# Q: How to read this bode plot to find transfer function?

I am working on a blackbox system and I would like to make a Bode plot to find its transfer function. Since I do not have a lot of information on the system I am trying to input a series of sin waves of a known amplitude & frequency to find the corresponding gain and phase. I made the Bode plot manually with Matlab and I don't really know how to interpret it :

I measured the points several times to make sure it was correct, but it does not look like the plots I usually see, I don't know if I made a mistake somewhere. For the gain I used $$20\times log_{10}({\frac{Amplitude_{Out}}{Amplitude_{In}}})$$ and for the phase $$360 \times {\frac{Delay_{InputToOutput}}{InputPeriod}}$$

I usually see the phase converge at -90 or -180° of phase but this one looks like it's completely falling at higher frequencies. Also I don't seem to have a -20dB slope at $$\10^{-3}\$$. Does my plot look wrong ? How do you read such a plot ? Thank you.

• You don't really have enough points to make a correct guess, and I'm referring here mostly to the 1e-2...5e-3 range, where there seem to be only two points. That means you're likely to miss important variations by linearly interpolating. At any rate, you're measuring mHz, which means the settling time should be in the order of tens...hundreds of seconds, did you really wait that long before making the measurements? What generator did you use? What measurements: voltmeter, oscilloscope, ...? Did you correctly account for I/O impedances? Mar 24 at 16:31