# Re: lumped circuits and trasmission lines

Home Forums Transmission Lines lumped circuits and trasmission lines Re: lumped circuits and trasmission lines

#16357

No macro. There are three types of models in PSpice:
T, that is the easiest way to model a loseless transmission line,
you have to insert characteristic impedance Zo value and
the electrical lenght TD (for example if the signal velocity in a cable is V= 0.75*c, where
c is the speed of light, TD=20ns it means that 0.75*c*20ns= 4.5mtr ).
Often Zo is known, or for a coax cable you can find out as SQRT(L/C) where L and C are provided
by vendor or as Zo= 60/SQRT(Îµr) * ln(b/a) where Îµr is the dielectric constant, a is the diameter of the inner conductor and b is the diameter of the inside surface.
Tlossy models a lossy transmission line and a loseless line
with R=G=0. For a coax cable R is modeled as Rs/6.28 *(1/b+ 1/a), where Rs is the
skin effect resistivity [Ohm], the G parameter is 6.28* Ïƒ/(ln(b/a)), where Ïƒ is the conductivity [Siemens/m].
At last we have the Tlumped model.This is made by a finite numbers of sections of cable and can be used when
TR/TD >=4 in the single section.(TR is the rise time of the pulse). More are the sections and more accurate is the result.