Reflection Measurements    Related Topics

In a reflection measurement, the analyzer transmits a stimulus signal to the input port of the device under test (DUT) and measures the reflected wave. A number of trace formats allow you to express and display the results. depending on what you want to learn from the data. Only one analyzer test port is required for reflection measurements.

In the following example, the analyzer is set up for a reflection measurement, a frequency sweep range and measurement parameter is selected, the instrument is calibrated and the result is evaluated using various formats.  

Instrument setup for reflection measurements

Parameter and sweep range selection

Calibration

Evaluation of data

Saving and printing data

Instrument Setup for Reflection Measurements

In order to prepare a reflection measurement, you have to connect your DUT (which is assumed to have a male N 50 W connector) to one of the (equivalent) analyzer test ports. Besides, it is recommended to preset the instrument in order to set it to a definite, known state.

 

  1. Proceed as described in section Starting the Analyzer and Shutting Down to switch on the instrument and start the NWA application.  

  2. Connect the input port of your DUT to test port 1 of the network analyzer.

  3. Press the PRESET key in the SYSTEM keypad to perform a factory preset of the analyzer.  

The analyzer is now set to its default state. The default measured quantity is the transmission S-parameter S21. This quantity is zero in the current test setup, so the trace shows the noise level.

Press the TRACE SELECT key in the TRACE keypad and use the softkeys in the Trace Select menu if you wish to create a new trace or a new diagram area.

Parameter and Sweep Range Selection

After preset the display shows a diagram with a dB Mag scale. The sweep range (scale of the horizontal axis) is equal to the maximum frequency range of the analyzer, and the S-parameter S12 is selected as a measurement parameter.

To obtain information about the reflection characteristics of your DUT you have to select an appropriate measurement parameter and specify the sweep range.

 

  1. In the CHANNEL keypad, press START CENTER and enter the lowest frequency you want to measure in the Start Frequency numeric entry bar  (e.g. 5 GHz).

If you use the DATA ENTRY keys at the front panel for data entry, simply type 5 and terminate the entry with the G/n key. Refer to section Data Entry to learn more about entering numeric values and characters.

 

  1. Press STOP SPAN and enter the highest frequency you want to measure in the Stop Frequency numeric entry bar  (e.g. 5.5 GHz).  

In addition to the linear frequency sweep considered in this example the analyzer provides various other sweep types. Refer to Setting up a Sweep for more application examples.

  1. In the TRACE keypad, press MEAS and select the forward reflection coefficient S11 as a measurement parameter.  

  1. In the TRACE keypad, press SCALE and activate the Autoscale function.  

The analyzer adjusts the scale of the diagram to fit in the entire S11 trace, leaving an appropriate display margin.

Refer to section Scaling Diagrams to learn more about the different methods and tools for diagram scaling.

Instrument Calibration

The analyzer provides a wide range of sophisticated calibration methods for all types of measurements. Which calibration method is selected depends on the expected system errors, the accuracy requirements of the measurement, on the test setup and on the types of calibration standards available.

In the following we assume that the calibration kit ZV-Z21 contains an appropriate male short standard with known physical properties. With a single short standard, it is possible to perform a normalization, compensating for a frequency-dependent attenuation and phase shift in the signal path.

Due to the analyzer's calibration wizard, calibration is a straightforward, menu-guided process.

  1. Unscrew the DUT and connect the male short standard from calibration kit ZV-Z21.

  2. In the CHANNEL keypad, press CAL to open the calibration menu.

  3. Activate Start Cal – One Port P1 – Normalization (Short) to open the calibration wizard for the selected calibration type.

 

  1. In the first dialog of the wizard, select the calibration kit (here: ZV-Z21) and the test port connector (here: N 50 W (f), corresponding to a male calibration standard), and click Next.   

The next dialog of the calibration wizard shows that only a single calibration standard needs to be measured.  

  1. Click the box Short (m)... to initiate the measurement of the connected short standard.

The analyzer performs a calibration sweep and displays a message box with a progress bar. After completing the sweep the analyzer generates a short sound and a green checkmark appears in the checkbox.  

  1. Click Apply to close the wizard, calculate and store the system error correction data and apply them to the current measurement.

  2. Remove the short standard and connect the DUT again.

Evaluation of Data

The analyzer provides various tools to optimize the display and analyze the measurement data. For instance, you can use markers determine the maximum of the reflection coefficient, and change the display format to obtain information about the phase shift of the reflected wave and the impedance of your DUT.

  1. In the TRACE keypad, press MARKER. This places Marker 1 to its default position (center of the sweep range).

A marker symbol (triangle) appears on the trace. The stimulus value (frequency) and response value (magnitude of the reflection coefficient converted to a dB value) at the marker position is displayed in the marker info field in the upper right corner of the diagram.

  1. Press MARKER FUNCT and activate Min Search.  

The marker jumps to the absolute minimum of the curve in the entire sweep range. The marker info field shows the coordinates of the new marker position.

  1. In the TRACE keypad, press FORMAT and select the Phase of the reflection coefficient to be displayed.

The phase is shown in a Cartesian diagram with a default vertical scale of –225 deg to +225 deg. The marker info field shows the frequency and phase at the marker position.

  1. Still in the FORMAT menu, select Smith.  

The Smith chart shows lines of constant real and imaginary part of the impedance in the reflection coefficient plane.

Refer to section Display Formats and Diagram Types to learn more about the diagram properties.

Saving and Printing Data

The analyzer provides standard functions for saving measurement settings and for printing the results. You can use these functions as if you were working on a standard PC. Moreover you can export your trace data to an ASCII file and reuse it in a later session or in an external application.

Data transfer is made easier if external accessories are connected to the analyzer or if the instrument is integrated into a LAN. Refer to sections External Accessories and Remote Control in a LAN to obtain information about the necessary steps.

  1. Press TRACE FUNCT and activate Import/Export Data – Export.

  2. In the Export Data dialog opened, select a file location, format and name and activate Save.  

The active trace data is written to an ASCII file.

Refer to section Trace File Formats to learn more about trace files and their use.

  1. Press PRINT in the SYSTEM keypad and select Print Now to create a hardcopy of your diagram.  

  2. Select Print to File... or Print to Clipboard to copy the diagram to a file or an external application.  

  1. Press SAVE in the SYSTEM keypad.  

  2. In the Save As dialog opened, select a file location, format and name and activate Save.

The active setup is stored to a file and can be reused in a later session.

Proceed as described in section Starting the Analyzer, Shutdown to shut down your analyzer.