[SI-LIST] Re: Right Angle Bends
- From: "Brad Brim" <bradb@xxxxxxxxxxx>
- To: "'Jeff Walden'" <jwalden@xxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 15 Jul 2011 11:56:33 -0700
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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