[Blabber] Help with microstrip traces

Guan Yang guan at yang.dk
Tue May 14 13:45:18 UTC 2013


It doesn't depend on how far apart they are and dielectric etc?

For what it's worth, when I use a random online calculator and double the
trace width, the characteristic impedance is not halved.


On Tue, May 14, 2013 at 9:31 AM, Chris Stratton <cs07024 at gmail.com> wrote:

> It still combines the same way.
> On May 14, 2013 9:28 AM, "Guan Yang" <guan at yang.dk> wrote:
>
>> Dave, characteristic impedance is not the same thing as DC resistance.
>>
>>
>> On Tue, May 14, 2013 at 8:26 AM, David Reeves <drreeves at gmail.com> wrote:
>>
>>> Could you make 2 parallel traces of 100 ohm? From 1/((1/R1)+(1/R2)) that
>>> would be 50 ohm.
>>>
>>>
>>> On Tue, May 14, 2013 at 8:22 AM, Ian Harris <imharris at gmail.com> wrote:
>>>
>>>> Hi guys,
>>>>
>>>> So I'm stuck.  I'm building a wifi module and I need to create a 50 ohm
>>>> trace from the module to the antenna.
>>>>
>>>> Looking on the internet, I can find impedance calculators, and after
>>>> poking around it seems that I need to use a four layer board (since in two
>>>> layer boards the dielectric is just too thick).
>>>>
>>>> So in a four layer board, I can find examples where the dielectric<http://www.expresspcb.com/expresspcbhtm/specs4layerstandard.htm>is 0.017" (17mil).  Copper is, say 1oz (which the internet
>>>> tells me <http://www.pcbuniverse.com/articles.php?a=4> is 1.37mil
>>>> "high").  The dielectric constant is 4.6 (same source).
>>>>
>>>> Now according to a calculator<http://emclab.mst.edu/pcbtlc2/microstrip/>,
>>>> the required strip width (trial and error) for 50R is around 30mil.
>>>>
>>>> However, on the demo board for this module, the antenna trace, while a
>>>> reasonably solid trace is *almost* the SMD pad width - but not quite -
>>>> and the SMD pad width is 27mil. If I had to guess, I'd say it's around
>>>> 20mil.
>>>>
>>>> So how do they get the impedance down to 50R? The calculator says that
>>>> a 20mil trace / 1.37mil copper / dielectric (17mil) / 4.6 constant = 88R.
>>>>  I'm presuming that's not close enough.
>>>>
>>>> Change copper from 1oz to 0.5oz makes almost no difference.
>>>>
>>>> Any ideas?
>>>>
>>>> kind regards
>>>> Ian.
>>>>
>>>>
>>>>
>>>>
>>>>
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>>>
>>>
>>
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