Hi Gary
If the question was whether this has ever been done, the answer is yes.
Some very cheap set-top-boxes have used this method in the past.
I do not know what tools would have been used to determine the track and
gap figures, more likely they were worked out by the silicon vendor who may
also have provided a reference design to copy. As the volumes of such
products are large the cost of top-end SI tools can make this a lower cost
route by taking the price out of the laminate.
But the question whether you should do this on your own board can probably
only be answered by you putting in the numbers into simulators capable of
giving good approximations to the real board performance and then you
deciding if they are good enough for your needs. As you say, the free
simulators have rules beyond which their approximations break down. Other
simulators can handle this, but I'm not aware of any free ones.
Regards
Jonathan
On 7 April 2017 at 20:44, Alan Hilton-Nickel <alan@xxxxxxxxxxxxxxxxx> wrote:
Hi Gary,
If you're looking for permission, you're asking the wrong group. ;-)
There is nothing inherently "wrong" with doing what you're suggesting.
However, you need to be aware of a few things:
1. Are you maintaining a consistent physical relationship between
your signal and ground? If this is a mass-produced item,
manufacturing variation may make it difficult to maintain that
relationship precisely between iterations of the same product.
2. Are you maintaining that relationship as much as possible when
you change structures, say to a via to another layer?
3. What is happening on the adjacent layer? There may be other
signals crossing the path, or there may be another plane influencing
the impedance of your signal. A thick PCB is no guarantee of isolation.
4. How are you getting power to the electronics?
5. Is this the only high speed signal? If not, how dense is the high
speed routing?
These questions, and their answers, change SIGNIFICANTLY with the a)
frequency/edge rate, b) whether this is a clock or NRZ signal, and c)
the power requirements of the board.
Generally speaking, unless you are under extreme pressure to reduce
cost, and have a thorough understanding of the pitfalls in your
signaling environment, I doubt there is a compelling reason to run a
coplanar return path instead of a stripline or microstrip. I also think
it will be tricky to get a good differential trace this way, at least if
you want it to be tightly (or even somewhat) coupled - common-mode
currents behave more predictably and favorably when the path is
symmetrical.
In the end, it's up to you. How comfortable are YOU with this setup?
Alan Hilton-Nickel
On 4/7/2017 12:13 PM, Gary Giust wrote:
When laying out microstrip 50 Ohm single-ended or 100 Ohm differential
traces, it is OK to use the same layer as the reference for targeting
the trace impedance (if the laminate is very thick between adjacent
signal layers in a 2-layer stack up, e.g. 55 mils)? The signal traces
are routed next to ground on the same layer. I've only seen online PCB
calculators with a plane below the signal trace(s). Are there any online
calculators to compute the impedance when the ground reference is on the
same layer?
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