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Envelope Tracking Using SCPI

Envelope tracking is an enhancement for DUT nonlinear correction, providing the digital predistortion (DPD) correction for power amplifier (PA) testing. The following sections in this topic describe envelope tracking using remote SCPI programming.

Envelope Generation

Voltage Mapping

Generate

Abort

Input File

Output File

Oversampling Ratio

Input Waveform Sample Rate

Envelope Waveform Sample Rate

Conversion Input Type

Conversion RF Power

Shaping Table Data

ETPS Gain

Vcc Offset

Vcc Clipping

Vcc Max

Vcc Min

Peak Differential Voltage

DUT Nonlinear Correction

Generate

Abort

Input File

Output File

Input Waveform Sample Rate

DUT Nonlinear Correction Mode

Lookup Table Data

Lookup Table Type

Lookup Table Power

Lookup Table Apply Mode

Memory Polynomial Coefficients

Volterra Series Coefficients

Memory Order

Nonlinear Order

Odd Order Only

Scale

Cross Term Order

Envelope Generation

The envelope generation utility generates the envelope waveform of the input waveform. Any waveform file format that can be read by the Waveform File mode of Signal block is acceptable.

This envelope generation engine, ported from the Keysight N7614C Signal Studio for Power Amplifier Test application, generates the envelope waveform required by the MIPI eTrak specification.

In typical envelope tracking measurements, the digital predistorted (DPD) waveform is used as input as well as the user’s original waveform. If the DPD waveform is required, DUT Nonlinear Correction utility can generate it. The following pseudo SCPI listing shows how the envelope of the DPD waveform is created from the user’s original waveform.

# Send the waveform to VXG

# This waveform data can be used for both group1 and group2.

:MMEMory:DATA "SNVWFM:myWaveform", ...

# DUT Nonlinear Correction to generate the DPD waveform

:UTILity:NCORrection:DUT:FILE "SNVWFM:myWaveform"

:UTILity:NCORrection:DUT:FILE:OUT "SNVWFM:myWaveform_DPD"

:UTILity:NCORrection:DUT:MODE LUT

...

# Generate (overlapped command)

:UTILity:NCORrection:DUT:GENerate  

*OPC?

# Envelope Generation

:UTILity:NCORrection:ENVelope:FILE "SNVWFM:myWaveform_DPD"

:UTILity:NCORrection:ENVelope:FILE:OUT "SNVWFM:myWaveform_DPD_ENV"

:UTILity:NCORrection:ENVelope:OSRatio 3

...

# Generate (overlapped command)

:UTILity:NCORrection:ENVelope:GENerate

*OPC?

# The peak voltage should be set to

# I Output Amplitude (:IQOut1:IQADjustment:EXTernal:AMPlitude:I)

:UTILity:NCORrection:ENVelope:PVOLtage?

## Group 1 (Envelope) ##

:GROup1:SIGNal1:WAVeform:FILE "SNVWFM:myWaveform_DPD_ENV"

:GROup1:SIGNal1:STATe ON

## Group 2 ###

:GROup2::SIGNal1:WAVeform:FILE "SNVWFM:myWaveform_DPD"

:GROup2:SIGNal1:NCORection:DUT OFF

:GROup2:SIGNal1:STATe ON

Voltage Mapping

Voltage mapping is an important concept and digital process in the envelope tracking measurement. This section illustrates the calculation flow and the relationships of the related settings. Most of the envelope tracking algorithm and terminology used in this section are compliant to MIPI eTrak spec.

Voltage mapping conceptually consists of the three parts described below. Its settings are categorized into two parts: one is related to envelope tracking power supply (ETPS) and the other is shown in the above drawing.

• In the V_CC waveform generation part, the input signal waveform is converted to V_CC waveform using the conversion data given by Shaping Table Data. The input to the shaping table can be normalized or scaled with a power value given by Conversion RF Power. The waveform is oversampled to the sample rate specified by Oversampling Ratio. The value of the output waveform is returned via Envelope Waveform Sample Rate.

• In the V_CC clipping part, the VCC waveform is clipped to the max and min values when Vcc Clipping is on.

• Lastly, the waveform is shifted and scaled by Vcc Offset and ETPS Gain. The outputs from this part are the envelope waveform and the peak voltage. The waveform itself is scaled by the peak amplitude in the waveform. The expected V_REF voltage can be achieved by setting I Output Amplitude to the peak voltage value returned via Peak Differential Voltage.

The following equations show the relationship between the input and output of EPTS.

where,

V_cc is the supply voltage of the PA, which is also called V_{ETPS_OUT} in eTrak spec.

V_{cc_offset} is the offset of V_cc. This parameter can be measured at the ETPS output by setting V_REF to 0.

ETPS_GAIN is the amplification of the differential supply voltage.

V_ETP is the positive signal output of the envelope waveform.

V_ETN is the negative signal output of the envelope waveform.

V_REF is used as the reference for ETPS.

V_CM is the common mode voltage as defined in MIPI eTrak spec.

The following figure illustrates the relationship between V_ETP, V_ETN, V_REF, and V_CM clearly.

Generate

This overlapped command initiates the envelope generation using the file specified by the Input File setting. The generated file is saved to the file specified by the Output File setting.

You can abort the generation using the Abort command.

Abort

Aborts the envelope generation started by the Generate command.

Input File

Allows you to select the input file for the envelope generation.

The following MSUS can be used.

• SNVWFM

• NVWFM

• NVCSVWFM

• SNVWFM<channel>

• NVWFM<channel> 

• NVCSVWFM<channel> 

The Auto Folder Selection by Extension works in the same way as File.

Output File

Allows you to select the output file for the envelope generation. Only the secure waveform file format (*.wfm file) is supported and SNVWFM and SNVWFM<channel> MSUS can be used for this setting.

