Diodes – Diodes in Parallel or Series

Ethernet uses \$\pm0.85\;\mathrm{V}\$ signaling so a signal will only be able to "activate" one diode pair. So any transmission only has a chance to reach the two immediate neighbors (no current will flow more than 1 "hop" round the loop).

Now you can notice the trick which will work only with 3 devices: both of computer A TX lines' have RX lines from computers B and C in immediate neighborhood and it's own RX lines on the far side of the loop. You really don't want to hear what you are sending since it would trigger the collision detection algorithm.

Two interesting points:

1. The diodes will heavily attenuate the signal so it probably won't work over longer distances.

2. You cannot use resistors since they would linearly attenuate the signal and after going all the way around the loop it would finally reach your own RX line. It would be attenuated but the receiver circuit is very sensitive so it would still be able to detect is as a collision. You need a nonlinear element (like a diode) that provides a sharp cutoff.

PS. It is a really clever circuit. I cannot imagine inventing something like that on my own. :)

It's not clear how you plan to use this diode since "measurement setup for capacitor leakage" doesn't explain anything. However, in general, the "speed" of a diode is it's reverse recovery time.

Power rectifier diodes are generally slow, but are optimized for handling large current and substantial reverse voltage. "Switching" diodes or "high speed signal" diodes are more optimized for speed, like the common 1N4148.

The reverse recovery time of a diode is the time it takes to stop conducting after being switched from forward to reverse biased. This is a issue with semiconductor diodes because any charges in the depletion region will continue to allow conduction even when reverse biased. It takes a finite amount of time to sweep out these charges.

Schottky diodes are generally much faster, usually to the point you can think of them as instantaneous for the purpose of most circuits. This is because they have a much smaller depletion region inherent in their construction. Intead of a N-P junction, they are a semiconductor-metal junction. The flip side is that they also leak more and are unavailable or very expensive above about 100V.

Vacuum tube diodes work on a completely different principle and therefore have their own set of characteristics.