My understanding is that the trace length between the magnetics and connector should be as short as possible (< 25 mm) and not deviate to much from 100Ω while the length between the PHY and magnetics should be longer than 25 mm.
This is a myth and, I think we went through it semi-recently but I don't recall the thread. Or maybe it was on Stack...
Perhaps someone else can find the thread.
PHY to magnetics only matters if there's troublesome noise coming from the magnetics, and that somehow manages to couple into the PHY. Toroid cores are used so the external magnetic field is minimal; this would only be due to common mode, electric field from the media side itself. So it seems a stretch that it would be a problem, even in the worst case (very sensitive PHY, noisy wire right over top it).
And this is irrelevant for integrated magnetics (the connector is shielded), for example. I suppose you'll be using discrete magnetics with the non-standard connector, so that doesn't apply at least. But it still seems unlikely there would be a problem.
Conversely, PHY to magnetics distance is constrained by stub length, particularly for traditional 10/100 wiring (CT to AVCC, either side to termination resistors to PHY). It might be strictly better for this distance to be short... but the length scale that actually matters is still quite large (i.e. a fractional bit time, or say a ns or so -- 10s of cm).
I forget if there were other opinions about this, regarding the purported 25mm limit.
To be clear: in general, my knee-jerk reaction is to seriously doubt or disregard anything that 1. isn't justified by realistic and quantitative (even if rather loose) measures or explanation, and 2. that no such explanation is obviously given by the quantity in question. Given, of course, that I have adequate knowledge about possible explanations to meet #2 -- this accounts for the Dunning-Kruger gap, at least past the initial peak.
So, if something says "keep 25mm" but doesn't explain why, I would just as well ignore it. Why, does it have cooties? Is it radioactive? Does it have radioactive cooties? Where does such an arbitrary number come from? Why is it so suspiciously close to a round, hand-wavey number (25.4mm == 1in)? How long has this number been repeated, can earlier references be found? Maybe one of them had an explanation. Maybe the original case is completely inapplicable and no one remembers that.
Basic critical thinking stuff. But, it helps also that I'm generally experienced enough to have a feel for what signals, attenuations, distances, etc. are needed in a given application, or at least in the applications I've worked on specifically; that's experience that's unfortunately not so easy to confer by text alone. So, you can only take this so seriously, I'm afraid.
The issue is that I don't want to run both isolated signals and non-isolated signals in the same FFC especially when one is "high speed" ETH (10 Mbit/s).
And running high speed PHY through a FFC cant be to good either? The specifications & design guidelines for RMII doesn't seem to crazy. But it might just be my lack of knowledge with ETH.
Definitely don't put RMII on it, that's a huge EMI/RFI mess already. Those signals are simply LVCMOS logic level, so, 3.3V 20mA ~1ns rise sorts of things. Even with a ground plane, putting it over a flex cable is dubious. Plain unshielded, suicide. (You would at least interleave ground between every signal, as for any other ribbon cable; perhaps it would pass tests alright for say a modest length (up to 10"?) and with a few ferrite beads clipped on, and preferably with the boards sharing an RF ground some other way (Y caps and standoffs?) to reduce the common-mode burden on the cable.)
And I would avoid PHY for the above reason (stub length).
Media however, I mean, it's just extending cable, who freaking cares. That's the other thing about the "advice", distance from magnetics to connector is irrelevant, it's all the same wiring. Keep the differential impedance around 100 ohms and nothing's the wiser. Well, keep CM impedance high, don't run over circuit GND plane (can make local plane referenced to centertap / Bob Smith terminators though), simple enough. Maybe getting Z_DM low enough is hard for planar (diff pair no GND plane), in which case length should be kept short, that's fine.
Tim