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One for the Ethernet grey beards - magnetics

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Hi all. I've been looking into the parts I need to build an Ethernet adapter for a Motorola 68k system I have been working on, and I'm up to the part where I need to find suitable magnetics.

Having some past experience with the Am79C90 Ethernet controller I'm going to use that. This requires an external serialiser for which I am looking to use an LXT901.

The datasheet for the LXT901 says it needs a 1:1 transformer on the RX side, and a 1:√2 transformer on the TX side.

The interesting thing about this is I have a device that also uses an LXT907 (very similar to the LXT901 with the same requirements), but this is using 1:1 transformers on both the TX and RX.

My question is, how much do the transformer winding ratios matter? And if the datasheet says it needs 1:√2, what will be the side effects of using 1:1 in its place?

Basically, I've found somewhere to buy a small quantity of each of the above chips plus the same model magnetics as used in this other device (from an eBay seller that I have had good success with in the past), but I don't want to buy the magnetics if they simply aren't going to work.


Easiest is to just buy an RJ PCB connector with integrated magnetics. No need to fiddle with the transformers yourself.

I am aware of jacks with built-in magnetics, but that isnt really answering the question of what effect a 1:1 transformer has when the datasheet calls for 1:√2.

Have seen that ratio listed in the usual places (DigiKey etc.), though I've never had a need to look at them and see what actual availability or other specs are like.  Assuming they're available -- what's wrong with that?

Otherwise, if you don't mind the reduced maximum range, I would expect it's fine (going with 1:1)?  Says the driver has extremely low output resistance, as CMOS output pins go (5Ω typ?), so put in whatever series termination resistors (half and half each, mind) make up the difference for your medium, and that should do.

Not sure why they specify 24.9Ω resistors when the driver should be accounting for a couple of those ohms.  And arguably it's better to be on the low side, helps account for HF losses in the media.  I'd probably go with 22Ω, myself?

Or why an "irrational transformer" caught on, an obvious impossibility; a 5:7 or whatever ratio is well defined and only off by 2% (51Ω primary side).

Note that they don't show CMCs on the transformers; I wouldn't be surprised if this is easy to get away with at a mere 10Mb.  Shouldn't hurt to include it (and the grounded (at AC, usually through a 75R + 1.5nF "Howard Johnson termination") tap), so aside from the ratio, off-the-shelf parts shouldn't have any problems.

Mind, if you're going for absolutely period-correct capability, and you're going to be making design changes like these -- you'll probably have to get the contemporary standards, or diagnostic tools or whatever, to check it all out.  If you're only interfacing with modern hardware (sensitive receiver, lots of line compensation tricks), and don't need long runs, I doubt you'll notice anything different, hell, run it over some dumbass audio transformers for almost all that matters, 10BASE-T is terribly tolerant of poor conditions. :)


Thanks Tim.

I suppose I am curious why a big name brand networking company (in this case, Cisco) would go against the datasheet recommendations. Analog is not my strong point, and this falls well within that realm.  ^-^

But its also highly likely that "they know something that I dont", or had some people with the brains and equipment to figure out what would work. There are a bunch of resistors and capacitors between the transceiver and the transformers, so maybe they are doing something there to "fix things up", although the topology and values look very much the same as the datasheet examples but with a couple of capacitors sprinkled around.

There is also a CMC between the transformers and the jack.

The device is an old ethernet router, its probably something that would be somewhat unlikely to hang off the end of a very long run of network cable. It has both 8P8C 10bT and AUI connectors, so presumably if it was going to sit on the end of a long cable run you would have used an optical AUI adapter instead of copper.

But if what youre saying is correct - that you just lose a little bit of range by using 1:1 on the TX side - and that 10bT is quite tolerant of poor conditions - then maybe they determined that it would be "good enough".

I would simply be inclined to use the same part because it is in a known working configuration (and although it is a surface mount part, it has 0.1" pin pitch which is relatively friendly). :)

EDIT: incorrect diagram/schematic, see later post


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