# specify resistor name followed by three nodes

Discussion in 'CAD' started by maya, Aug 6, 2007.

1. ### mayaGuest

how to define in spice for specify resistor name followed by three
nodes, model name and a user
defined equation to get resistance value.

2. ### Kevin AylwardGuest

The simplest way is to use the Spice "URC" - Uniform distributed RC
transmission line:

3.3.5. Uniform Distributed RC Model (URC)

The URC model is derived from a model proposed by L.
Gertzberrg in 1974. The model is accomplished by a subcir-
cuit type expansion of the URC line into a network of lumped
RC segments with internally generated nodes. The RC seg-
ments are in a geometric progression, increasing toward the
middle of the URC line, with K as a proportionality con-
stant. The number of lumped segments used, if not specified
for the URC line device, is determined by the following for-
mula:
2
| R C |(K-1)| |
_ _ 2
log|F 2 J L |-----| |
max
| L L | K | |
N = ------------------------------
log K
The URC line is made up strictly of resistor and capa-
citor segments unless the ISPERL parameter is given a non-
zero value, in which case the capacitors are replaced with
reverse biased diodes with a zero-bias junction capacitance
equivalent to the capacitance replaced, and with a satura-
tion current of ISPERL amps per meter of transmission line
and an optional series resistance equivalent to RSPERL ohms
per meter.
name parameter units default
example area
1 K Propagation Constant - 2.0
1.2 -
2 FMAX Maximum Frequency of interest Hz 1.0G
6.5Meg -
3 RPERL Resistance per unit length Z/m 1000
-
4 CPERL Capacitance per unit length F/m 1.0e-15
-
5 ISPERL Saturation Current per unit length A/m
- -
6 RSPERL Diode Resistance per unit length Z/m
- -

and called by in the spice netlist:

3.3.4. Uniform Distributed RC Lines (Lossy)
General form:
UXXXXXXX N1 N2 N3 MNAME L=LEN <N=LUMPS>
Examples:
U1 1 2 0 URCMOD L=50U
URC2 1 12 2 UMODL l=1MIL N=6
N1 and N2 are the two element nodes the RC line con-
nects, while N3 is the node to which the capacitances are
connected. MNAME is the model name, LEN is the length of
the RC line in meters. LUMPS, if specified, is the number
of lumped segments to use in modeling the RC line (see the
model description for the action taken if this parameter is
omitted).
e.g..MODEL ResisterInTankURC_XN urc(rperl=1G cperl=100p)Most GUI spice
should allow you do do this all automatically. e.g SuperSpice has an
example, and a GUI setup for that model.