Complex electrical term/ratio/quantity with symbol, Y, analogous to the inverse of impedance, Z, of a circuit, i.e., the ration of the current to the voltage in a circuit taking into account phase. Y = I / V
A standard physics term for the velocity of light in vacuum/air (absolute). c = 3 x 108 m/sec. Also inversely related to absolute To (Air Time Delay) = 3.3333 ns/m.
Critical Length is the unique length, Lcrit, and demarcation point if the end of a Short Line and the beginning of a Long Line (i.e. a transmission line); where Lcrit = Trise / (2 To e**1/2), where Trise is the rise time of the digital signal; and To is the line time delay of air, and e is the material dielectric constant of the interconnect medium. When L of an interconnect < Lcrit, the interconnect is a Short Line. When L > Lcrit then the Interconnect is a Long Line or a Transmission Line; and must be modeled and designed as such in a signal line analysis and design.
Critical Length Ratio Function
Mathematically the function can be expressed as (2 TD / Trise) where TD is the electrical length in "time" of the interconnect; and Trise is the characteristic Signal Rise Time.
Free space impedance (also known as free space intrinsic) is 377 ohms. The value derived from the square root of the ratio of the permeability constant of vacuum ( mu o = 4 pi x 10**-7 Henries/meter) to the permittivity constant of vacuum (epsilon o = 8.854 x 10**-12 Farads/meter), the absolute values of free space or vacuum. "Intrinsic" relates to a given pure material medium.
Length to Critical Length Ratio
This normalized length ratio (either physical or electrical permissible) - when the ratio is < 1, then the interconnect must modeled and treated as a Short Line or a lumped element circuit; and if <<1 , it is then a Very Short Line and can be neglected in transmission line analysis (the signal therefore does not see this interconnect in its path and it can be safely ignored). If the ratio is > 1, then it must be treated and modeled as a Long Line or a distributed-element transmission line.
Interconnect of length, L, where must be modeled with distributed elements and as a transmission line. Ratio of its length to a critical length ((L/Lcrit) >1); where L / Lcrit = 2 TD / Trise; where TD is the interconnect's electrical length; and T rise is the rise time of the digital signal.
Greek letter used to represent the ratio of reflected voltage measurement to the initial or launch voltage (Reflection Coefficient of an Interconnect within a transmission line). Also equal to the ratio of the Characteristic Impedances in the following expression: = (Z2 - Zo) / (Z2 + Zo) ; where Zo is the line impedance and Z2 is the interconnect or mismatch impedance of the discontinuity.
Saturated NEXT or Saturated Near End Cross Talk is also a function of the Critical Length Ratio. For the victim line with a length (physical or electrical) and having a Critical Length Ratio < 1, then the victim line is unsaturated in NEXT. With a victim line having a Critical Length Ratio > 1, then the victim line is with its maximum NEXT or is fully saturated in NEXT.
Interconnect discontinuity adequately modeled by Lumped Element Circuits; a line of length, L ,when ratioed to a Critical Line, then ((L / Lcrit) < 1): where L / Lcrit = 2 TD / Trise. TD is the electrical length of the interconnect = L / Vp; Vp = Interconnect line velocity of propagation; Trise is the digital signal rise time.
Normalized Time Delay (usually in nanoseconds/meter) which characterizes the delay per unit length of a signal traveling along a transmission line in a medium of dielectric constant (e). Specifically, Td = To (air) x square root of e (ns/m), where e is the material dielectric constant. For air, e = 1.
An interconnect of length (L) that is smaller than 1/20 of a signal's effective wavelength.
Signal Velocity of Propagation. An absolute term for velocity of a signal in a given transmission line material medium (meters/nanosecond). Vp = c when medium is equal to air. Vp is inversely related to Td (Time Delay - Normalized).
Relative Signal Velocity of Propagation. Sometimes multiplied by 100 in order to be expressed as a percentage. vp = Vp / c