Ether coaxial receiver-transmitter interface for driving 75 ohm coaxial cable

https://patents.google.com/patent/CN2409714Y/en

The utility model discloses an ethernet coaxial transceiver interface for driving 75 ohm coaxial cables. A direct current biasing voltage device is connected with the collision detection interception port (CDS base pin) of a DP 8392 (National Semiconductor chip) or DP 8392 compatible chip (the chip is provided with a CDS collision detection interception base pin) in a transmitting-receiving interface circuit of the coaxial cable of the thick cable of a baseband of 10M ethernet and the thin cable of a baseband of 500M (10 base 5) /10 M ethernet which conform to the 802.3 standard of the international institute of electrical and electronics engineers (IEEE). The utility model provides the transmission of an ethernet signal on the 75 ohm coaxial cable, and can extend the transmission distance of the ethernet into the distance more than 2 km from 500 m and 185 m under the condition without a relay.

R−ZoR+Zo

Where Zo is the impedance of the cable and R is the source or load resistance.

And, for your 50/75 ohm setup will be -0.2. So the signal you put down the cable of (say) 3Vp-p will produce a reflection of 0.6Vp-p. Is this too much? It’s not great but it’s certainly not terrible.

https://electronics.stackexchange.com/questions/130655/effects-of-impedance-matching-between-50-and-75-ohm-coaxial-cables-for-10-mbit-s

This is quite a bunch of text because I have included plenty of background info. However, there will finally be a good, and precise question: Should I use an impedance matching network when connecting cables of different impedance such as 50 Ω and 75 Ω? Possible answers will likely start with “It depends…”, and this is why I provide a ton of background info first.

We’re looking at a combination of the signals transmitted and received by the ethernet hub near the oscilloscope. Judging by the “clean” part, the transmitted signal has approx. 1.9 Vpkpk, and the received signal has 1.6 Vpkpk. If it’s safe to assume that both drivers have an output of the same amplitude, we can even calculate the loss introduced by the cable: 20×log(1.6/1.9)dB = 1.5 dB. Good enough, because the calculation for 15 m of typical coax with 6.6 dB/100 m yields 1 dB.

The noise is greatly reduced when a matching network is inserted at the near or far ends of the 75 Ω part of the coax. It looks like this (Credits to this source)…

.. there are still some reflections visible travelling back from the unmatched far end.

With the matching network at the far end, there must also be reflections along the comparatively short 50 Ω cable between the hub and the discontinuity labeled “near”, but as I’ve learned from a friend, the scope can’t “see” them, because they are absorbed by the driver. Also, a part of the signal from the “far” driver is reflected and travels back along the 75 Ω cable, and gets terminated into the matching network on the far end

Compared to the unmatched setup, the amplitude of the signal from the far end is approximately halved (-6 dB), and this is in good agreement with the theory that predicts a loss of 5.6 dB over the network and the impedance it “looks” into.

Now, why not use two matching networks at “near” and “far”? Well, 10base2 is designed for a maximum length of 185 m of RG58, having a loss of 6.6 dB/100 m or 12.2 dB/185 m. Therefore, two of my resistive matching networks would already eat almost all the signal and bring me so close to the allowed limit that, including the cable, there is too much loss altogether. I am still in doubt that a low-loss, transformer-based solution would work because I think 10base2 (“cheapernet”) needs a DC path: “DC LEVEL: The DC component of the signal has to be between 37 mA and 45 mA. The tolerance here is tight since collisions are detected by monitoring the average DC level on the coax.” (Source: p.4; also backed up by this data sheet) Then again; the resistive matching network will also put any DC bias in trouble…

After all,

… the short question again: Should I use an impedance matching network when connecting cables of different impedance such as 50 Ω and 75 Ω?

Anything between “I prefer the unmatched/matched setup because I like this/that oscillogram better” to answers with plenty of background info on RF or the low-level hardware of 10base2 is greatly appreciated.

Edit

If you have access to the inside of the Coaxial Transceiver Interface (CTI), you can modify the circuit between the chip (8392 seems to be the type made by a large variety of manufacturers and also the type that’s used almost exclusively for pretty much any interface made by anyone for 10base2 adapters) and the BNC connector. A trade-off for cables with 75 Ω and 93 Ω is possible at the cost of allowed bus length. National Semiconductor made an Application Note on this topic, called AN-620 (pdf, Sept. 1992).

But even after finding this app’note, it would be great to find some background info about what’s inside an 8392, i.e. what one would have to use to build the interface using discrete parts and maybe some glue logic and opamps.

bitsavers.trailing-edge.com/pdf/national/_appNotes/AN-0620.pdf