Dissection of the LiHeat flexible PCIe riser

iFreilicht

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Feb 28, 2015
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Welp, here goes nothing.

As you all know, run-of-the-mill PCIe ribbon cables have all sorts of issues regarding EMI, effectively reducing the maximum possible bandwidth, apart from looking like IDE cables from the 90s.
LiHeat promises to offer a reliable and better looking but affordable alternative that is shielded and braided.

Due to a recent discussion in the !nverse thread, I offered to take one of my risers apart and take a look inside. Here are the results of that investigation:


So it seems like this cable counteracts most crosstalk and EMI problems you'd usually see with relatively simple measures. I particularly like how they use some wires of the ribbon cable to divide it up into differential pairs, that's quite ingenious.
3M does the same by using micro-coax cables, but that's a lot more expensive.
 

Phuncz

Lord of the Boards
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May 9, 2015
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Amazing that you did this, all in the name of the mad science we call SFF.
 

iFreilicht

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A mad science indeed. But it's extremely interesting to see how they achieve the better reliability and I do have two of those cables, so it's not like I'd have to buy a new one now. It's also way easier slicing up a 30€ cable than slicing up an 80€ cable.
 

QinX

Master of Cramming
kees
Mar 2, 2015
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Very interesting to see, the SFF gods are thankful for your sacrifice.

Just a question, but what do you mean in the last picture with
"It would be interesting to know whether the contact pairs that leave the connector are the ones that are designed as differential pairs in the PCIe spec"
 

Hahutzy

Airflow Optimizer
Sep 9, 2015
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So from what I can tell, its cross section is:

Tape -> cable -> copper foil -> cable -> Tape

And the copper foil is there to prevent crosstalk between the 2 ribbon cables.

Now my question is, given those outer tapes are electrically conductive (that's the only way for them to prevent external EMI), but in that last photo, it looks like the tape goes (lengthwise) all the way to the leads on the PCB on both ends. Wouldn't that short something? Or somehow, they managed for the tape to touch only the ground leads, and nothing else?
 

EdZ

Virtual Realist
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May 11, 2015
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That's interesting, the Tx and Rx lanes are shielded from each other, and both are shielded from external EMF, but there appears to be effectively no crosstalk protection between adjacent pairs (pairs are not themselves shielded, no alternating-prime twist ratios).
However, if the root of the issue is near-end crosstalk between the 'high' power Tx lane and it's adjacent Rx lane (i.e a device puts energy into the Tx line, this energises the adjacent Rx line, and the device 'receives' it's own outgoing signals at a higher level than the incoming 'real' signals) that may be sufficient for the relatively short ranges these extenders operate at (i.e. extenders not long enough for pair-to-pair crosstalk to be an issue).
 

iFreilicht

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Very interesting to see, the SFF gods are thankful for your sacrifice.

Just a question, but what do you mean in the last picture with
"It would be interesting to know whether the contact pairs that leave the connector are the ones that are designed as differential pairs in the PCIe spec"
So in the picture you can see that most contacts have the pattern of two being connected to traces on the board that lead to two pins which are close together and the next two being connected to the ground plane.
PCIe uses differential signaling to reduce the influence of EMI on the signal quality, so for every transmitter or receiver (which together form one lane), the connector has two pins, each of which carries the negative voltage of its counterpart.

What I deduct from that is that the two traces that lead to two adjacent wires in the ribbon cable are correspondent to the pins that form those differential signal pairs.
I just looked at the PCIe spec and the pinout seems to correspond pretty much exactly with the ribbon cable and the traces on the PCB, so maybe I should remove that comment from the picture.

So from what I can tell, its cross section is:

Tape -> cable -> copper foil -> cable -> Tape

And the copper foil is there to prevent crosstalk between the 2 ribbon cables.

Now my question is, given those outer tapes are electrically conductive (that's the only way for them to prevent external EMI), but in that last photo, it looks like the tape goes (lengthwise) all the way to the leads on the PCB on both ends. Wouldn't that short something? Or somehow, they managed for the tape to touch only the ground leads, and nothing else?
As you can see in picture 6 ("The braiding"), the conductive layer on the tape isn't covering the whole tape. It is a separate layer that is glued on to the outer, non-conductive braiding during assembly. Where it could come into contact with the solder connections, it is not present on the braiding.

That's interesting, the Tx and Rx lanes are shielded from each other, and both are shielded from external EMF, but there appears to be effectively no crosstalk protection between adjacent pairs (pairs are not themselves shielded, no alternating-prime twist ratios).
However, if the root of the issue is near-end crosstalk between the 'high' power Tx lane and it's adjacent Rx lane (i.e a device puts energy into the Tx line, this energises the adjacent Rx line, and the device 'receives' it's own outgoing signals at a higher level than the incoming 'real' signals) that may be sufficient for the relatively short ranges these extenders operate at (i.e. extenders not long enough for pair-to-pair crosstalk to be an issue).
Well, as you can see in the last picture, there are always at least two grounded wires between each differential pair, which corresponds to the pinout of the connector specified by the PCIe spec, so that should prevent most crosstalk between those pairs. It is true that there is no twisting going on in this ribbon, though.
From what we know, this riser performs a little less well than the 3M one, so that might be the reason why.

The shielding between Rx and Tx is the most basic thing I expect from such a riser, the PCIe connector is designed in such a way that you have Rx on one and Tx on the other side, so if you're using a triple-layer board, having a ground plane in the middle already prevents such crosstalk. The same can be said for rigid PCIe risers, and those aren't shielded against pair-crosstalk either, are they?
 

iFreilicht

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This question was posted on the Brevis S thread, but it seems more fitting here.

Hi iFreilicht,

I will also use the Liheat riser type A from my project, which consists in fitting a full size 750 Ti (I need indeed a 6 Pin powered model) into a Streacom F7C case:

http://www.ebay.com/itm/PCI-E-Express-Gen-3-0-Riser-Card-300mm-A-type-black-/121883960550?hash=item1c60d9a4e6:g:Rz8AAOSwDNdVvult

Could you please give me the dimensions of the PCB supporting the PCIe slot, as well as the mounting holes position/diameter ?
Liheat contact is not really helpful...

I have other questions regarding the ribbon but I'll start a proper thread for my "mod".

Thanks !
The PCB is 125mm long and 12mm wide. If you take a look at the PCIe CEM, the edge of the PCB is perfectly aligned with the slanted edge of the PCIe slot.
The mounting holes are 113mm spaced apart and have a distance of ~5.5mm from the top edge of the PCB. I can't perfectly measure the diameter of them but it seems to be 4mm. An M3 screw fits very loosely.
Keep in mind that from the top edge (the one where the ribbon is NOT), you need at least 23mm of space for the ribbon to bend, better 24 or 25.

You can ask any other questions about this cable here as well, so that people can later on find all the information about it in this location.
 

aquelito

King of Cable Management
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Feb 16, 2016
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Thanks a lot ! That's great info.

Those 25 mm bending are not a great news (link to my build) ; I thought it would be more around 20 mm according to the info I could gather around...