I want to calculate the Q-factor using the S-parameter results. However, it came to my attention results significantly is different when I use the s-parameter magnitude (range [0 1]) compared with when I use dB scale.
Calculation method:
1. Magnitude
Q=f0/(FWHM)
2. dB scale
Q=f0/(df)
df=f2-f1 where f1 and f2 have 3dB difference that the resonance strength in f0.
The difference between these two are so huge. For example while the Q-factor appears to be 1700 when I use dB scale, the result for Q-factor calculation using magnitude is around 60-80.
What is my mistake?
What did you use for FWHM? This means the bandwidth for half the power. Did you use half the amplitude (root power) instead of half power? 3 dB from the peak should be the same frequencies as half the power, because they mean the same thing. (or 3.0103 dB to be more accurate).
If you display the amplitude (root power) then it needs to be the 71% point (1/root 2) not the 50% point. 50% of amplitude is 25% (-6 dB) of the power, and the curve will be a lot wider which could explain the low Q.
Malcolm White You are right. The real problem regarding the calculations was the lack of necessary data points. As I increased the data points the difference gap closed to a significant extent.
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