In my previous article, I introduced SCPI commands and their applications. The original idea of SCPI is to specify a common syntax, command structure, and data formats to be used by all test and measurement instruments. It does so by introducing many subsystem keywords for interfacing such as SYSTem and CONFigure. These intermediatory keywords helps the developers to access the functions of the instruments through simple scripting.
With the release of Touchstone File Viewer v1.8, the Vector Network Analyzer (VNA) Connect is introduced as a beta feature. This feature helps users to save and view SNP file in a quicker way by directly connecting the VNA to users’ computer. With this feature, users do not need to manually export the SNP file from the VNA to view it in personal computers. Users just have to link the Viewer to the VNA and directly read the file.
With great features comes great challenges. In the developing process of this feature, it is not without its challenges. The challenge comes in the form of integrating the SCPI commands of VNA from the different well-known manufacturers such as Agilent/Keysight, Rhode & Schwarz, and Anritsu.
Same Same, But Different
SCPI commands are created to standardize the command format for instruments. However, though most instruments adhere to SCPI, the same function of different instruments may not necessarily have the exact same SCPI commands. To be fair, it is reasonable when SCPI commands of a digital multimeter is different from a signal generator because this two instruments serve different purposes.
The challenge faced as developers is when same type of instrument from different manufactures or the same manufacturers, different series, uses different SCPI commands.
Take the VNA as an example, the command that gets the formatted data array for the selected active trace can be CALCulate:DATA:FDATa? for Agilent ENA model but CALCulate:DATA? FDATA for R&S ZVx model. Although both commands are under CALCulate subsystem, they are not universal commands.
The same issue not only happen for models from different manufacturers. The Keysight PNA N52xxA series and Keysight PNA N52xxB series are having CALCulate:DATA? FDATA and CALCulate:MEASure1:DATA:FDATa? as their respective function to gets the formatted data array. As you can see, both are Keysight instruments, but different models may use slightly modified SCPI commands.
The situation becomes more complicated when setting up parameter traces. Some VNA models such as Agilent/Keysight ENA series requires selecting the number of traces before setting up the trace parameters; some VNA models such as R&S ZVx series and Keysight PNA series are able to set up the traces directly by defining it and feeding it into the user interface.
Why Is SCPI Commands Not Fully Universal?
There are two causes for this issue.
Firstly, the different manufacturers want to distinguish themselves when designing their own VNA. The user interface, hardware, and workflow of each function are slightly different than others. Bear in mind, SCPI commands are written based on each function’s requirement of the VNA. When VNA models provide the unique functions that other VNA doesn’t have, the designer only need to make a small change to SCPI commands to avoid the commands overlapping with other functions.
Secondly, the numbers of subsystem keywords of SCPI commands have limited combinations to cover every new function. This limitation becomes more obvious when more and more advanced VNA models were introduced to support different applications in the RF industry. To cover all the new functions, the designers need to introduce new subsystem keywords for the particular SCPI commands.
The Workaround
With the existence of old and new VNA models in the industrial market, creating a SCPI related application such as FILPAL’s Touchstone File Viewer has become more challenging. However, even though SCPI commands are not exactly the same among all VNA models, the standard of subsystem keywords still guide developers to obtain the desired commands.
As developers, it is not possible to change the SCPI commands of VNA that were built-in when it was designed. Because of that, we came out with some alternatives to work around this challenge and ensure our Touchstone File Viewer is able to support the popular VNA models. Currently, the new feature of the application supports the following models:
- Agilent PNA E83xxB/C Series
- Anritsu MS20xxB/C Series
- Anritsu MS46xxA/B Series
- Keysight ENA E506xA/B Series
- Keysight ENA E507xA/B Series
- Keysight ENA E508xA/B Series
- Keysight N99xxA/B Series
- Keysight PNA N52xxA/B Series
- R&S ZNx/ZNxx Series
- R&S ZVx Series
- Tektronix TTR50xA Series
More VNA models support will be added in upcoming version. Meanwhile, try out the feature in your work! You can download it for FREE in FILPAL’s Touchstone File Viewer.
Originally published at http://filpal.wordpress.com on September 26, 2021.
