1Mbps push-pull discrete transceiver

281

04:43:10

20m CAT-5 driven with a long-tailed pair and push-pull outputs. Receiver is 2 more long-tailed pairs putting out 5V logic levels. I'm in love with long-tailed pairs, LOL! They work wonders.

The data source represents ASCII 'u' in 8 bits with 1 start and 1 stop bit at 0-level: (0)01110101(0).

It's mostly the same as my previous 1Mbps transceiver. The difference is the new driver stage, which doesn't improve much on its own (in simulation) but makes source termination possible. Source termination (the 100Ω resistor on the right) improves signal quality significantly. 

The other circuit had common-collector drivers. I was told they didn't need source termination. The truth was they did, but whatever I tried interfered with the current flow.

I was going to do more work on this, trying to get higher speeds, but I kept putting it off. I decided to just publish this and leave it, as I'm getting more interested in software again anyway. I suspect higher speeds would be better handled by a receiver using a transformer for CMRR; that's what real Ethernet does, but that's a whole new learning curve for me.

published 8 years ago

rich11292000

8 years ago

Check out @flowDAQ's work. Looks like he has a professional product on a fancy website. http://everycircuit.com/circuit/5814610404835328

eekee

8 years ago

@rich that circuit you linked is 100 times slower than this. :) flowDAQ does have some interesting stuff, I will have to look closer later, but a quick look showed nothing this fast at all.

eekee

8 years ago

Apparently, my cable simulation is wrong. By lumping the capacitance and inductance into just two groups, I've made it "too pessimistic." I'm told what really happens is you only drive a small section at the front of the cable, and then the cable carries it to the end. If this means my circuit is better than I thought, I'm happy!

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