Automatic Fixture Removal (AFR)

AFR has the following enhancements with the PLTS 2016 release:

• Added file version number for AFR extracted fixtures

• When loading standards from files, PLTS assumes a characteristic system impedance of 50 Ω

• Added a marker for finding fixture impedance

• Auto iterate feature to find fixture impedance automatically

• Enable calibration reference to system impedance in batched AFR

• AFR gating on DUT measurement to get S11a and S22b

AFR has the following enhancements with the PLTS 2018 release:

• Support for 64 ports

Tip:

If you do NOT have a 2X Thru calibration board built, then try the 1-port AFR method first and use the fixture with an open at the desired reference plane as a start. This is an extremely fast and easy test to see if the fixture open works well or not.

If the fixture open radiates too much (like a SMA male connector would), then the next step is to try a 1-port AFR with a short at the fixture output. These are both very easy to test without building special calibration standards. If neither of these two methods work, then the best practice might be to build a 2X Thru standard and use following the 2X Thru AFR method.

Test fixture

2X Thru standard

OR

Left or Right halves of 2X Thru standard

Supports 1-port, 2-port, and multi-port single-ended DUT configurations.

When used with a multi-port DUT configuration, the trace coupling is not removed.

Test fixture

2X Thru standard

OR

Left or Right halves of Thru standard

Differential configurations also remove the effects of coupling between the differential traces.

Automatic Fixture Removal Wizard

Note: These dialog boxes show images for a Single ended DUT, but Differential works exactly the same, only with Differential S4P files.

For best results, follow the AFR Wizard tabs from steps 1 through 5 by either clicking Next > or clicking the tabs.

In this section:

1. Describe Fixture

2. Specify Standards

3. Measure Standards

4. Remove Fixture

5. Save Fixture

1. Describe Fixture

The choices that you make in the dialog are reflected in the diagram and text. The version number for AFR is shown in the upper-left portion of the dialog and is added to the AFR extracted touchstone file. This information helps to determine which algorithm was used.

My fixture inputs are:

• Single Ended - The fixture and DUT have single-ended inputs and outputs.

• Differential - The fixture and DUT have differential inputs and outputs.

My measurement is (Single Ended):

• 1-port - such as S11

• 2-port - such as S21

• Multiport / Use THRUs- select up to 64 ports. Check Use THRUs to use THRU fixture.

My measurement is (Differential):

• 2-port - such as SDD11

• 4-port - such as SDD21

• Multiport / Use THRUs- select up to 64 ports. Check Use THRUs to use THRU fixture.

Advanced Settings (click ^ to show and hide)

After fixture removal set Calibration Reference Z0 to: (Choose one of these settings)

• System Z0 - Sets impedance to the System Impedance setting. Learn how to set System Z0.

• Measured Fixture Z0 - Sets system impedance to the value that is measured during the AFR process. For example, if de-embedding fixtures designed for a device having an input and output impedance of 42.5 Ω, the system impedance will be set to 42.5 Ω automatically. The input and output impedance must be the same to use this function.
  
  Not allowed when 'band limited' is selected below.

• <nn> ohms - Sets impedance to an arbitrary value.

• Set 'System Z0' to Calibration Reference Z0 - When the impedance is measured or set to an arbitrary value, check to also set the System Z0 to the same value.

Note: Changing the System Z0 setting does NOT change measured S-parameter data. Changing the port impedance setting WILL change the measured S-parameter data. For additional technical detail on non 50-ohm AFR applications, see this white paper (Internet connection required).

Select all that apply:

• Check if this is true: I want to correct for when the match (Return Loss) of Fixture A is NOT equal to the match of Fixture B.

• Check if this is true: I want to correct for when the electrical length of Fixture A is NOT equal to the electrical length of Fixture B.

• Check if this is true: My fixture is band limited. Bandpass mode will be used during the Time Domain measurement. If NOT checked, then Lowpass mode is used. Learn more about these settings.

