11
Figure 6. Single RC Lump.
OrCAD provides the following RC lumped models in the transmission line library:
Model Description Parameters
TLURC1 1 RC Lump R, C, LENGTH
TLURC2 2 RC Lumps R, C, LENGTH
TLURC4 4 RC Lumps R, C, LENGTH
TLURC8 8 RC Lumps R, C, LENGTH
TLURC16 16 RC Lumps R, C, LENGTH
TLURC32 32 RC Lumps R, C, LENGTH
TLURC64 64 RC Lumps R, C, LENGTH
TLURC128 128 RC Lumps R, C, LENGTH
Table 3. RC Lumped models.
For TLURC64, the value of R for one lump is R*LENGTH/64.
A distributed RC line can be realized with a cascade of T-sections, each T like that of figure 6. If the
number of sections were infinite, the governing equations would be
These combine to produce the diffusion equation
The "Elmore delay" of this line is
d
=0.5 R C
L
2
+ R C
L
*L + R
d
*C
L
+ R
d
C
L
, where R
d
is the driver
resistance, C
L
is the load capacitance, and L is the line length. PSpice can be used to predict this delay
for heterogeneous networks.
Lumped RLCG models (TLUMPx)
When L/R>>1 or C/G>>1, the per unit length inductance should be included in the lumped model.
dt
dV
C
dz
dI
RI
dz
dV
dt
dV
RC
dz
V d
2
2
