I faced a very simple yet problematic phenomena when trying to find the bode plot of an unknown system with oscilloscope.
as we know we can simply inject a signal to a system by a signal generator and swipe the frequency then measure the input and output of the system and then by comparing the gain and phase shift plot the bode diagram.
here is the problem. when you have an unknown system with no prior knowledge. how can you find that the phase shift is positive or negative. as it can be seen in the picture the phase shift both can be considered +20 and -160
Your '-160' must be below -180 as you pass simple inversion, I would say -240.. Also, your '+20' is more like 120 degress or so (90 degree shift = top coninceds with 0 crossing).
120+240 =360.
One should note it is 360 deg (full period ) between sinusoiidal peaks, and you may choose to represent phase as 0..360 or +-180 degress
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