• Check if this is true: My characterization fixture ≠ DUT measurement fixture. In some cases, the 2x thru is different from the actual DUT measurement fixture but the 2x thru is used to characterize the fixture. In these cases, gating on the DUT measurement fixture is performed that accounts for the fixture input reflection (S11a) and output reflection (S22b) to improve accuracy.

AFR Configuration

The AFR Configuration dialog can be launched by clicking on the gear icon (shown below) or by using the AFRCfg.exe application located in the directory where the PLTS application is stored (the default is C:\Program Files\Keysight\PLTS20xx). Double-clicking this file opens the AFR Configuration dialog.

The AFR Configuration dialog has three tabs:

• 1 Port Char. (Using OPEN/SHORT only)

• 2 Ports char. (Using 2X Thru)

• General

1 Port Char. (Using OPEN/SHORT only)

Access the following settings in the 1 Port Char tab of the AFR Configuration dialog.

• Spike tolerance parameter - Include the top N peaks of impulse response and determine which is the reflection position.   This parameter should be limited between 1 and 10.

• Start/Stop Position - Sets the start/stop time of the TDR.
  Minimum: N - If fixture length is bigger than N * RiseTime, then move gate close to OPEN/SHORT
  Normal - Put gate at halfway of T11
  Custom - Put gate between Positon1 and Position2

• Low Frequencies - Compensate for roll off at low frequency range.
  Data Range - Sets the ratio range for fitting.
  Merge Point - Sets ratio count of the merge point.
  Merge Span – Sets ratio count of the merge span.

• High Frequencies - Compensate for roll off at high frequency range.
  Data Range - Sets the ratio range for fitting.
  Merge Point - Sets ratio count of the merge point.
  Merge Span – Sets ratio count of the merge span.

2 Ports Char. (Using 2X Thru)

Access the following settings in the 2 Ports Char tab of the AFR Configuration dialog.

• Gating Algorithm.
  PLTS (gating in time domain) - Apply time domain gating for low pass cases.
  PNA (gating in frequency domain) - Apply frequency domain gating for low pass cases.

• Data Integrity Enforcements
  Passivity - Apply passivity enforcement.
  Reciprocity - Apply reciprocity enforcement.
  Import/Recall fixture-Dut file created by data integrity enforcement - Import in PLTS or recall in VNA FW the enforced fixture-Dut file for de-embedding.

General

Access the following settings in the General tab of the AFR Configuration dialog.

Set manual time domain start and stop settings for corner cases shown under Time Domain Settings.

Use Full Alias Free 1/(freq.step) - Check to set the stop time of impedance trace from 1/(2*freq.step) to 1/(freq.step) on dialogue Measure Fixture Impedance TDR data.

Manual Start/Stop Time - Check to set the manual start/stop time of impedance trace on dialogue Measure Fixture Impedance TDR data.

Window Coefficient - Check to use the manual window type.

DUT Gate Coefficient - Check to use DUT gate coefficient settings.

Select Single-Ended or Balanced.

Only Reflection Parameters - Check to apply DUT gate on reflection parameters only.

Fitting Thrus at high freq. ranges - Check to compensate for roll off at high frequency range.

Data Range - Sets the data range.

Merge Point - Sets the merge point.

Merge Span - Sets the merge span.

The Tolerance (in ohms) and Number of Iterations can be set under Wizard Tab 3, Edit Window, Measured Fixture Impedance Iteration. Also, two conditions for automatic iterations can be enabled via checkbox:

• Automatically iterate when Impedance Method Auto is checked

• Automatically iterate when Calibration Reference Z0 is set to System Z0 or custom value

Also, check the Automatically Iterate when Calibration Reference Z0 is set to System Z0 or custom value box to automatically iterate in that condition.

Impedance Correction for DUT Measurement Fixture - Check to correct the fixture impedance when the impedances of fixture and fixture-DUT don’t match.

