[SI-LIST] Re: Right Angle Bends
- From: Jeff Walden <jwalden@xxxxxxxx>
- To: Scott McMorrow <scott@xxxxxxxxxxxxx>
- Date: Fri, 15 Jul 2011 19:29:40 -0400
@Scott hahaha .. cocktail time on a Friday eh? Current PCB manufacturing
capability as well as simulation technology really negates and obsoletes
decades old technology. With great caution do I ever consider relevant
"papers" over 5 years old. I know owners of improved via tech, for instance,
but are mechanical guys and not getting it out there very well ==
frustrating.
On Fri, Jul 15, 2011 at 6:56 PM, Scott McMorrow <scott@xxxxxxxxxxxxx> wrote:
> Lee
>
> Agreed. Vias are much more worserer than right angle bends, whether you
> are dealing with digital or RF signals. This is why low noise RF
> designs are primarily microstrip.
>
>
> regards,
>
> Scott
>
>
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> (401) 284-1827 Business
> (401) 284-1840 Fax
>
> http://www.teraspeed.com
>
> Teraspeed® is the registered service mark of
> Teraspeed Consulting Group LLC
>
>
> On 7/15/2011 6:47 PM, Lee Ritchey wrote:
> > It is odd that those who fret over right angle bends seem to ignore
> > the fact that there is at least on at every via where a trace
> > connects. My old friend, Dan Murphy, called this "looking where the
> > light is good."
> >
> > The same engineers used dual in line IC packages by the millions with
> > as many as three right angle bends in each lead. They weren't
> > visible, so they didn't count!
> >
> > --------------------------------------------------
> > From: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
> > Sent: Friday, July 15, 2011 2:02 PM
> > To: "Julian Ferry" <julian.ferry@xxxxxxxxxx>
> > Cc: <si-list@xxxxxxxxxxxxx>
> > Subject: [SI-LIST] Re: Right Angle Bends
> >
> >> It depends on who you ask.
> >>
> >>
> >> Scott McMorrow
> >> Teraspeed Consulting Group LLC
> >> 121 North River Drive
> >> Narragansett, RI 02882
> >> (401) 284-1827 Business
> >> (401) 284-1840 Fax
> >>
> >> http://www.teraspeed.com
> >>
> >> Teraspeed® is the registered service mark of
> >> Teraspeed Consulting Group LLC
> >>
> >>
> >> On 7/15/2011 4:53 PM, Julian Ferry wrote:
> >>> That's a good point.
> >>>
> >>> Who's that guy who always says "It depends?".....
> >>>
> >>>
> >>>
> >>> -----Original Message-----
> >>> From: si-list-bounce@xxxxxxxxxxxxx
> >>> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Scott McMorrow
> >>> Sent: Friday, July 15, 2011 4:39 PM
> >>> To: si-list@xxxxxxxxxxxxx
> >>> Subject: [SI-LIST] Re: Right Angle Bends
> >>>
> >>> Julian
> >>>
> >>> Actually, it depends on the frequency and the trace thickness, since
> >>> capacitive coupling on the sidewall of the trace at the corner can
> >>> cause
> >>> a net decrease in capacitance at some frequencies.
> >>>
> >>>
> >>> Scott McMorrow
> >>> Teraspeed Consulting Group LLC
> >>> 121 North River Drive
> >>> Narragansett, RI 02882
> >>> (401) 284-1827 Business
> >>> (401) 284-1840 Fax
> >>>
> >>> http://www.teraspeed.com
> >>>
> >>> Teraspeed(r) is the registered service mark of
> >>> Teraspeed Consulting Group LLC
> >>>
> >>>
> >>> On 7/15/2011 4:05 PM, Julian Ferry wrote:
> >>>> OK, looks like I made one little not-so minor mistake in my
> >>>> previous email:
> >>>>
> >>>> The capacitance goes UP, the inductance goes DOWN.
> >>>>
> >>>> Sorry for any confusion I might have caused!
> >>>>
> >>>> Julian Ferry
> >>>> High Speed Engineering Manager
> >>>> Samtec, Inc
> >>>>
> >>>> -----Original Message-----
> >>>> From: si-list-bounce@xxxxxxxxxxxxx
> >>>> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Julian Ferry
> >>>> Sent: Friday, July 15, 2011 3:48 PM
> >>>> To: Brad Brim; 'Jeff Walden'; si-list@xxxxxxxxxxxxx
> >>>> Subject: [SI-LIST] Re: Right Angle Bends
> >>>>
> >>>>
> >>>> I think one of the problems here is that like with most myths or
> >>>> urban legends, there is a kernel of truth to it.
> >>>>
> >>>> It is pure, unarguable physics that the capacitance will go down
> >>>> and the inductance will go up in a right angle bend. The question
> >>>> is whether that change matters in a particular application.
