Vector Signal Generators (VSG) Generating Low Output Power

Updated Dec 15, 2017

Reported In

Hardware

  • PXIe-5644
  • PXIe-5645
  • PXI-5670
  • PXI-5671
  • PXIe-5672
  • PXIe-5673
  • PXIe-5673E

Issue Details

The output power is significantly lower than expected based on the level set in the application (up to 6 dB less when using 1 V modulating signal). 

This issue is likely to occur, while using Modulation Toolkit RFSG examples shipped with LabVIEW, e.g. MT RFSG Generate AM.vi. Affected devices: NI PXIe-5644R/5645R, NI PXI-5670/5671, NI PXIe-5672/5673/5673E
 

Solution

Hardware Cosiderations
NI 5670/5671: While in Script mode, if the Power Level Type property is set to Average Power, the driver scales each waveform so that all waveforms have the same average power. The average power level of each waveform matches the value set in the Power Level property. You can disable this scaling operation by setting the Power Level Type property to Peak Power. 

NI 5644R/5645R/5672/5673/5673E: This property must be set to Peak Power when the Generation Mode property is set to Script. 
 
 
How to achieve correct behaviour
If the Power Level Type is set to Average Power it can be safely passed to the driver and the output power should have expected value. In this case the power value should be automatically normalised between -1 and 1. 
 
However, if the Power Level Type is set to Peak Power, its value should be converted using MT Calculate Power Adjustment and PAPR.vi from the Modulation Toolkit before being passed to the driver: 



Alternatively, the user can implement his own mechanism of conversion from Average Power value to Peak Power following the equations given in RF:Power Level Type Property
 
Frequency Modulation Schemes 
Moreover, to ensure waveform correctness in case of frequency modulation schemes (FSK, MSK, and CPM) used in Peak Power mode, users should account for additional factors such as resampling or filtering effects by using MT Calculate Worst Case Scaling Factor (Modulation and Resample).vi
The modulation VIs included in Modulation Toolikt only accept normalised signals on the input with values ranging from -1 to 1. Therefore, the implicit assumption behind the RFSG examples shipped with LabVIEW is that signal generators are always set to the Average Power mode rather than Peak Power. It is up to the user to ensure the power settings are aligned and values passed are within specified limits.

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