AFR Mode Conversion - AFR mode conversion will turn on or off automatically to make the AFR result more precise with non-single ended data.

Checking In PLTS show menu item in the General tab will allow this dialog to be opened from within the PLTS screen by selecting Utilities -> Automatic Fixture Removal -> Advanced AFR Configure.

User Preset

This button opens the User Preset dialog where you can save, load, and enable your fixture settings as an AFR User Preset.

2. Specify Standards

Selecting 2 single-ended ports, or 4 differential ports allows a 2X Thru choice.

Checking Use THRUs in the 1. Describe Fixture tab allows 2X Thru choice for a Multiport measurement.

If the Use same thru for all groups is checked, only one cal standards need to Load or Measure instead of repeating the same steps for each group.

If My characterization fixture ≠ DUT measurement fixture is checked in the 1. Describe Fixture tab, the Fixtured DUT choice is displayed and checked.

Otherwise, the fixture MUST be characterized using 1-port fixture measurements using either OPEN or SHORT terminations.

Note: The term 'Standards' is used here because this process can be thought of as the second calibration in a '2-tier' calibration process. The first tier of the calibration must already be performed (the VNA calibrated) before starting the AFR process. Another way of describing this step would be:

"How will you be measuring or loading the characterization of the Thru standard?"

• 2X Thru - Both halves together in one fixture.

• Both 1X Thru halves separately (also known as 1-port AFR). Specify Open and/or Short at the end of each half of the standard.

Notes:

• This image:appears as the 2X Thru when you select 1. Describe Fixture, My measurement is 1 port. This means the left and right fixtures are mirror images of each other and have the same S-parameters. The cascaded combination of the two fixtures are Fixture A + Reversed Fixture A, or A'.

• If both halves are identical and you do not have a 2X Thru, then only Fixture A measurements are necessary.

Advanced Settings

These settings are used to describe any ADDITIONAL length between the halves of the 2X Thru or added to either of the individual halves.

If the electrical length of the Thru standard is identical to the test fixture, then make no changes to the default settings (Known length = 0).

My Thru fixture has:

• Known thru length - Enter the length in nanoseconds. See a simulated length in the diagram between the two halves of the 2X Thru.

• Unknown thru length computed using reflects.  This setting requires the two halves of the Thru fixture be characterized separately with a reflect standard.

• Unknown thru length computed using fixtured DUT measurements.  This setting requires an additional characterization of the Fixture + DUT.

3. Measure Standards

The following dialog is for Multiport 2X Thru measurements when Use THRUs is checked in the 1. Describe Fixture tab.

This step characterizes the 1X or 2X Thru standards. This is done by either performing measurements or by loading one or more *.snp files that describe the characterization of fixtures.

Note:  When loading standards from files, the typical system characterization impedance (Z0) value is used, which is 50 Ω.

Click Measure to see the following dialog:

For best results, the analyzer should be calibrated. Also, the measurement Start and Step frequencies should be equal. This is necessary for TDR measurements.

Connect the specified standard at the PNA port. Then click Measure.

OR click Load and navigate to the *.snp file that describes the standard.

For 2X Thru standards, the following dialog allows you to optionally remap the fixture ports.

Calculated Fixture Characteristics

The loaded or measured Impedance and Electrical Length of the fixture are calculated and displayed here.

The preview button allows you to visualize the time domain characteristics of the fixture model that will be used for de-embedding. By using the editing features noted below, the output match, length, and impedance of the fixture model can be optimized. This allows flexibility for including or excluding physical characteristics such as excess inductance of plug fingers, excess capacitance of receptacle pads or any other anomalies not desired in the fixture model. This advanced feature should be used with care as misuse could cause passivity or causality errors in the resultant fixture model.

N

Click Edit to start the following, interactive Measure Fixture Impedance TDR Data dialog.

This dialog allows you to change the Impedance and/or Fixture Length of the saved *.snp data by moving a marker to the desired location on the TDR plot.

