RPL1116 (MSO1000Z) and PLA2216 (MSO5000) Active Logic Probe teardown

[ebclr]

Will HDMI or CAT 7 connectors be a good choice to break in 4 channels pods?

[dren.dk]

Regarding pod-cabling:
* RJ45 doesn't really work, it's only 4 pairs and that's too little for even a 4 channel pod.
* HDMI could work for a 4 channel pod as there are 4 pairs and a number of extra conductors, but you'd still have a problem if children start plugging monitors into the breakout board, though.

Regarding length matching the LVDS pairs, I don't buy that length matching matters at all, because at 1 GSa/s nobody is going to be able to be able to measure a sub-cm length difference in signal paths.

Looking at the traced near the LA connector in the scope does not reveal any meandering to equalize the quite uneven lengths of traces going to the LA connector, sure, they could do that on an inner layer, but doing that near the connector where there's plenty of room would make more sense to me, so I suspect that it wasn't a consideration at all.

[wulfman]

Length matching was not my concern as much as impedance matching with the distance between dif traces.

[dren.dk]

Ok, it seems like you're aiming for a straddle mount socket for the scope connector, what kind are you using?

[Muza]

Regarding pod-cabling:
* RJ45 doesn't really work, it's only 4 pairs and that's too little for even a 4 channel pod.
* HDMI could work for a 4 channel pod as there are 4 pairs and a number of extra conductors, but you'd still have a problem if children start plugging monitors into the breakout board, though.

What about full-feature USB 3.1 Type-C cables?
Plenty of shielded data lines and an abundance of cables...

Hopefully some one chimes in on the pin configuration, so that when accidentally plugged into a computer the device just gets ignored.

I kinda like the idea of sleek pods with low profile connectors such as Type-C...

[Muza]

A quick'n'dirty render of what I meant. Last number is of course 15, not 16...
Essentially, it does not matter if we decide to use any interim connectors between the scope and the pod, in any case I was planning to 3D print a case and then make a silicone mold of out of it.
Afterwards, casing can be poured in PUR or Polyester molding the idc connector with pcb or idc connector with the flat ribbon cable.

[wulfman]

Ok, it seems like you're aiming for a straddle mount socket for the scope connector, what kind are you using?

Like this  https://www.mouser.com/ProductDetail/3M-Electronic-Solutions-Division/9150-4500PL?qs=sGAEpiMZZMs%252bGHln7q6pmzuniGC9vGvgdUx8jl58Y%252bI%3d

I am sure they are found cheaper on ebay

[Muza]

That's the 3M 9150-4500 Series i used the CAD from

[dren.dk]

I did look into USB-C, it does have 4 pairs, but it's not keyed and supporting rotation of the connector would be somewhat hard to support the reversible connector.

USB-C also has built in chips that make it harder to abuse for something like this.

Natually it's possible to use the 4 pairs for the data and then implement a proper +5V power supply that generates the +4V, +2V5 and -2V5 rails and to feed the vref through one of the connections, but it's also somewhat hard and not all that elegant.

I think someone who wanted to (ab)use a consumer cable for a 4 channel pod would be best served with HDMI, though pure mini and micro variants don't seem to be very common...

[Muza]

That's a valid point regarding the polarity with USB-C... which with correct layout would just mean that two channels would be swapped if inserted "upside-down". Not an ideal case.

Going back to the 50-wire flat ribbon cable: maybe can just stack two narrower ribbons on top of each other to make the connection between the scope and the pod less bulky just like the Rigol RPL2316

[wulfman]

The cable issue is why i like the SCSI III cabling

https://www.ebay.com/itm/Internal-SCSI-Ribbon-Cable-2-Drive-with-LVD-SE-Terminator-32-Inch/161643732466?hash=item25a2b79df2:g:HCkAAOSw34FVCz4Z

cut off the extra connector and terminator. 3' cable

[Rerouter]

While I try and figure out how I am going to route the 1.27mm header, here is an 8 Channel pod using LMH7324's,

Not the smallest of things at 41x33mm, and the ribbon is most of that width, but the smaller header / ribbon version is on the todo list.

at a glance it should be possible to make this a 2 layer PCB, but I'm not up for that challenge just yet.

[Rerouter]

And the 1.27mm header version, for a 0.635mm pitch ribbon. This requires a 0.6mm thick PCB to inline the header.

If you want to go more compact than this, it would need to use smaller components, At present I have been working to keep the 0603 minimum as set earlier.

Edit: missed a via, board files have been updated.

[oliv3r]

For the cheap guys: Just solder the amount of chips you have/want on the pcb.. You only have to watch out for the correct pin.

I think  you are hitting a good point. I understand where everybody wants to optimizethe design for their own use case. But going with 4 or 8 channel pods for the 'cheap' crowd, makes little sense, as it's far easier to just not get the LMH chips.

Even with the 4 channel chips the same goes Cheaper = don't solder the chips.

I think Rigol knows what they are doing...

Do we really think that? I mean that Rigol is knowing what they are doing


In any case, good progress is being made.

[Rerouter]

I was working on 8 channel pods, mainly for ease of routing, they have a wrapped around pin order for a dual side loaded PCB, so if stuck to single side loaded PCB's, its easy to break into 2 logical units, but I am including all the design files. so people are free to edit them to there own requirements.

