Here is a link to *another* document which mentions bridge taps, but this one has a couple of decent examples:
https://www.broadband-forum.org/technical/download/TR-197.pdfThis document is TR-197 from the Broadband Forum, discussing DSL quality factors. The relevant section is on "dual-ended line tests" that are performed in some DSL modems.
Figure 10, page 51, shows the Hlog for a line in 3 states:
- Perfectly normal
- With a 30m bridge tap, first dip somewhere around tone 520. We get to see one further dip.
- With a 200m bridge tap, first dip somewhere around tone 55. We get to see 13 further dips.
Sanity check of the previous formula:
- Tone 520 = 2.25MHz. Tap length = (160 ÷ 2.25) ft = 71 ft = 22m.
- Tone 55 = 0.24MHz. Tap length = (160 ÷ 0.24) ft = 667 ft = 203m
The sanity check isn't far off, telling us the formula is a reasonable estimate. In that case, we just measure the first dip, and ignore all the rest.
With gazaai's first dip occuring at tone 164, I calculate:
- Tone 164 = 0.71MHz. Tap length = (160 ÷ 0.71) ft = 225 ft = 69m
I think that gives us the answer for the total tap length.
That backs up (nearly) just reading data from table 2 (in the original JDSU document) as the only step needed. I don't think you need to perform averages over the extra dips (except for the fact it might give you a better accuracy for where the first dip really was), nor to multiply by the number of visible dips (after all, in reality, those dips go on ad-infinitum).
I also think I know how to read the example in that 1st document better now ... but I'll have to do that in a separate post, likely tomorrow.