FGEN 5402 Amplitude at low frequencies

[VikHof]

Hello,

Sorry, I don't know the right location...

I am working with the NI-PXI-5402 FGEN.

My measurement is between 1000Hz to 1Hz. My example amplitude is 1V.

The FGEN makes 1V from 1000-15Hz. After 15Hz the amplitude goes done to 700mV (10Hz), 500mV (5Hz) an so on.

Is that the normal behavior?

How can i regulate the voltage to constantly 1V?

I'm working with Labview, by the way.

Thanks!

[A_Donfigulation_Error_Has]

Hi VikHof,

First go to the Start Menu and then navigate to the NI FGEN SFP. To find it, go to All Programs -> National Instruments -> NI-FGEN -> NI FGEN Soft Front Panel. When it first comes up, the default settings are for a 1Vpkpk Sine wave @ 1MHz. Change the Amplitude pkpk to 2Vpkpk, and then click on the play button to begin generation. While the device is generating, change the frequency on-the-fly to lower frequencies and see if the amplitude degrades.

If the signal is still degraded, then how are you measuring the output of the 5402? What device are you using to measure this? What is the Output Impedance of the 5402 and what are you setting as the input impedance of your measuring device? 

If the signal is NOT degraded, then check your code. This is not a HW issue. 

Jason L

[VikHof]

Hey,

thanks, for the fast answer!

My measuring device is a NI-PXI-5105.

I tried something else:

I opened the NI FGEN SFP

and i opened the NI Scope SFP. Now i connected the output Ch0 from the FGEN to the Ch0 input of the Digitizer.

Now i configured an amplitude of 30mV.

The NI Scope tells me following:

1000-40Hz more or less an amplitude of 30mV

40-20Hz 25mV

10Hz 20mV

4Hz 10mV

and so on

Actually I don't know how to see the impedance config.

  Edit: Output impedance of 50Ohm

Is that helpfully?

[A_Donfigulation_Error_Has]

Hi Vikhof 

Can you provide screenshots of your NI SCOPE Soft Front Panel showing the waveforms at different frequencies? If the voltage is low, then you probably need to RMA the board.

Jason L

[VikHof]

Here are the Screenshots:

By the way, a similar behavior at 1V

[A_Donfigulation_Error_Has · Accepted solution]

Turn off AC coupling on the Scope. The cutoff of the 5105 is at 12Hz. 

http://www.ni.com/pdf/manuals/374403b.pdf#page=4

Jason L

[VikHof]

Thanks!

The Labview Code works with the AC config too.

I think the solution will be to turn it into DC.

The problem is, that Labview don't want to change the VI after update...

But i will get a newer version. Hopefully its changeable and it works.

Otherwise i will ask again.

But thanks till now!

[A_Donfigulation_Error_Has]

You should be able to change the Coupling through the NI SCOPE API in your LabVIEW code. Use the niScope Configure Vertical.vi

http://zone.ni.com/reference/en-XX/help/370592Y-01/scopeviref/niscope_configure_vertical/

Jason L