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copper traces to nowhere on MSI motherboard
I have an MSI Z68A-G43(G3) desktop motherboard with an Asmedia ASM1083 PCI-E to PCI bridge chip. That chip seems to be designed to connect to 2 thin serpentine copper traces, each about 2" long, that go nowhere. On this particular motherboard they're not used at all because the jumpers that normally connect to the chip (labeled SIM1 and SIM2 in the picture below) aren't installed, and neither are the header pins for the jumpers. Notice that each serpentine trace has 2 traces adjacent to it that follow it, only they're designed to not be connected to anything at either end.
Asmedia hasn't replied about their chip, and I can't find a datasheet or pinout, but does anybody know the purpose of unconnected serpentine traces like these? |
#2
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copper traces to nowhere on MSI motherboard
I forgot the link to the photos of the traces to nowhe
http://imageshack.us/photo/my-images...tonowhere.jpg/ |
#3
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copper traces to nowhere on MSI motherboard
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#4
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copper traces to nowhere on MSI motherboard
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#5
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copper traces to nowhere on MSI motherboard
wrote:
I have an MSI Z68A-G43(G3) desktop motherboard with an Asmedia ASM1083 PCI-E to PCI bridge chip. That chip seems to be designed to connect to 2 thin serpentine copper traces, each about 2" long, that go nowhere. On this particular motherboard they're not used at all because the jumpers that normally connect to the chip (labeled SIM1 and SIM2 in the picture below) aren't installed, and neither are the header pins for the jumpers. Notice that each serpentine trace has 2 traces adjacent to it that follow it, only they're designed to not be connected to anything at either end. Asmedia hasn't replied about their chip, and I can't find a datasheet or pinout, but does anybody know the purpose of unconnected serpentine traces like these? At first I thought differential TDR and test coupon. (If you're bored...) http://www.ewh.ieee.org/r5/denver/ro...00/diffimp.pdf But the inner layers of that four layer stackup, should be copper planes, and the layup would likely be microstrip. Meaning the top and bottom traces can't "see" each other. - ----- VCC ----- GND - And that tells me, it's more likely to be two single-ended TDR measurements, to verify single ended impedance on the top and bottom layer microstrip. If they'd put more tracks on the top layer, then it might be a test coupon for differential impedance check. The outer tracks could have been intended as guard traces, but with no pads to make connections to them at all, I'm not at all sure what their purpose would be. Could you do diff TDR between the outer traces ? Would the center copper track invalidate the results ? With no pads on the ends of the tracks, it's hard to see how they'd connect the test head. Diff TDR? Clamp right to the track ? | | v v - - - ----- VCC ----- GND - - - Just a guess, Paul |
#6
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copper traces to nowhere on MSI motherboard
On Mon, 10 Dec 2012 11:33:36 -0800, larrymoencurly wrote:
I forgot the link to the photos of the traces to nowhe http://imageshack.us/photo/my-images...tonowhere.jpg/ Impedance matching stub? |
#7
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copper traces to nowhere on MSI motherboard
Why wibbly wobbly? Something tells me that this forms an inductance.... but going to nowhere! Ah, but wait! Notice the outer tracks... this would form a capacitance to the opposite layer, if UHF circuitry this is what they do to form 'tuned circuits' in UHF circuits so we'd end up with a self resonant series tuned circuit - these are used in traps etc, to null a specific frequency. It could of course form a capacitance which would only be a pF (pico Farad) or so. Mobo's use multilayer boards, if there was anything connected to this there *certainly* have been a via - a plated through hole connection, it could also add coupling to whats on the other side of the board, the board forming the dielectric of a capacitor. If it was an antenna then it certainly would not have had a guard track on the outside. The length, the shape and the outer track are telling something, let you decide. davy |
#8
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copper traces to nowhere on MSI motherboard
davy wrote:
Why wibbly wobbly? Something tells me that this forms an inductance.... but going to nowhere! Ah, but wait! Notice the outer tracks... this would form a capacitance to the opposite layer, if UHF circuitry this is what they do to form 'tuned circuits' in UHF circuits so we'd end up with a self resonant series tuned circuit - these are used in traps etc, to null a specific frequency. It could of course form a capacitance which would only be a pF (pico Farad) or so. Mobo's use multilayer boards, if there was anything connected to this there *certainly* have been a via - a plated through hole connection, it could also add coupling to whats on the other side of the board, the board forming the dielectric of a capacitor. If it was an antenna then it certainly would not have had a guard track on the outside. The length, the shape and the outer track are telling something, let you