Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements
11-21
From these calculations, it can be concluded that lower impedance lines result
in longer line lengths before termination is required. The lower impedance lines
are preferred over higher impedance lines because longer open lines are
possible, and the propagation delay down the line is reduced. In addition, more
stubbed-off gate loads can be driven with a terminated line due to its higher
capacitance per unit length.
Microstrip Transmission Line Techniques Evaluated
Using TDR Measurements
The time domain reflectometer (TDR) employs a step generator and an
oscilloscope in a system which might be described as "closed-loop radar." Refer
to Figure 11-7. In operation, a voltage step is propagated down the transmission
line under investigation. Both the incident and reflected voltage waves are
monitored on the oscilloscope at a particular point on the line.
Figure 11-7
Time Domain Reflectometer
For the examples the incident voltage setup, E1, is a positive edge with an
amplitude of 1 V and a risetime of 30 ps. It is generated from a source impedance
of 50 Ω. Also, the output edge has very little overshoot (less than ±5%).
This TDR technique reveals the characteristic impedance of the line under test.
It shows both the position and the nature (resistive, inductive, or capacitive) of
each discontinuity along the line, and signifies whether losses in a transmission
system are series losses or shunt losses. All of this information is immediately
available from the oscilloscope's display. An example of a microstrip line
evaluated with TDR techniques is shown below:
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