> >>>>
> >>>> For some applications, like in high power microwave or RF
> >>>> applications, this effect can be significant enough relative to
> >>>> other factors that it is definitely worth worrying about.
> >>>>
> >>>> But in the grand scheme of things in the current SI world, this
> >>>> change is way down on the list of potential problems. It is
> >>>> effectively swamped out by many other effects (like our relatively
> >>>> crappy connectors, for one example...)
> >>>>
> >>>> I think these guys wrote a pretty decent little paper with some
> >>>> math that can help determine whether you might need to worry about
> >>>> bends.
> >>>>
> >>>>
> http://www.millertechinc.com/pdf_files/mti_tn063_microstrip_right_angle_bends.pdf
> >>>>
> >>>>
> >>>>
> >>>> Also keep in mind that we in the SI world are mostly dealing with
> >>>> pulsed signals, where a large portion of the energy is contained in
> >>>> the lower frequency components. The excess capacitance will only
> >>>> affect the very high frequency components. But in the microwave
> >>>> environment, it's all about the high frequency signal, so a bend
> >>>> can be a much greater concern.
> >>>>
> >>>>
> >>>>
> >>>> Julian Ferry
> >>>> High Speed Engineering Manager
> >>>> Samtec, Inc
> >>>>
> >>>> -----Original Message-----
> >>>> From: si-list-bounce@xxxxxxxxxxxxx
> >>>> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Brad Brim
> >>>> Sent: Friday, July 15, 2011 2:57 PM
> >>>> To: 'Jeff Walden'; si-list@xxxxxxxxxxxxx
> >>>> Subject: [SI-LIST] Re: Right Angle Bends
> >>>>
> >>>> exactly, Jeff ... and the traces that connect to the bends are also
> >>>> MUCH
> >>>> longer than typical for the RF/microwave case.
> >>>>
> >>>> In RF/mw ckt sim libraries/layouts a bend is a separate
> >>>> "component". The
> >>>> reference planes (i.e. where the traces connect to the bend) are at
> >>>> the edge
> >>>> of this component. For example, the 90-degree bend reference planes
> >>>> are at
> >>>> the edge of the square representing the area of the bend. For most SI
> >>>> applications the bend is NOT a separate component and the two
> >>>> traces simply
> >>>> meet at a single node. Having worked way too many hours to
> >>>> implement and
> >>>> test RF/mw ckt sim bend models over a dozen years ago I observed the
> >>>> parasitics are a delay of length on the order of the node-to-node
> >>>> distance
> >>>> with additional capacitive parasitics for sharp bends and inductive
> >>>> parasitics for aggressively chamfered bends. For a 90 degree bend the
> >>>> different definitions of reference plane imply 2*(W/2) additional
> >>>> length
> >>>> trace for the SI case. Given approximate parasitic delay of
> >>>> sqrt(2)*W/2, all
> >>>> implies doing nothing for SI applications is still on the order of
> >>>> only 30%
> >>>> phase delay error versus a much more precise parasitic model (for
> >>>> an already
> >>>> small parasitic). The phase delay dominates because bend capacitive
> >>>> parasitics are small for SI apps relative to other capacitive
> >>>> parasitics not
> >>>> modeled throughout the system.
> >>>>
> >>>> Therefore, if the trace are not wide (low impedance) and their lengths
> >>>> coming in/out of the bend are long relative to the linewidth, then
> >>>> ignoring
> >>>> the bend is obviously the correct choice.
> >>>>
> >>>> Where SI apps might not always want to ignore bends is for tight
> >>>> meander
> >>>> structures used to accumulate phase delay and balance skew. These
> >>>> geometries
> >>>> sometimes have short distance between bends and could therefore
> >>>> lose some
> >>>> accuracy from ignoring bend parasitics. In these cases it is
> >>>> probably more
> >>>> important to include coupling amongst the parallel traces. As we
> >>>> all might
> >>>> guess, if you need to know a meander behavior accurately you may
> >>>> wish to
> >>>> model it as a single component with more detailed simulation rather
> >>>> than
> >>>> treat it as a collection of traces (with or without bend parasitics).
> >>>>
> >>>> cheers,
> >>>> -Brad
> >>>>
> >>>>> -----Original Message-----
> >>>>> From: si-list-bounce@xxxxxxxxxxxxx
> >>>>> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Jeff Walden
> >>>>> Sent: Friday, July 15, 2011 10:55 AM
> >>>>> To: si-list@xxxxxxxxxxxxx
> >>>>> Subject: [SI-LIST] Re: Right Angle Bends
> >>>>>
> >>>>> The difference is that today's SI traces are significantly
> >>>>> narrower than the typical "RF" microstrip of 30 years ago.
> >>>>> -Jeff
> >>>>>
> >>>>>
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--
Jeffrey L Walden
EMC/SI RF analysis and product development
jwalden@xxxxxxxx
(866)547-5365
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