An error is issued if the output file is empty or the file path or name is invalid.

The Auto Folder Selection by Extension works in the same way as File.

Oversampling Ratio

Sets the oversampling ratio for the envelope signal.

The final sample rate should not exceed the limit of the hardware sample rate.

Input Waveform Sample Rate

Sets the sample rate of the input waveform.

It is set to the sample rate in the waveform's header when a valid file is selected for Input File and the sample rate value is available in the header.

Envelope Waveform Sample Rate is calculated from Oversampling Ratio and this value.

Envelope Waveform Sample Rate

Returns the sample rate of the generated envelope signal. This is a query only SCPI.

This query always returns Input Waveform Sample Rate OSR.

Conversion Input Type

Selects the data type for the input of the shaping table.

If the Override Absolute RF is selected, the Override Absolute RF Amplitude value is used for shaping table input calculation.

• Normalized I/Q Amplitude – The peak amplitude of the original waveform is scaled to 1.

• Conversion RF Power – The input trace is scaled to the voltage at the RF Signal Generator’s RF Output connector using the signal’s RMS and Conversion RF Power setting. The RMS value in the waveform header is used if it is available. If not, the RMS is calculated from the waveform.
  

Conversion RF Power

Sets the RF Power value used to scale the input of the shaping table when Conversion Input Type is Conversion RF Power.

Shaping Table Data

Sets the shaping table used to convert the signal’s envelope to the waveform of V_CC. The table consists of two columns.

The first column is the input voltage. When Conversion Input Type is Normalized, it’s the normalized envelope value scaled such that the peak power becomes 1.0. When Conversion Input Type is Override Absolute RF, the input voltage is calculated from the signal’s RMS and RF Power given by Override Absolute RF Amplitude.

The second column is the Vcc voltage, the power supply of the PA, which is converted via the external ETPS that takes +I and -I Out outputs. ETPS Gain and Vcc Offset are taken into account to calculate the Vcc voltage.

This command takes the array of real values interleaved by the first and second column values. If the number of values is not a multiple of 2, the data is rejected and not set.

For example, with the following SCPI command

:UTILity:NCORrection:ENVelope:STABle:DATA 0.0, 0.7, 1.0, 3.8

the shaping table becomes as follows.

The Vcc voltage is extrapolated from the two outermost points when the input value is out of the range given by the first column of table.

ETPS Gain

Sets the ETPS Gain.

Please refer to the Voltage Mapping section to see how ETPS Gain is used in the envelope calculation.

Vcc Offset

Sets the Vcc Offset.

Please refer to the Voltage Mapping section to see how V_{CC_Offset} is used in the envelope calculation.

Vcc Clipping

Selects whether the Vcc is clipped using the Vcc Max and Vcc Min values.

Vcc Max

Sets the maximum value of V_CC when Vcc Clipping is on.

Vcc Min

Sets the minimum value of V_CC when Vcc Clipping is on.

Peak Differential Voltage

Query only command. Returns the calculated peak voltage of the waveform given to the ETPS. This value needs to be set to I Output Amplitude to scale the I Out voltage.

DUT Nonlinear Correction

All settings in this section are remote command only.

This DUT Nonlinear Correction (DNC) utility has the same capability as DNC under the Signal block. (8.1.8.5.2 in GPSS PD)

The major difference is whether the corrected waveform is saved to the non-volatile memory or not. This utility outputs to the non-volatile memory. On the other hand, the DNC under the Signal block uploads the corrected waveform directly to the volatile (arb) memory.

Generate

This overlapped command initiates the generation of corrected waveform using the file specified by the Input File setting. The generated file is saved to the file specified by the Output File setting.

You can abort the corrected waveform generation using the Abort command. 

Abort

Aborts the corrected waveform generation started by the Generate command.

Input File

Allows you to select the input file for the DUT Nonlinear Correction.

The following MSUS can be used.

• SNVWFM

• NVWFM

• NVCSVWFM

• SNVWFM<channel>

• NVWFM<channel> 

• NVCSVWFM<channel> 

The Auto Folder Selection by Extension works in the the same way as File.

Output File

Allows you to select the output file for the envelope generation. Only the secure waveform file format (*.wfm file) is supported and SNVWFM and SNVWFM<channel> MSUS can be used for this setting.

The Auto Folder Selection by Extension works in the same way as File.

Input Waveform Sample Rate

Sets the sample rate of the input waveform.

It is set to the sample rate in the waveform's header when a valid file is selected for Input File and the sample rate value is available in the header.

This value is saved in the header of the output waveform file.

DUT Nonlinear Correction Mode

Selects the DUT Nonlinear Correction mode.

• Lookup Table: DPD is applied using the lookup table data provided in the file specified by LUT File Setting.

• Memory Polynomial

• Volterra Series

Lookup Table Data

Sets the lookup table data.

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 will be

:UTIL:NCOR:DUT: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 the AM/PM has two rows, the SCPI command becomes as follows:

:UTIL:NCOR:DUT:LUT:DATA 0.1,0.01,0.2,0.02,NONE,NONE,0.4,0.04 

The maximum number of data accepted by this SCPI is 3000 sets or 12000 numbers in the list.

Lookup Table Type

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

• 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 max 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 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 Power

Sets the power value used when Lookup Table Type is Absolute.

Lookup Table Apply Mode

Selects 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

Memory Polynomial Coefficients

Accepts complex coefficients for Memory Polynomial DPD. Thus, the number of values given to this command needs to be even. An error is issued when it's an odd number. The maximum number of coefficients accepted by this 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 doesn’t 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. An error is issued when it's an odd number. The maximum number of coefficients accepted by this SCPI 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 doesn’t 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 non-linear 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.

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