HP 54753A User Manual page 245

Signal Propagation Delay for Microstrip and Strip Lines with Distributed or Lumped
Loads
An equation for maximum open line length for a strip line (using G-10 fiber glass
epoxy material) can be written in a similar fashion as follows:
C
D
2
l + ------ - l
max
max
C
o
Using the lattice diagram, it has been found that the rule of thumb used to derive
equations 12 and 13 should be modified for an open line because the incident
voltage doubles at the end of the line. This results in a faster risetime at the
receiving end of an unloaded line than at the driving end. An approximate value
of maximum open line length can be generated from equations 12 and 13 if the
risetime that is substituted into the equations is multiplied by an adjustment
factor, 0.75. This maintains an approximate overshoot and undershoot of less
than 35% and 12% respectively.
To demonstrate how equations 12 and 13 may be used, the maximum open line
length will be computed for a 50 Ω line with a fanout of one MECL 10K gate.
Using the equation t
pd
C = 2.96 pF/in for microstrip, and C
one, C is equal to 5 pF when the device is in a socket. The risetime for MECL
D
10K is 3.5 ns which means that a value of t
in the equations. Solving equations 12 and 13, l
= 6.2 inches for a 50 Ω strip line.
and l
max
Equations 12 and 13 can be very useful in finding the approximate maximum
line length under various conditions. Suggested maximum open line lengths for
MECL 10K/10KH and MECL III are tabulated in tables Table 11-1, Table 11-2,
and Table 11-3 for various fanouts and line impedances. For these tables, line
lengths are chosen to limit overshoot to 3.5% of logic swing and undershoot to
12%. Note that the tables give the maximum line lengths for fanouts of 1, 2, 4,
and 8 for various types of lines with a wide range of characteristic impedances.
Transmission Line Theory Applied to Digital Systems
2
– 7.1t = 0 (for strip lines)
r
= Z C , the line capacitance, C
o o
= 3.76 pF/in for strip line. For a fanout of
o
= 0.75 X 3.5 = 2.6 ns should be used
r
, is found to be
o
for a 50 Ω microstrip line
max
(13)
11-17