From: Georg Ramsch (Georg.Ramsch@t-online.de)
Date: Wed May 16 2001 - 00:43:03 PDT
Hello Xiao !
Principally you are right; impedance "bumps" occur between 10-20 ohm at the vias
and 50 ohm on the transmission line.
But that is only really true for very short pulses (ex: TDR / 30 psec risetime ).
Longer risetimes cause waves to "ignore" such a "bumpy road"; or lets say bumps
become diffused as risetimes (resp. wavelenghts )
A comparision to water waves may be very imaginative: assume a wave travelling
along a river. Short wide gaps of river width will not affect the propagation of
the wave, but longer ones (at least 1/20 of the wavelength) will have an impact.
Same you can do with plane wave effects (power planes / INoise); when throwing
stones in a pond with a certain frequency, you will get a wave interference
pattern, which may be similiar to what happens between Vcc-Gnd-planes when exited
by current injections.
At frequencies we regard, those capacitive bumps can be regarded as integral part
of the transmisssion line ( not valid for higher frequencies, where the length of
the capacitive impact is in the range of a wavelength; here you have to apply
3D-field solvers to get the real effects, which also include inductive and
resistive stray elements).
Looking upon a via as an integral capacitive part of a transmission line is a rule
of thumb for engineers to make live easier and get faster results.
Not everybody can afford 2D- or 3D-field solvers @ 20.000 $ and up for 1 license.
Just look upon this as a "cooking receipt" to get an instant result instead of
doing a biochemical analysis of the whole meal.
By the way: it will not matter, whether you choose a 40 ohm transmission line or
65 ohm transmission line; both will work as the "impedance"
of the line goes down to 25 / 12.5 ohm with the effects of the drivers
50 ohm is chosen by many people as this has been done since the last 50 years and
only a few put this into question.
Long ago, scientists found, that at 33 ohm the energy throughput for a cylidrical
transmission line is a maximum, but attenuation is best at 75 ohm.
As the transmitted energy and received energy had to be guided on the same line, a
good compromise was to choose 50 ohm as the "middle".
So thats an old rule, which may need a new interpretation for todays needs, as it
came into existence when copper pipes were used as transmission lines
and design tools were slegde hammers and pipe wrenches.
Look at Futurebus physical design: terminations are defined 33 ohm at both ends,
which will be the effective impedance of a transmission line with vias and driver
cap. icluded with this slotpitch and connector type.
From my point of view I would choose 40 ohm, as manufacturing tolerances are
decreasing their effects at this level. Production yield will go up, thickness of
pcb will go down, PCB will become cheaper.
Hope, this explains a little bit the background of bussed lines.
Wang Xiao-yun wrote:
> Hello, George:
> I'm a bit confused with your explanation and here is my question.
> When a via is put on a transmission line of nominal 50 ohm, the impedance
> at that point and only that point is changed and therefore the signal will see
> an impedance mismatch at exactly the place where the via is. The rest of the
> segments of the line still has 50 ohm impedance.
> I agree the manufacturing tolerance of the impedance will range around
> +/- 10% at least, but it shouldn't be the reason to increase the impedance
> because it does no help. The only thing I can do is to route the tracks with
> 50 ohm target and simulate them with manufacturing tolerance in mind.
> I also agree with you that lower impedance will induced less crosstalk, and
> as a result I will be much more happy with a design with unique impedance of
> 50 ohm wherever possible. If I were asked to have some traces of 65 ohm, I
> would like to find out wether it's really necessary. Sometimes you may even
> have to do some extra work to accomplish it, for example, change your
> stackup a bit.
> Comments are appreciated.
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