweep and am executing the example found here: http://zone.ni.com/devzone/cda/epd/p/id/3820
Frequency sweeping works well for the SINE output, but the voltage levels are much lower than expected. The SINE output for CH 0 is at a maximum ±30 mV, and the CLKOUT output for CH 0 is a DC signal at whichever high voltage I specify.
05-16-2012 02:54 PM
Remember that the amplitude you choose for the sine wave corresponds to the peak-to-peak voltage, so at 30 mV your signal will range from -15 mV to 15 mV. Also, these values are assuming termination to a 50 Ohm load. The Channel 0 Clock Output can be set to 1.8, 3.3, or 5 V assuming you have a high impedance load. If this is not the cause of the discrepancy, can you please describe your setup in more detail? What are you connecting the output to, and how you measuring the output?
05-16-2012 04:54 PM
The NI Signal Generators Help specifies that a square wave is generated on the CLOCK Out line and that the voltage settings are assuming high imedance termination. Therefore, I'm guessing right now that your Ch. 0 scope input (the black line) is simply acting as a floating input, and that the Ch. 1 (red line) is showing a highly reflected, inverted square wave from CLOCK Out. Try setting the impedance of Ch. 0 to 1 MOhm and seeing if the square wave is a larger amplitude.
05-16-2012 05:00 PM
CH1 is the Black Line, the Red Line (CH0) is showing a sine wave with 60 mVPP.
I've varied impedances and even looked at the output through different oscilloscopes and they are the same.
05-17-2012 03:46 PM
To isolate whether the problem could be with the hardware, let's try to replicate the behavior using two of the NI-FGEN example programs without modifying their block diagrams. These can be found under LabVIEW Help>>Find Examples>>Hardware Input and Output>>Modular Instruments>>NI-FGEN>>Standard Function Generation>>
Fgen 5404 Clock Generation.vi
Fgen 5404 Frequency Sweep.vi
Please also use the Scope Soft Front Panel from Start>>All Programs>>National Instruments>>NI-SCOPE to read the inputs from each channel independently for each FGEN example. Set the termination impedance to 50 Ohms for the CH 0 SINE Out connection, and 1 MOhm for the CH 0 CLOCK Out connection. If you see similar results, could you please attach a screenshot of your readings?
05-18-2012 08:11 AM
I used the Fgen 5404 Clock Generation example with the Soft Scope, and the screenshot is in the previous post. I noticed that changing the frequency in the Fgen 5404 Clock Generation front panel changes both the frequency and amplitude of the measured signal.
05-18-2012 01:59 PM - edited 05-18-2012 02:02 PM
I have tested the PXI-5404 and PXI-5105 on our test chassis and see that the output works properly. For this test, I had connected the CH 0 SINE output to CH 0 of the PXI-5105 and CH 0 CLOCK Out to CH 1 of the PXI-5105. CH 0 impedance was set to 50 Ohms and CH 1 to 1 MOhm.
So it appears that there is either an issue with your SMB connection or the PXI-5404 card itself. You mentioned that you tried a different scope and saw the same results so I assume that the problem is not with the PXI-5105. If you try another cable and ensure a proper connection, but still see the same results you more than likely have a broken PXI-5404. In this case you can create an RMA request by calling 1-866-ASK-MYNI (1-866-275-6964).