Datasheet

SCEA022
7–258
Achieving Maximum Speed on Parallel Buses With Gunning Transceiver Logic (GTLP)
2 Physical Properties and Limitations of Bus Lines
The basic composition of a line consisting of capacitive and inductive replacement components
is shown in Figure 1. In the case of static conditions, line impedance primarily is determined by
the ohmic resistance and/or the parallel conductance of C′ of the line. These are not of any
consequence at a frequency of just a few kilohertz, because the frequency is in the term
ω = 2 × π × f.
R, G
At high frequencies, transmission-line losses on printed circuit boards in digital systems can be neglected.
R′/2 L′/2
C′ G′
L′/2 R′/2 L′/2
C′
L′/2
L′ = Characteristic Inductance Per Unit Length (nH/cm)
C′ = Characteristic Capacitance Per Unit Length (pF/cm)
R′ = Characteristic Resistance Per Unit Length (Ω/cm)
G′ = Characteristic Conductance Per Unit Length (S/cm)
With R′ << jωL′ and G′ << jωC′:
Line Impedance
Propagation Time
Z
0
+
LȀ
CȀ
Ǹ
(
real number !
)
t + LȀ CȀ
Ǹ
Z
³
0
+
jwLȀ)RȀ
jwCȀ)GȀ
Ǹ
Complex Line Impedance
Figure 1. Ideal Transmission Line With Negligible Conductance and Resistance
Comparing the apparent impedance for a specified frequency (e.g., 1 MHz) with the parallel
conductance G′ and the series resistance R′ shows that G′ and R′ are negligible compared with
the line impedance Z
o
.
The simplified formulae for line impedance and the propagation delay time per unit length are
dependent only on the inductive and capacitive layer of the line. In practice, calculations are
easy to handle. In the case of a homogeneous line, which represents a connection between a
transmitter and a receiver, its capacitive and inductive layers determine the line property.
However, if we look at bus lines, such as the one in Figure 2, the line impedance no longer is
constant and is dependent on the number of inserted modules.