Digital Signal Processing Reference
In-Depth Information
the edge, while the other, identical trace is 100 mils (2.5 mm) from the other edge.
A side view appears in Figure 14.3(b).
The inductance of the stripline routed close to the board edge is about 10%
higher and the capacitance is over 15% lower than the other stripline. This causes
the impedance to grow by nearly 14% and the delay to fall by nearly 3%. Although
not shown, the impedance and delay of a 4-mil-wide, 50
stripline (which is sig-
nificantly closer to the return plane then the traces in Figure 14.3) only change by
about 2% under these same conditions.
Microstrip behaves in the same general way as stripline, with the biggest effects
seen on wide, high-impedance traces. Often 2D field solvers tacitly assume the re-
turn plane is very much wider than the trace. Figure 14.3 shows that circuit models
created with this assumption will not represent the characteristics of the traces
routed close to the edge. This can be a significant problem when signaling at high
speeds, particularly if the signaling is occurring with differential pairs.
Ω
14.4 Serpentine Traces
Often high-performance integrated circuits include delay locked loops (DLLs) or
similar circuits to digitally adjust the timing between signals [11, 12]. When these
circuits are not present or when it is necessary match the arrival of individual sig-
nals within a bus, a long trace can be used to add a known delay based on the trans-
mission lines time of flight (
tpd
). Because tpd is typically hundreds of picoseconds
per inch, adjusting timing usually requires several inches of trace.
These delay lines are created by folding the trace back and forth to create a
serpentine trace. They are also referred to as meander or trombone traces. A five-
segment serpentine is diagrammed in Figure 14.4.
Each of the five segments has a delay equal to
tpd
, making the expected total
delay 5
tpd
plus the delays of the short vertical connecting straps. However, the
actual delay will be less than this unless care is taken with the layout.
1
2
Kb1,2
3
4
Kb1,5
5
tpd
Figure 14.4
Five-segment serpentine delay line. Each long segment has a delay of
tpd
.
