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Thursday, 12 October 2017

VMA Simple Spectrum Analyser - Reflectometer Measurement

Hello,

In the past I wrote two blog entries about a reflectometer measurement with the SMA device:

http://vma-satellite.blogspot.pt/2016/08/reflectometer-measurements-with-sma.html
http://vma-satellite.blogspot.pt/2016/08/reflectometer-measurements-part-2.html

Since then I wanted to implement this as an automatic measurement function within the VMA Simple Spectrum Analyser software and, well, here it is!



Cycle through the measurement screens until you reach Measurements - Reflectometer:


Activate the Reflectometer and select the used Cable Type. The Distance to fault will show the a value in meter: this is the length of the unterminated cable or the distance to the fault in a terminated cable.

The cable type is important, as the propagation speed of the electric signal depends on the medium. In vaccum, the speed would be the sped of light, but in a medium with a foam dielectric, it may only reach 82% of that speed!

The manufacturer of the cable should normally indicate the propagation speed of the cable. This parameter is called VoP (Velocity of Propagation) or v/c (ratio of the propagation speed in the cable and the speed of light).

What if you don't know the VoP of your cable? Well, you can measure it yourself. Just take a cable of the same type and measure it, in order to know its length in meter. Click the "Calculate v/c" button and let the software determnine the VoP of your cable!

The software looks for a file called "Reflectometer VoP.ini", which contains pre-configured cable types. You can edit this file, so that your cables are listed. The default cable type is always the first in this list. Of course, if you determine the VoP as described above, you can select to have it added to the list automatically.


What do you need to carry out reflectometer measurements?

Apart from the obvious (SMA/NWT device and the VMA Simple Spectrum Analyser software), you need a "T" connector and in case of the SMA devices with only one ADF4350/ADF4351/MAX2870, you need a noise generator. NWT users can use the OUT connector.

Interested in further reading about reflectometer measurements?

Here are some links:


Regards,
Vitor



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