Using Power Spectrum to Manually Make an SEM Measurement

While you can still manually make SEM (Spectral Emission Mask) measurements using Power Spectrum, the Keysight 89600 VSA software now includes a Spectral Emissions Mask measurement type to greatly simplify the process of configuring and performing SEM measurements.

Introduction

This topic uses an example to show you how to use the Power Spectrum measurement option to manually make an SEM measurement. Spectral emissions mask is also called transmit spectrum mask.

The example signal used is an 802.11ac WLAN WLAN - Wireless Local Area Network: A system that includes the distribution system (DS), access points (APs), and portal entities. It is also the logical location of distribution and integration service functions of an extended service set (ESS). A WLAN system contains one or more APs and zero or more portals in addition to the DS. standard using a bandwidth of 160 MHz Megahertz: A unit of frequency equal to one million hertz or cycles per second..  Test conditions and limits for this signal are specified in IEEE Institute of Electrical and Electronics Engineers. A US-based membership organisation that includes engineers, scientists, and students in electronics and related fields. The IEEE developed the 802 series wired and wireless LAN standards. Visit the IEEE at http://www.ieee.org 802.11ac 2013 IEEE Standard for Information Technology.  The standard requires measuring over a 240 MHz frequency span using a 100 kHz kiloHertz: A radio frequency measurement (one kilohertz = one thousand cycles per second). resolution bandwidth and 30 kHz video bandwidth. 

Required Hardware and Software

The 89600 VSA software and M9391A or M9393A vector signal analyzer are used to make the spectral emission mask measurement.  The 89600 VSA software requires an 89601B-SSA/89601200C license to enable power spectrum measurements.  If an 802.11ac test device is not available, the M9381A vector signal generator can also be used and has an example 802.11ac WLAN signal as an arbitrary waveform source.

Generator setup

If a 802.11ac test device is not available, start the M9381A software and perform the following steps:

  1. Set the ARB source to the WLAN_11ac_64QAM_160MHz.wfm example waveform found in \Program Files (x86)\Keysight\M938x\Example Waveforms\WLAN_11ac_64QAM_160MHz.wfm.

    This waveform is a 56 µs WLAN burst.

  2. Set the frequency to 5.25 GHz Gigahertz: A frequency measurement which equals one billion hertz..
  3. Set the amplitude to 0 dBm deciBels referenced to a milliWatt: dB relative to 1 milliwatt dissipated in the nominal input impedance of the analyzer.
  4. Check the ARB on checkbox to enable modulation
  5. Set the RF Radio Frequency: A generic term for radio-based technologies, operating between the Low Frequency range (30k Hz) and the Extra High Frequency range (300 GHz). on checkbox to generate the signal. 

  6. Connect the VSG RF Out to the VSA RF input.  
  7. Connect the VSG sync output trigger (Trig 2 on the M9311A module) to the VSA external trigger input (Trig 1 on the M9214A module).

89600 VSA setup

  1. Start the 89600 VSA Software
  2. Click MeasSetup->Measurement Type->Power Spectrum to set the measurement type to power spectrum.
  3. Click MeasSetup->Frequency and set the center frequency to 5.25 GHz and span to 400 MHz.


  4. Click MeasSetup->ResBW and set the resolution bandwidth to 100 kHz and the video bandwidth to 30 kHz.


  5. Click Input->Analog and set the input range to 10 dBm. 
  6. Click Input->Gate Trigger and select external gating by checking the Gate Enabled and Gate View Result Available check boxes.


  7. Click the arrow on the right side of the Trace B tab and select Gate Time to view the Gate Time trace.


  8. Click Input->Gate Trigger and set Style to External trigger, Length to 60 us, Delay to -2 us, and Holdoff to 1 µs.  These settings ensure measurement data is acquired during burst on time.


  9. Click Utilities->Limit Tests and click New to enable limit lines for the spectral emission test. This opens the Limit Test Editor window. Enter a name for the limit test.
  10. In the Limit Test Editor window, click New to display the Limit Line Editor. Check the Test Displayed Limits check box. Enter the limit line name in the Name field.


  11. Click the Units tab and set X Reference to Relative to Center and Y Reference to Relative to Reference Level


  12. Click the Points tab, then click New… to add a limit line point. Enter the x value and y value for the limit line.


  13. Enter the following cutoff values into the Limit Line Editor.  These are the limit line cutoffs in dBr at different frequencies for 802.11ac 160 MHz WLAN:

Offset from center frequency (MHz)

dBr

-240

-40

-160

-28

-81

-20

-79

0

79

0

81

-20

160

-28

240

-40

 

  1. Click Markers->Limits to display the limit line on the waveform.  Check the Limit Test check box and select the name of the limit test in the name dropdown box. Make sure the limit line is displayed in the correct frequency spectrum Trace by selecting the correct trace from the trace dropdown menu in the Markers->Limits window. In this case it is Trace A.





Creating a Macro to Align the Limit Line

This section shows you how to create a macro that aligns the limit line with the top of the measured waveform.  The macro takes a marker and places it at the maximum point on the viewed signal. It then records the y-axis dBm value and sets this as the new reference level. Because the limit line is set to be positioned on the reference level, the top of the signal is at the top of the limit line. The script then sets the reference position to be 90% of the trace window to make it easier to see the limit line and waveform.

 

  1. Click Utilities->Macros, then click Create New. Enter a name in the macro name window and click OK. Then select the new macro in the Macros window and click edit.


  2. Enter the following code in the macro edit window.

 

// -----------------------------------------------------------------------------

// This is a demo macro to show how to align the limit line with the top of the

// waveform being measured.

// -----------------------------------------------------------------------------

namespace Agilent.SA.Vsa.Macro

{

using System;

using System.Collections.Generic;

using System.Collections.ObjectModel;

using Agilent.SA.Vsa;

public class C_TransmitSpectrumMask : MarshalByRefObject, IMacroRun

{

public void Run(Application app)

{

// Display a marker on the selected trace

app.Display.Traces.SelectedItem.Markers.SelectedItem.IsVisible = true;

 

// Move the marker to the highest peak in the signal

app.Display.TracesSelectedItem.Markers.SelectedItem.MoveTo(MarkerMoveType.Peak);

 

//Record the dBm value of this marker

double y_dbm = App.Display.Traces.SelectedItem.markers.SelectedItem.Y;

 

//Set the trace reference level to this marker value

app.Display.Traces.SelectedItem.YRefLevel = y_dbm;

 

//Set the reference level at 90% of the trace display window

app.Display.Traces.SelectedItem.YRefPosition = 90D;

}

}

}

 

 

 

  1. Click the Save icon and close the Code Editor window.
  2. In the Macros window, click Build & Run. The limit line should now be lined up with the signal.

Once the macro has been created, it can be executed via the Window->Macros dropdown menu. This brings up a window containing a list of macros that can be run.

Summary

If the signal passes the spectral emissions mask test, the 89600 VSA software displays pass in the lower right corner of the message bar.