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DUT Nonlinear Correction

DUT nonlinear correction predistorts the signal applied to the DUT. This method is primarily used in power-amplifier testing. The algorithm for DUT nonlinear correction is ported from the N7614C Signal Studio for Power Amplifier Test application.

This topic provides basic user-interface and SCPI descriptions.

How it Works

Applying digital predistortion (DPD) to your waveform is a two-step process. You must first enable nonlinear correction and then enable vector modulation, which initiates both the DPD calculation and waveform upload operations. The IQ data is therefore modified during the upload process, creating the predistorted waveform that will reside in the signal generator's volatile (arb) memory.

Digital Predistortion (DPD) is applied to the waveform before uploading it to the arb memory. For example, when DUT Nonlinear Correction is enabled, the signal generator internally creates the predistorted waveform as a part of the waveform uploading operation. Therefore, the predistorted waveform is not saved in the non-volatile memory.

Note that the predistortion calculation doesn’t run unless Signal upload occurs. For example, the waveform is not uploaded when Enable Vector Modulation Signal is off. Which means no DPD calculation runs at this moment. When Enable Vector Modulation Signal is checked (turned on), the waveform upload is triggered and DPD is applied as a part of the upload sequence.It should also be noted that DPD cannot be applied to a waveform already in the arb memory. This is because IQ data in the arb memory cannot be modified on the signal generator.

When nonlinear correction is enabled, an icon appears at the end of the Type value in the Signal Block, as shown below.

D indicates DUT nonlinear correction.

I indicates instrument nonlinear correction.

Settings

Enable DUT Nonlinear Correction

Setup

DUT Nonlinear Correction Mode

Memory Polynomial Coefficients

Volterra Series Coefficients 

Memory Order

Nonlinear Order

Odd Order Only

ScaleVolterra Series Coefficients 

Cross Term Order

Lookup Table Type

Lookup Table Apply Mode

Lookup Table Data

Enable Gain Expansion

Gain Expansion

Enable Gain Expansion (Utility)

Gain Expansion (Utility)

Enable DUT Nonlinear Correction

Turns DUT Nonlinear Correction on or off. When turned on, the DUT nonlinear correction is applied to the selected waveform file in the non-volatile memory before it is uploaded to the arb memory. A valid Lookup Table, Memory Polynomial, or Volterra Series must be loaded into the instrument before DUT Nonlinear Correction can be enabled.

Note that this feature cannot be turned on when a waveform in the arb memory is selected, a sequence file is selected, or Configuration is not in Independent.

RMS of Waveform File Mode is updated to the one calculated from the predistorted waveform when the DPD calculation succeeds. When DUT Nonlinear Correction is turned off, the previous RMS value is restored.

For the uploaded file, the suffix "_DPD" is added to its name in the arb memory. For example, if "MyWaveform.wfm" is uploaded with Enable DUT Nonlinear Correction on, the waveform file name will be "MyWaveform_DPD.wfm" in the arb memory. You can confirm the waveform file name using :MMEMory:CATalog? "SWFM1" query if you are using channel 1.

Since the original waveform and the predistorted waveform are uploaded with different names,you can change the waveforms quickly using "SWFM" MSUS without uploading them. The following SCPI listing shows you how to do it.

*RST

:GROup:SIGNal:WAVeform:SELect "SNVWFM:MyWaveform"

# "SWFM1:MyWaveform.wfm" is uploaded and played.

:GROup:SIGNal:STATe ON

# Set up LUT DPD

...

# "SWFM1:MyWaveform_DPD.wfm" is uploaded and played.

:GROup:SIGNal:NCORrection:DUT:STATe ON

# You can see those waveforms exist in the arb memory.

:MMEMory:CATalog? "SWFM1"

# You can play the original file by turning off DUT Nonlinear Correction.

# It's also possible to play the original file by selecting the arb file directly.

# In this way, the waveform is played quickly because unnecessary waveform upload

# doesn't occur.

:GROup:SIGNal:WAVeform:SELect "SWFM1:MyWaveform.wfm"

# Please note that DUT Nonlinear Correction is turned off

# when a file in the arb memory is selected.

:GROup:SIGNal:NCORrection:DUT:STATe?

# You can play the corrected waveform again without uploading any file

# by selecting "MyWaveform_DPD.wfm" in the arb memory.

:GROup:SIGNal:WAVeform:SELect "SWFM1:MyWaveform_DPD.wfm"

# To turn on DUT Nonlinear Correction again,

# select the file in the non-volatile memory first, and then turn on the state.

# Otherwise, the state is not changed, and the Setting Conflict error is issued.

:GROup:SIGNal:WAVeform:SELect "SNVWFM:MyWaveform"

:GROup:SIGNal:NCORrection:DUT:STATe ON

A predistorted waveform's RMS value updates whenever the DPD calculation succeeds. When DUT Nonlinear Correction is turned off, the previous RMS value is restored.

