NI RF Signal Generators bad output

[LVuser14]

Hello.

I have a few NI chassis (PXI-1075, PXI-1044) with RF signal generators PXI-5650 and PXI-5652. Output from the generator does not look good. (I connect the output to the oscilloscope with 1 MOhm input resistance — PXI-5114). Also check with other external analog and digital oscilloscopes. What could be wrong? The resulting waveforms are shown below...

[USBarnes]

Hi LVuser14,

Most RF generators have an output impedance of 50 Ohms. Is there a reason you're using the 1 MOhm input impedance setting on your scope? What happens if you change it to 50 Ohms. Also, are you using 50 ohm cables? Is your cabling going directly between the generator and the scope or are there other connections being made? Also, are you generating the signals through the RFSG Soft Front Panels or through another method? I think some more information would help me and others make better suggestions. Thanks for posting!

[LVuser14]

As I expected, when connected to the 50 ohm input, amplitude changes only. Notch remains. Cable connects the generator output and the input of the oscilloscope directly. These cables are tested with other generators. Images shown previously obtained using scope and RFSG Soft Panels (latest and some previous versions). Checked also with the hand made VI — the same result.

[USBarnes]

LVuser14,

What information does an FFT give? If you click the Freq button on the Scope Soft Front Panel, what does it show? It would be nice to know what Frequency components are being added to the signal.

Also, what is connected to Channel 1 of your scope? If you disable Channel 1, does anything change?

[LVuser14]

Hello.

When we disable the other channels of the oscilloscope, nothing changes.

Below are the results obtained on the spectrum analyzer. We see that the distorted signal produces a strong odd harmonics. Apparently, the output stage of the NI generator (looks like the behavior of the opamp) is not very good.

Without distortion:

Distorted:

[USBarnes]

This does seem like odd behavior.

Could you create a service request with our support? I think that would be a good avenue for troubleshooting this. If it does turn out to be a malfunction, we can set up a return or repair if necessary.