Editing Mode.  (For 2X Thru, Fixture Length can NOT be changed.)

Choose one or more of the following to edit the data in resulting *.snp files:

• Impedance - move the M1 marker to edit the data as though the measurement was made in the specified characteristic impedance. The Z marker is used to measure the fixture impedance.

• Gate - move the flag to 'gate out' a part of the fixture measurement, usually close to the transition. The Z marker automatically changes to the location of the gate. The gate flag cannot move further than the fixture length diamond marker.

• Fixture Length - move the diamond marker to change only the electrical length of the fixture. The fixture length cannot be shortened to a value less than the current gate value. In this way, the gate value will never be greater than the electrical length.

Impedance Editing Mode

Select Fixture Impedance - Select the A or B fixture.

Impedance Method - Select Auto then click Iterate to measure the fixture Z0 automatically and return fixture impedance back to 50 Ω. If more than one fixture impedance needs to be measured, check All to measure all impedances listed in Select Fixture Impedance. Select Marker to measure the fixture Z0 manually using the Z marker. The Set At field shows the X-axis position of the Z marker in ns. The Set Marker At Default returns the Z marker and fixture impedance to the default value.

Measured Fixture Z0 - Shows the measured fixture impedance. When Marker is selected as the Impedance Method, this field displays the current Z marker value or a value can be entered directly in this field. When Auto is selected as the Impedance Method, this field displays the fixture impedance value that was found automatically after clicking the Iterate button. In Auto, moving the Z marker will not change this value.

Diff: ZA: - Enter the AFR fixture impedance manually using the text box.

Fixture Length Editing Mode

Select Fixture Impedance - Select the A or B fixture.

Set Marker Position, Gate Position, or Fixture Length depending on the selected Editing Mode. Move the marker (M1, Gate flag, or diamond) to the desired response on the trace.

Set At Default - PLTS chooses an appropriate length at which to set the initial impedance or length. Click to return the marker to this location.

Preview ON/off - Select Off, then ON to preview the new measurement.

Click OK when finished. When saved, the new Impedance and Length results are copied to the *.snp files.

Time Domain Settings

• Step Rise Time - Check to set the step rise time of gating window.

4. Remove Fixture

When using a PXIe system and the installed options are not sufficient to perform online characterization, the following dialog is displayed to prevent the system from hanging:

Note: First choose Select correction method, then make other selections, then click Apply Correction.

While all three choices are valid, the most common choice is De-embedding for PLTS measurements. All three operations can be performed, but only one at a time.

• De-embedding for PLTS measurements (default) - Then select the following:

• • Select ports to be corrected - For each port, check to correct or clear to NOT correct.

  • Select the PLTS measurements to be corrected.

  • • All of the open files or measurements are displayed.

    • All of the displayed traces for each checked file or measurement will be corrected.

    • Beginning with PLTS 2022, the measurement file can support three port  fixture s3p file.

• Turning on Fixturing/de-embedding for channels. Usually only one channel is being used on the PNA, so only one channel will appear in the choices of channels to be corrected.  

• • Select the channels to be corrected.

  • • Fixturing and de-embedding will be enabled for the selected channels in the PNA.

  • Advanced settings

  • • Enable Extrapolation - When fixture data is loaded from a file and the frequency span of the data is not as wide as that of the channel, check this box to calculate and use linearly-extrapolated fixture data.

    • Compensate for power - When checked, test port power is increased to compensate for loss in the fixture.

• Modifying the calset(s) used on the channels. Usually only one calset is in use on the PNA, so only one choice would be available.

• • Advanced settings

  • • Enable Extrapolation - When fixture data is loaded from a file and the frequency span of the data is not as wide as that of the measurement, check this box to calculate and use linearly-extrapolated fixture data.

    • Compensate for power - When checked, test port power is increased to compensate for loss in the fixture.

