Routing ethernet through a 5mm hole - Splitting phy + magnetics? - Page 1
Forum section: electronics / projects

author: adamarrrrr
author_rank: Newbie
timestamp: March 10, 2022, 12:08:23 pm
content: I'm making an ethernet connected guitar pickup (yes I know, it's weird, and I will explain myself later) that runs at 100Mbs. Most guitars have ridiculously small cavities to route the pickup wires through (<6mm) and a regular ethernet cable will not fit. A slimline ethernet cable could probably fit, however, the connectors definitely do not. I cannot route the signals directly from the pickup coils themselves through this hole, because there are roughly 33 wires I will need to route.I'm planning to use 0.5 mm flat flex to route it through the body to an ethercon jack, but I'm wondering if I should split up the PHY + magnetics or keep them on the same PCB (the main PCB sits directly under the guitar pickup). There is roughly 20-30cm of distance between the pickup and the ethercon jack on the main body of the guitar.I've read posts before that say to keep everything on the main PCB due to EMI/SI but I've never seen the reasoning explained. I've also seen posts that advise to do exactly the opposite? My original plan was to split the PHY and magnetics, and use an 8 pin FFC with GND-TX-TX-GND-RX-RX-GND-POWER to get a crappy diff impedance of around 115 ohms on the RX/TX lines. I would also place the magnetics as close to the jack as possible to make sure the common mode chokes are being put to good use.My concern with placing everything on one board is that the diff impedance on the TX/RX pairs will be larger due to no reference plane, plus random noise can be injected into the ethernet lines for the 20-30 cm before it can reach the actual ethernet jack. From looking at 24/48 port switches, it looks like they seem to favour placing magnetics super close to the ethernet jacks, and the main PHY IC(s) are up to 10-20 cm away from the magnetics.The reason these pickups connect via ethernet is because I have a fair amount of experience in AES/DANTE ethernet connected audio, these pickups are polyphonic, they detect horizontal and vertical deflections separately (so anywhere from 12-16 channels of audio), POE is extremely convenient, all in a single standardised format. As to the reason WHY.. it's because I'm a crazy person

author: Berni
author_rank: Super Contributor
timestamp: March 10, 2022, 12:21:46 pm
content: Yes the most noise resilient are wires after the magnetics, but its not like it would not work without it being close.The signals going from the PHY to the ethernet transformer are still mostly differential, so they still have a fair bit of noise immunity. As long as you route the pairs tightly together you should be fine even with meters of distance between the PHY and transformer.In fact there are some cheep garbage USB to Ethernet dongles floating around on AliExpress that don't even have a Ethernet transformer inside. They just connect the PHY directly to the RJ45 pins and it still works (tho it is pretty likely to die once the first good thunder storm comes by, maybe even frying the PC too)

author: Ian.M
author_rank: Super Contributor
timestamp: March 10, 2022, 12:53:06 pm
content: Got enough room in the pickup area of the neck for the Ethernet magnetics? If so, as the problem with ordinary Cat 5E UTP cable is the OD, and the internal wiring isn't going to be exposed, simply strip the jacket to get the diameter under 5mm, use whatever connector you like at the PCB end, or even solder it direct to pads, and use a Cat 5E RJ45 keystone jack mounted in the body to connect to a regular external cable. If you need to keep the de-jacketed cable neater, do a non-overlapping spiral wrap of narrow Kapton tape round it to keep the pairs bundled.

author: magic
author_rank: Super Contributor
timestamp: March 10, 2022, 01:00:04 pm
content: You only need two pairs for 100Mb/s operation, so perhaps some other type of twisted pair cable with similar impedance could be (ab)used, particularly if it doesn't have to run for 100 meters.

author: Ian.M
author_rank: Super Contributor
timestamp: March 10, 2022, 01:15:01 pm
content: Unfortunately two pair Ethernet seriously limits your options for POE.

author: adamarrrrr
author_rank: Newbie
timestamp: March 10, 2022, 02:00:00 pm
content: Yeah, in my initial design I had all the POE/magnetics right next to the main ethernet jack, mainly because vendors are so inconsistent in what POE spec they use, so it's safer to use all 4 pairs. I think I might just move all the POE/magnetics to the main board and cannibalise one of those slim ethernet cables and manually solder it up (or strip a fat one). I'm pretty sure everything should fit, and if they don't, I will make themIt also has the added benefit on not splitting components/BOM, plus it makes me feel warm and fuzzy inside knowing my ethernet is all in one place, relatively safe from the EMI demons. Apparently I can also buy super thin (1mm) twin axial cable for something like 50 cents a foot, which I had no idea was so cheap, so that may also be viable. It looks like if I want to use flat flex anything, I'll probably have to invest in a custom FPC ($$$). All the off the shelf solutions like LVDS cables seem to have 15/31/47 or some really esoteric number of wires (maybe not so esoteric if you're an LCD).

author: Terry Bites
author_rank: Super Contributor
timestamp: March 10, 2022, 04:19:53 pm
content: http://www.comnen.com/product/small-diameter-cat5e-cat6-utp-ultra-thin-patch-cord/