SIMPLORER User Manual V6.0 - FER-a

SIMPLORER 6.0 — Manual 165
The input voltage is a 1V pulse, 1ns wide. For the case of R=0, the input appears at the far
end, delayed by 1ns (LC curve). There is no attenuation because the line R is zero. There is no
reflection because the terminating resistance is equal to the characteristic impedance.
With high series resistance, the input is delayed 1ns as before, but distortion and attenuation
occur (RLC curve). With only series resistance and a very low shunt conductance, there is no
delay or distortion, but the output is attenuated (RG curve). Finally, with L=0 there is a delay
from the RC time constant, but the delay is not 1-ns (RC curve).
When the input pulse is very narrow compared to the line’s travel time, and the time step increases
during simulation, it is possible for the pulse to become lost. It never appears at the
far end. The breakpoint mechanism in the LTRA model does not fully address this problem,
even when running in SPICE itself. A very small resistance value (for example R=1u) usually
solves this problem.
See also Roychowdhury, J. S., and D. O. Pederson: Efficient Transient Simulation of Lossy Interconnect,
San Francisco, CA: ACM/IEEE Design Automation Conference, June 17-21, 1991
Line Model Applications
SPICE 3f5 implements three different line models:
• LTRA - the lossy transmission line model, implemented with convolution of the impulse
response.
• T - the lossless transmission line, implementing an idealized traveling wave behavior.
• URC - the uniform RC line, sometimes useful in low-frequency applications.
For the lossless case, LTRA is typically more accurate and efficient than the older T model.
The URC model automatically creates an internal lumped RC network for SPICE, which is
rather inefficient, and again, less accurate than the truly distributed-parameter LTRA model.
Therefore, only the LTRA model has been provided for SIMPLORER. All instances of the T and
URC model should be converted to use the LTRA model.
Converting Lossless T Components to LTRA
The basic T component parameters are the characteristic impedance, and the line travel or
delay time. To derive LTRA parameters, it's necessary to assume a wave velocity. For example,
3e8 m/s (speed of light) will be assumed here. A different velocity may be assumed, and the
derived parameters would be different but produce the same results in circuit simulation.
The characteristic impedance is directly available as Z0. The travel time may be input directly
as TD, or indirectly as a frequency and normalized line length. If necessary, obtain the travel
time as:
• TD = NRMLEN / FREQ
If not specified, NRMLEN = 0.25 for a quarter wavelength.
Next, obtain the length and the other LTRA parameters as:
• LEN = 3e8 * TD
• L = Z0 / 3e8
• C = 1 / (Z0 * 3e8)
• R = G = 0
For example, given Z0 = 50 and TD = 1n, we would obtain LEN = 0.3, L = 1.667e-7, and C
= 6.667e-11. Assuming a velocity of 2e8 instead of 3e8, we would obtain LEN = 0.2, L = 2.5e-
7, and C = 1e-10 as in the example of Figure 2. Both parameter sets would give the same results
in SIMPLORER.