Reference Level Configuration for Optimal EVM

You may configure the reference level on the vector signal analyzer for optimal EVM on PXIe 5646R, PXIe 5840, PXIe 5830, and PXIe 5831 devices. The technique uses inherent room in the dynamic range of the device. The optimization lowers the reference level from user-specified peak power by a factor that depends upon the user-specified peak power and carrier frequency.

You must ensure that the peak power of the incoming signal is specified accurately when you configure the reference level for optimal EVM using the following VI or function:

LabVIEW—niWLANA RFSA Configure Optimal Reference Level for EVM VI.

C—niWLANA_RFSAConfigureOptimalEVMReferenceLevel function.

To compute peak power accurately, you may perform Auto Level on the analyzer while the signal to be measured is present at its RF IN port. Use the following VI or function to perform Auto Level:

LabVIEW—niWLANA RFSA Auto Level v2 VI.

C—niWLANA_RFSAAutoLevelv2 function.

To avoid signal overflow or clipping during signal acquisition, you may provide an additional margin to reference level using the following property or attribute:

LabVIEW—Optimize Reference Level for EVM Margin property.

C—NIWLANA_OPTIMIZE_REFERENCE_LEVEL_FOR_EVM_MARGIN attribute.

Noise Compensation for EVM

You may use noise compensation for EVM to further improve residual EVM of vector signal analyzers on PXIe 5646R and PXIe 5840 devices. Due to additional calibration requirements and certain measurement restrictions, the manufacturer recommends that you use noise compensation for EVM only if the residual EVM performance without noise compensation does not meet your EVM performance requirements.

Noise compensation involves measuring noise floor of the instrument and using the measured noise floor data to compensate measured EVM. Perform noise floor measurement only if you want to apply noise compensation to EVM. Refer to the Calibration of Vector Signal Transceiver Devices for WLAN Measurements topic to understand how to perform noise floor measurement.

The toolkit makes use of the noise floor data measured using noise floor measurement to compensate EVM results and traces during OFDM Demod measurement for instrument noise floor. Follow these instructions to apply noise compensation to EVM:

• Ensure that LO connection and configuration settings remain the same between the noise floor measurement and the OFDM Demod measurement.
• To compensate measured EVM results and traces, you must set the following property or attribute:

  • LabVIEW—Noise Compensation Enabled.

  • C—NIWLANA_NOISE_COMPENSATION_ENABLED.

The manufacturer recommends using the following settings during OFDM Demod measurement for reliable results with noise compensation:

• Number of symbols ≥ 16.
• Number of averages ≥ 20.