[SI-LIST] Re: Right Angle Bends
- From: Karthik Raj Guruchandran <karthik.guruchandran@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 15 Jul 2011 23:04:07 +0100
Bends certainly seemed to have an impact on the near field associated with
the trace which might turn out into an EMC issue if a radiating trace is
routed next to a tight bend. I did some simulations before but sorry dont
have any measurement data to back up.
Here is the link to the article if anyone's interested :
http://www.basebandhub.com/pdfs/bends.pdf
Cheers,
Karthik
On Fri, Jul 15, 2011 at 9:38 PM, Scott McMorrow <scott@xxxxxxxxxxxxx> wrote:
> 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® 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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