What is the difference in Fsignal and F1?

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30 Dec 2009
Posts:

What is the difference in Fsignal and F1 in Signal information graph? Same times they show very different value, some times not. Even, when they ""would like"" to show the same value, they show not exactly the same, for ex., when 997Hz signal applied, F1 usually shows 997Hz, but Fsignal shows 1000Hz or 998Hz or something near the real value. Why the Fsignal usually miss (especially on low level signal)? Is the math of calculation differ?
And one more cosmetic question :).
Why in Signal information logging graph the Fsignal and Vsignal change they name to Fmax and Vmax?
Thank you.

5 Feb 2010
Posts: 396

Hello Malva,
This is tricky, and perhaps we will change how we do it. We have two quite seperate process going on here. The Fsignal/Vsignal value is derived from the Spectrum graph as you have it displayed. It finds the highest amplitude signal, and then calculates the frequency based on the frequency bin it came from. It does no interpolation. Say you are looking at a 1 kHz sine wave, and the spectrum graph shows a Frequency Span of 200 khz, with a resolution of 391 Hz. That means the interval between frequency bins is 391 Hz. The 1 kHz signal shows up in bin 3, so the frequency is shown as 3 x 391 = 1173 Hz.

The F1/F2/F3 frequencies are calculated using a harmonic analyser. It uses interpolation to find the actual frequency of a peak that might be placed between two bin frequencies, and then measures the amplitudes of the 2nd and 3rd harmonics. It then looks at the next highest amplitude signal, and repeats this process. The signal with the highest level harmonics is chosen as the harmonic signal. This means if you have a square wave of one frequency, and a sine wave of another frequency mixed together, the harmonic analyser will find the square wave.

Because the harmonic analyser has more information available (the harmonics) and uses interpolation, it results in a more accurate indication of the actual frequeny.

Perhaps we should consider getting rid of the Fsignal value - though in the mixed signal case, it will show you the highest amplitude signal.

You are right, we need to name Fsignal and Fmax the same. Originally they were both Fmax, but that got confusing for some people. So we changed it to Fsignal, and forgot to do it for the logger.
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