I am in the need of a piece of test equipment (that doesn't cost 5+ figures) that will help me verify signal integrity through various midspan Gigabit Ethernet power injection designs.
To provide some background for those who don't completely understand what I'm talking about - when you inject power for a modern Power over Ethernet (PoE) device you generally use some sort of power injector. Back before Gigabit Ethernet, if the power was being injected in the middle of the span (I.E. between the Ethernet switch and the powered device), you'd typically just use the spare pairs in the CAT5 cable since 10/100 Ethernet only used 2 of the 4 pairs in the CAT5 cable and you could use the other two for power. Validating and testing through one of these devices was rather simple since the data pairs were electrically connected end-to-end and most high-quality cable testers would do a 2 pair test to validate the quality of the signal connections through the injector.
Gigabit Ethernet is a different beast. Since all 4 pairs are needed for data for Gigabit, midspan power injectors use a set of Ethernet magnetics (aka a set of 4 isolation transformers) in the middle, and inject common mode voltage on the center taps of the windings closest to the powered device. This works well, but also causes the data pairs to be electrically isolated. To date, I haven't found a cable tester which will even try a signal integrity test since they notice that all of the pairs have a DC short on them (aka the transformer winding for that pair), and/or they are "open" to the far end (due to the isolation), which leaves me testing these by passing a lot of data through them and hoping that no data errors corresponds to no issues.
A similar set of issues occur when one is testing surge suppression devices as well.... but that's a different story.
What I need to be able to do is to test a given injector design and somehow determine how much impact the injector would have on the signal integrity of a Gigabit (or 10/100) Ethernet signal passing through it. In an ideal world, I'd go buy something like a Tektronix DSA8200 - but we're talking at least 20K for a used unit without the modules I'd probably need, and then I'd have to learn how to use it for this purpose (doesn't appear to be a small task). So I'm looking for something else.
So, I'd love to find one or more of the following:
- A cable tester which doesn't care if the pairs are shorted and open (as described above) but will still do a fairly detailed cable validation for gigabit signal integrity.
- Some other suitable piece of test equipment which will get me to where I need to be. (I.E. inexpensive transmission line tester for 100 ohm cable)
- A document or description of some alternative method of validating signal integrity... i.e. sweeping the cable+injector... and applying the result to gigabit signal integrity. (I'm thinking something like a bode plot here, but not quite sure how to equate the results to Ethernet)
- Something else I haven't thought of
Any ideas?
I know someone will ask the "what is your budget?". 20K is out. Anything less than that will be considered
One more note: I do have a fairly healthy test equipment collection, so there is every possibility I have the right stuff if I just wire it together correctly (For example, I'm going to be playing with making a TDR out of the pulse generator and the oscilloscope in the next few days, unless someone comes up with a better solution).