Digital Signal Processing Reference
In-Depth Information
transmission line having a delay of
5.6 ns/meter is found from (17.6) to be 482 nH/meter, and from (17.7) its capaci-
tance is 65 pF/meter.
For example, the inductance
L
of an 86
Ω
17.5 Finding Stripline Inductance and Capacitance When Impedance
Is Known
Because the dielectric is homogeneous in stripline (see Chapter 7), the inductance
and capacitance equations can be determined for that specific case when only the
impedance is known.
The inductance of a stripline is found with (17.8). Set
K
1 to 0.085 to obtain the
inductance in nH/inch. Use 0.033 to obtain the inductance in nH/cm.
Lsl
=×
Z
K
1
ε
(17.8)
o
r
The capacitance of a stripline is calculated with (17.9). Set
K
2 to 85 to obtain
the inductance in pF/inch. Use 33 to obtain the inductance in pF/cm.
K
2
ε
r
Csl
=
(17.9)
Z
o
stripline of any width or thickness is 3.4
pF/inch length when the dielectric constant (
For example, the capacitance of a 50
Ω
ε
r
) is 4.
17.6 Finding Stripline Inductance and Capacitance When Trace
Geometry Is Known
Inductance of a stripline trace can be estimated with (17.10). This equation has
been simplified and curve-fitted [3] from [5]. It assumes that the trace is centered
between the two planes. Since it does not account for trace thickness, it is only ac-
curate to within about 20% of the actual value.
b
(17.10)
LK
=×
+
3
wb
where
K
3 is 16 for results in nH/inch, or 6.3 for nH/cm.
The reciprocity principle [3] can be used to find the capacitance of stripline
(but not microstrip) from the inductance calculated in (17.10). Doing so yields
(17.11), and since it is derived from (17.10), it has the same level of accuracy.
wb
+
⎛
⎞
CK
=×
4
ε
(17.11)
⎜
⎟
r
⎝
⎠
b
