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Friday, 23 December 2016

VMA Simple Spectrum Analyser - Zero Span


Most spectrum analysers have a functionality called "Zero Span". What this does is to show the centre frequency in time domain, as opposed to the frequency domain of the spectrum: the result is similar to an oscillocope.

There are two ways to activate the zero span function:

  1. Just enter 0 MHz in the span frequency box
  2. Click twice on a given frequency, so that Marker 1 and Marker 2 have the same frequency value. Now select MARKER in the "Frequency Preset" group
Picture 1: I clicked twice on a transponder so that Marker 1 and Marker 2 are on the same frequency

Picture 2: The software recognizes the zero span entry and switches into time domain view on centre frequency

Before you get too excited, let me tell you that this function is rather useless... Why? Because the sample rate is much too slow for any useful application. Capturing 500 samples takes around 150ms, which equates to about 3000 Sample/s. The very popular hobby oscillocope Rigol DS1054Z has a sample rate of 1GS/S, that is over 300.000 times more!

The other problem is that the zero span serves to look at the signal modulated on the given frequency. However, the way the SMA/NWT devices work, they only offer 8 bit resolution for the whole signal, leaving little resolution for the modulated signal, itself.

So why did I implement this function? Here are my reasons:
  1. Doing so has allowed me to learn a lot about Zero Span in spectrum analysers!
  2. I consider that the VMA Simple Spectrum Analyser software is a great learning tool: if you learn how to use it, you will be able to use a regular spectrum analyser, so it does make sense to add this function, even if it's implementation doesn't allow a practical use.
  3. If future hardware devices of this family support a higher sampling rate, this function may become more useful.
  4. While the sampling rate is definitly insufficient to look for instance at analogue TV scanlines (a single scanline only takes 64ns!), there might be other applications where the current sampling rate is actually enough (if so, please let me know).
Other new features I implemented:
  • The Signal Source tab has now programmable preset buttons!
  • The frequency input is now verified - if you enter a frequency which is out of range or doesn't make sense, it will be ignored, instead of crashing/freezing the software.
Merry Christmas everyone!


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