[dren.dk]

Nice, but 0.6 mm 4 layer boards aren't all that cheap:) Perhaps it's ok to accept normal through hole mount, it's not like the straddle mount is that much smaller.

For me it's more important to be able to hand assemble the first 10 units and rework mistakes than it is to the get to the smallest possible size.     

I've been considering using a 2x8 female socket for the input with half of the pins dedicated to gnd. Probe wires with male ends are easy to find, but the really neat way to interface with that would be a single-use board with male headers that can be soldered to the DUT and left there.

I find I often have to solder wires to the DUT to be able to clip test leads or a scope probe on, so having the interface to the LA be soldering to a board seems like it would cut out the middle man.

[dren.dk]

bwt. rerouter: Nice looking layouts there.

I just did the core of my own 16 channel pod layout with lmh7324 yesterday, everything except connectors fitted nicely in 32x54 mm, even with 4mm of space around the ICs for rework, it's funny how easy it is to route on 4 layers, even when dedicating a whole layer to the ground plane.

I don't think I like the 16 channel pod as an idea at all, for three reasons:
* Pulling all the single-ended input connections together into one location might be tricky.
* Pulling the big fat uplink cable from scope to pod is going to be clunky, even if it is 0.025" pitch.
* Aside from the 68 wire scsi cable idea, sourcing a small amount of 50 wire ribbon cables and connectors is quite hard, while 26 pin 0.025" ribbons are much more accessible.

[dren.dk]

rerouter: I see what you were saying about routing the header for the 0.635 mm pitch ribbon, it's annoying as all hell not being able to put a wire between the pads.

[Rerouter]

As per the 0.6mm 4 layer pcb. I may be able to walk that back to a 2 layer 0.6mm pcb. As the thickness is low enough that the impedance widths are easier.

And yeah differential wiring the header was my nightmare when it wasnt inline. It can be done. But its not that easy and definatly seperates the differential pairs for a bit to ensure they are side by side on the ribbon

[wulfman]

Just placed my prototype order b board 2 layer was 50 cents each quantity 10, a board 4 layer was 2.90$ each  quantity 10

IF these work as advertised i will upload all the project files to gitlab for all to use. Or i will have 9 extra boards available 

[dren.dk]

Here's my take on an 8 channel lmh7324 based pod, highlights:

* Still single-sided load, for ease of assembly with a super-generous courtyard around the QFNs for ease of rework, should it be needed, but at 40x42 mm I don't really think it needs to be any smaller.
* I went with a 26 lead, 1 mm pitch ribbon and IDC connectors towards the scope, because I don't want to deal with the nightmare of laying out the 1.27mm pitch header and 26mm is narrow enough for me:)
* The inputs are via a 2x8 2.54 mm pitch female socket, with half of the pins being ground, that should make it easy to plug in a single-use adaptor board or individual probes, I want to plug in my inputs from the top, but it's just as easy to use the same foot print for an angled header or socket if front-entry is desired.
* I want to laser cut an acrylic sandwich case, which basically requires a hole for a screw in each corner, so the board grew 10 mm in width because of that.
* To use the space between the mounting holes for something I stuck a crude voltmeter on the board in the form of an ATtiny85 and 10 charlieplexed LEDs, so it's fast and easy to verify that the threshold voltage is set correctly.
* The Kicad project is on gitlab here: https://gitlab.com/dren.dk/mso5k-la-pod/tree/master/lmh7324-8ch

Roast my layout:)

[Rerouter]

Ok, 1.27mm non-inline header done, I'll make up my ribbon breakout board probably tonight.

[oliv3r]

Beautiful! and really good use of that attiny!! Just 2 question I have.

Why not make it a nice 50 x 50 mm, gives you more room to play with bigger ground cooling plane, and allt hese cheap PCB services houses have their minimum size set to 50 x 50 anyway. Size is probably smallest concern anyway, right?

One question remains, I do not recall the pinout from the scope-end; but do remember the original 2 channel LM's they 'split' the inputs between channel 0 and 8 for the first LM. The second LM goes to 1 and 9 and so on. Now if that's also how the signal enters, then it does not matter I think how you do it.

But with this new redisgn, you will end up with a really odd UI don't you think? So bit 0 - 8 on your probe, will correspond to the odd bits, and 9 - 16 to the even?

[Rerouter]

That's why the breakout board idea, to remap the channels back to the correct order, 2 layers is a pain for routing crossing differential pairs with decent impedance. So at this point I've drifted to 4 layer

[dren.dk]

I started out making my breakout board in 2 layers, but then I noticed that jlcpcb allows me to panelize two pods and a breakout for 15 usd, which means that I can get 4 layer boards for 20 pods and 10 breakout boards in black for 60 USD, so I'm going with 4 layers for my breakout too.

I've been trying to hit the correct impedance with the stackups offered by jlcpcb and I've taken the time to length match the wires of each pair, but not the inter-pair length.

Seeing that it was possible to route the 1.27mm pitch header, I switched to a 1.27 mm pitch header in smd and that freed up an enormous amount of space which made it much easier to route.

The idea with the breakout is to reorder the pin outs so the channel numbering on the scope fits with the numbering of the pods and channel 0-7 goes on one pod and 8-15 goes on the other

My breakout is designed to sit flat against the front of the scope with a tall female socket on the bottom, but it can also be used with a short cable to the scope.