The DUT Nonlinear Correction algorithm is ported from the N7614C Signal Studio for Power Amplifier Test.

Setup

Selecting the Setup button opens the DUT Nonlinear Correction Setup screen.

DUT Nonlinear Correction Mode

Selects the DUT Nonlinear Correction mode.

• Lookup Table

• Memory Polynomial

• Volterra Series

Memory Polynomial Coefficients

Remote command only.

Accepts complex coefficients for Memory Polynomial DPD. Thus, the number of values given to this command must be even. The maximum number of coefficients accepted by this function is 1500 complex coefficients or 3000 numbers in the list. Any double precision number can be accepted as a coefficient. If the number of the complex coefficients does not match the value determined by other DUT Nonlinear Correction settings, an error is issued.

Volterra Series Coefficients 

Remote command only.

Accepts complex coefficients for Volterra Series DPD. Thus, the number of values given to this command needs to be even. The maximum number of coefficients accepted by this remote command is 1500 complex coefficients or 3000 numbers in the list. Any double precision number can be accepted as a coefficient. If the number of the complex coefficients does not match the value determined by other DUT Nonlinear Correction settings, an error is issued.

Memory Order

Sets the memory depth of the DPD model. This value is used in Memory Polynomial and Volterra Series.

Nonlinear Order

Sets the nonlinear order of the DPD model. This value is used in Memory Polynomial and Volterra Series.

Odd Order Only

Specifies whether to use only the odd orders of the nonlinear terms or not. This value is used in Memory Polynomial and Volterra Series.

Scale

Sets the Scale used for Memory Polynomial and Volterra Series.

The Centaurus team recommended 0 to 10 as range. However, we know 0 doesn’t do anything. Then we set the min same as the resolution number. We don’t know what resolution is sufficient, but 1e-9 can be small enough.

Cross Term Order

Sets the order of the cross terms in the Volterra Series algorithm.

Lookup Table Type

Sets the type of values in the AM/AM, AM/PM in the lookup table (LUT).

Normalized: The input in the lookup table is a normalized value. IQ data are normalized with its peak magnitude before the lookup table is applied.

Absolute: The input in the lookup table is voltage. The IQ data is scaled with the rms output power before the lookup table is applied. Note that the rms output power doesn’t include Amplitude Offset and Amplitude Reference.

The scaled IQ data may exceed the maximum of the lookup table inputs. This should be avoided to make DPD work correctly. However, if it occurs, LUT is applied by either applying additional scaling or using the edge values and a status message is displayed. Specifically, for AM/AM conversion, the IQ data is additionally scaled to fit with the lookup table and for AM/PM conversion, the edge value is used. For example, if the magnitude of input data exceeds the maximum AM input value in the lookup table, the PM value paired with the maximum AM input value is used.

Lookup Table Apply Mode

Set the lookup table apply mode from the following selections.

• AM/AM, then AM/PM

• AM/PM, then AM/AM

• AM/AM Only

• AM/PM Only

Lookup Table Data

Remote command only.

Sets the lookup table data. It accepts the list of values in the order of <AM/AM input>, <AM/AM Correction>,<AM/PM input>, <AM/PM Correction (Degree)>. For example, if both the AM/AM and AM/PM have two rows as shown in the table below, the SCPI command will be:

GRO:SIGN:CORR:PRED:LUT:DATA 0.1,0.01,0.2,0.02,0.3,0.03,0.4,0.04 

If the numbers of rows of AM/AM and AM/PM tables are different, NONE needs to be used to fill in the unavailable rows.

For example, if the AM/AM has one row and AM/PM has two rows, the SCPI command becomes as follows:

GRO:SIGN:CORR:PRED:LUT:DATA 0.1,0.01,0.2,0.02,NONE,NONE,0.4,0.04

Enable Gain Expansion

Turns Gain Expansion on or off. When turned on, the value is applied to the predistorted waveform.

Gain Expansion

Sets the Gain Expansion for all DUT Non-linear Correction Modes.

Enable Gain Expansion (Utility)

Turns Gain Expansion on or off. When turned on, the value is applied to the predistorted waveform. This command is used by PathWave X-Series Measurement Applications to perform a calculation and obtain a result before changing the VXG waveform.

Gain Expansion (Utility)

Sets the Gain Expansion for all DUT Non-linear Correction Modes. This command is used by PathWave X-Series Measurement Applications to perform a calculation and obtain a result before changing the VXG waveform.

Related Topics

ARB Waveform Memory

Instrument Nonlinear Correction

Signal Summary