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Friday, 11 November 2016

Quick update: Full support for Spectral Inversion in SMA Simple Spectrum Analyser software


For some applications, it is required to work with spectral inversion.

If you are using for instance a C-band LNB, then it's LOF is commonly 5150MHz.

But instead of adding the intermediate frequency to that LOF, the intermediate frequency is obtained by subtraction!


Ku Low-Band: LOF = 9750 MHz

RF = 11383 MHz
IF = 11383 MHz - 9750 MHz = 1633 MHz

C Band: LOF = 5150 MHz with spectral inversion

RF = 3780 MHz
IF = 5150 MHz - 3780 MHz =  1370 MHz

Interestingly, because you are now subtracting the RF from the LOF, you end up with an inverted spectrum, where the left side has a higher frequency value than the right side!

Many SAT field meters just work like this (if they support spectral inversion at all), but it is kind of odd working on a mirrored spectrum.

In the last release of my software, this was already supported. All you had to do was to introduce a negative LOF value. This way, the software would recognize a spectral inversion and show the correct frequencies, however the spectrum was rendered mirrored from high frequecies to low frequencies.

I was not happy with that and thought that the spectrum should be rendered correctly.

So I sat down yesterday and spend a whole lot of time fixing this.

The result is a new version with FULL SUPPORT for spectral inversion. Just enter a negative LOF value and you will get an inverted spectrum, which none the less is rendered correctly: lower frequencies on the left side, higher frequencies on the right side!

In the process I found an existing bug for marker 2, which got fixed (and gave me a lot of headache while implementing the spectral inversion).

Here is the high band spectrum of Hispasat 30.0W, shown with the LOF set to 10600 MHz.

The same spectrum rendered with a LOF set to -5150 MHz, which is equivalent to an inverted spectrum. Notice how the spectrum is shown mirrored relative to the previous picture:

Go ahead and download the latest release here:

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