TOPICS IN HIGH-SPEED CHANNEL MODELING - Advanced Signal Integrity for High-Speed Digital Design - page 425

C

D = 1 inch

TDR

TDT

V DD

50 Z 0 = 50

2 inch 2 inch

50

0.5 V

D = 1 inch

C

Ground

Figure 10-9 Transmission line routed onto a floating power island—a common

occurrence.

or an electromagnetic simulator can be used. However, for the purposes of this
example, the estimation is adequate. The propagation delay is therefore calculated
for both the microstrip and slotline:

√ 3 . 15

√ ε eff
c

10 − 12

t d = =

10 8 m/s ⇒

150 × s/in.

3 ×

Step 2: Create the equivalent circuit. Using the propagation delay, the electrical
length of each segment is calculated. The total length of the slot is the segment
between the decoupling capacitors. Since the capacitors are large, they will look
like a short for high-frequency return current. In the equivalent circuit, they are
placed at the end of the slot. The equivalent circuit is shown in Figure 10-10.
Step 3: Simulate the circuit model. The step response is shown in Figure 10-11.
The TDR is the waveform seen at the driver and the TDT is the waveform seen
at the termination resistor at the far end of the line (as labeled in Figure 10-10).

TDR TDT

V x

50

t d = 300 ps [VCVS] t d = 300 ps

Gain = 1

+ −

50 Ω 50 Ω

50

Driving line

I x

0.5 V

1 nF

t d = 150 p s [CCCS] t d = 150 ps

+

90 Ω 1 nF

1 nF 90 Ω

Slot V x I x

−

G = 1

Figure 10-10 Equivalent circuit of the structure shown in Figure 10-9.