decide. davy It's not an antenna. Of that, I'm sure. You could design a better antenna than that. I worked with an engineer that did 10GHz circuitry, and he used to draw structures in copper (like, filters), and his handy work looks nothing like that. That track was drawn by a digital designer, not an RF designer. An RF designer could not resist the temptation to augment that simple design. That's the kind of track I'd draw in a PCB (as a digital designer), as I know next to nothing about good microwave RF practice. The serpentine layout is typically used to equalize track length on multi-lane busses. The designer of that PCB, is using serpentine, in order to get a decent length of track for the TDR. The TDR needs enough track to work with, so you can see the "impedance bump" in the trace, and make sense of it. The serpentine uses "soft curves" so the width of the copper track will not vary at the corners. If you did a "square" layout for the serpentine, the track impedance would be disturbed above 1GHz (corners start to become an issue). I recognized immediately, the lack of vias. If there were vias, I would have dismissed the idea of TDR and test coupon. If there were vias present, then the track could be connected to some circuit. As near as I can tell, and especially with the "SIM1" thing printed on the silk screen, I don't get the impression it's intended to be part of the design. It's not a functional part. With no vias, it's a relatively clean setup to test the impedance of a track. If the engineer specifies "controlled impedance" when ordering the PCB, then both parties (PCB house, engineer) need a means to verify they got what was expected in the order. Normally, the test coupon appears outside the perimeter of the final board shipped to customers, for test coupons intended to verify the PCB shop fulfilled specs. I've had lots of PCBs that failed the test coupon (this was caught in production, and the check is done during incoming inspection of materials). The impedance might be controlled to around the 5% level. The shop making the PCBs, the operator there is very experienced in selecting materials for the PCB and ensuring they give the desired result. For example, I might specify a certain thickness for a layer, and the operator at the shop will use a $5000 software package, phone me up and tell my I'm full of crap. They also see lot to lot variation in materials, and the operator will adjust the stackup as he sees fit. But in the end, at the engineering end, the impedance can't vary too much from the target, or it could affect signal integrity, as high speed signals leave the PCB and go to add-in cards (like, PCI Express tracks). As long as those tracks show no signs of being connected to some other part of the board, then, the structure is for testing. Not functionality. The structure exists on both sides of the board, as if the designer wants to check layer 1 and layer 4 impedance. If the solder mask had been rolled back a bit, so the outer two tracks of the three track set was tinned on the end, I'd assume the intention was to connect to those tracks. But it just doesn't look like the designer intended for test equipment to be connected to those tracks. So I don't see what was intended for the outside tracks. You wouldn't want to have to scrape the solder mask off, to make connections to those outside tracks. Even tinning the ends of the tracks would help in that case. Paul |
#9
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copper traces to nowhere on MSI motherboard
On 12/11/2012 12:18 AM, Loren Pechtel wrote:
On Mon, 10 Dec 2012 11:33:36 -0800 (PST), wrote: I forgot the link to the photos of the traces to nowhe http://imageshack.us/photo/my-images...tonowhere.jpg/ I think that's probably to support something that's not installed on that board. How about thinking that this an Antenna??? |
#10
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copper traces to nowhere on MSI motherboard
hp wrote:
On 12/11/2012 12:18 AM, Loren Pechtel wrote: On Mon, 10 Dec 2012 11:33:36 -0800 (PST), wrote: I forgot the link to the photos of the traces to nowhe http://imageshack.us/photo/my-images...tonowhere.jpg/ I think that's probably to support something that's not installed on that board. How about thinking that this an Antenna??? To see if this is the case, why not do an "image search" on your favorite search engine, and see how patch antennas are constructed. Then compare to the pattern in the above image. It's not one of these. http://ts2.mm.bing.net/th?id=I.49835...64697&pid=15.1 This one has copper plates on the end of each squiggle. http://www.digdice.com/wp-content/up...el-antenna.jpg More plates on this one. Note the dimensions of the conductors are constantly changing. A microwave RF designer designed this. PCB CAD tools don't like to draw apertures like this, and this was "hand forced" into the CAD tool. http://ts3.mm.bing.net/th?id=H.48453...98262&pid=15.1 The antennas on things like USB Wifi, are rather timid looking. I would say a non-microwave designer drew this. No finesse. http://www.wireless-home-network-mad...fi-antenna.png Anyway, look for images of PCBs, with a built-in antenna, and see if you can match Larry's structure. One other thing. An antenna designer, would probably not use vias in the antenna path, and would draw the antenna on the same PCB layer, as the semiconductor feeding it. It generally isn't a good idea to have header pins on an RF trace. Paul |
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