    • Prompt for new calset names - When cleared, when you apply AFR to a calset, the calset is overwritten. Once done, this process is NOT reversible. When checked, you are prompted to enter a new calset name and the original is preserved. The new calset is written with AFR correction.

    • Apply AFR to other selected Calsets - When checked, allows you to apply AFR correction to other calsets not currently in use on the PNA. Click Browse, then navigate to the calsets to be corrected.

5. Save Fixture

Select File format to save fixture data:

1. • Touchstone (*.snp)

   • Touchstone 2 (*.ts)

   • Citifile (*.cti)

Make 4 port file from 2 port data - Available ONLY when two single-ended 2-port fixtures are selected in the Describe Fixture tab (as in the above image).  

• Check the box to save one *.s4p file.

• Clear the box to save two *s2p files.

Choose port assignment for save fixture files:

The port assignments are interpreted differently when the file is opened in each program.

Choose which program software you will be using to open the saved file: PLTS, PNA, ADS.

Choose the directory and base names for the saved files:

Click Browse to navigate to a directory folder.

With a base file name:  The resulting filename will appear as follows (assuming a Touchstone format):

• <base file name>_1.S4P - The left half of a Differential fixture.

• <base file name>_2.S4P - The right half of a Differential fixture.

• <base file name>_1.S2P - The left half of a single-ended fixture.

• <base file name>_2.S2P - The right half of a single-ended fixture.

Click Save Fixture Files to save the files to the specified directory.

Single-Ended / Differential AFR dialog help

Single-ended dialog; left and right halves NOT the same delay

Although Single-Ended is pictured here, the same dialog settings are used for Differential.

Do your left and right fixtures have the same electrical delay?

• If yes, specify only one 2X Thru *.S4P/*.S2P file. These same delay and loss values will be removed for both halves of the test fixture.

• If no, specify S2P files for both left and right 2X Thrus.

• • The first file describes left and right halves for the left fixture (Red in above image).

  • The second file describes left and right halves for the right fixture. (Green in above image).  

Select Thru File: Click to navigate to the *.S4P/*.S2P, *.DUT, or *.CTI file of the left, and possibly right, Thru fixture.

I want to compensate for asymmetric match terms  Check when the left-half and right-half of the fixtures have different return loss values. Clear when they are identical.

I want to compensate for non 50 ohm impedance  When checked, the following appears on the dialog:

Specify the impedance for the left half (Z1) and the right half (Z3) of the fixture.

Measure from 2X Thru TDR data  

Click Measure to load the *.S4P/*.S2P files that describe the 2X Thrus. The following dialog appears:

1. In the bottom left corner, select the fixture impedance to measure.

2. Move the marker to the location to use to describe the impedance.

3. Click OK. The values are loaded to the AFR dialog impedance values.

Save extracted files for future use

• When checked, the following files are saved when Apply is clicked.

• When cleared, files are NOT saved.

Save future files to: Click then navigate to the folder for files to be saved to.

Touchstone files of the Test Fixture are saved to the specified path and Base File Name:

• <Base File Name>_1.S4P - The left half of a Differential fixture.

• <Base File Name>_2.S4P - The right half of a Differential fixture.

• <Base File Name>_1.S2P - The left half of a single-ended fixture.

• <Base File Name>_2.S2P - The right half of a single-ended fixture.

Click Apply to remove the test fixture from the displayed measurement and optionally save files.

Click Undo to revert to the displayed measurement of test fixture and DUT.

Set as Default  When clicked, the dialog settings are saved and populate the dialog next time it is started.

16-Jan-2024

Added User Preset button for save and recall

3-Dec-2020

Added Diff: ZA

3-Dec-2020

Added AFR Mode Conversion section

24-Nov-2020

Added AFR Configuration section

6-Nov-2019

Added Multiport Use THRUs (2X THRU for N-Port)

1-Dec-2015

Wizard enhancements

12-June-2014

Wizard enhancements

5-Dec-2013

Added wizard