As I know this circuit has three bridge taps.

Well that is interesting but in my above analysis and calculation I saw no obvious sign of more than one tap. It just goes to show that we must consider all information about a circuit. If, for example, I saw that Hlog plot the first thing I would probably do is to make TDR measurements from both ends of the circuit. Look at the attached PDF document. In the example trace, the length of the tap is given by the difference between the minimum and the maximum whilst the location of the tap is given by the point where the plot starts to drop down to the minimum.

What do you say about difference between my usual hlog and hlog with target SNR margin 15 dB (and higher power output)? Do you see that better graph (hlog15db) has also higher attenuation at some frequiences? It implicates there are also measurements errors causing lowering attenuation value.

Please be aware that the transfer function is a negative value, so that when we examine a Hlog plot the zero point of the Y-axis is at the top. Thus greater attenuation is indicated by the curve being further down the Y-axis scale. Greater attenuation is lower down; lesser attenuation is higher up.

You have shown me similar plots before. I really do not know

*why* your circuit behaves that way. It would be nice if you could plot the normal in red and the abnormal in green on the same graph. That would make it so much easier to compare them both.

I also attach raw data.

I have downloaded the data for both Hlog plots. At the moment I'm not sure what I can do with them.

As I have spent hours performing the calculations and then typing up what I had found, earlier this evening, I just do not have any more time to spare.

There are other things in my life that require my time . . . so I am stopping looking at your other Hlog plots.

[Edited to correct a minor error in the description of the JDSU TDR plot example.]