HP 54753A User Manual page 258

Figure 11-15
Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements
A related experiment was performed to find the reflection due to a ground plane
near the active line, but not directly under it. The test configuration and test
results are shown in Figure 11-15.
Effects of Ground Plane Discontinuity
As indicated by the TDR measurement, the reflection is about 36%. Again using
equation 15, the impedance of the 2½ inch strip can be calculated:
Z =
line
The reason for the reflection is the change in the characteristic impedance along
with the line resulting from the ground plane not being under part of the active
line. In such a region, capacitance of the line to ground decreases while the
inductance of the line increases, the net result being a higher characteristic
impedance.
11-30
1 + 0.36
•
------------------- 50
= 106ohms
1 0.36 –