Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements
11-31
It must be remembered that the TDR input waveform has a risetime of 28 ps.
Consequently, in a real logic circuit situation where perhaps, a MECL III gate
with a 1 ns risetime is driving the line the reflection would actually be less than
27%, not 36% as in this example.
1
This can be determined by scaling the value
of ρ found with the TDR waveshape in Figure 11-15B, with a 1 ns risetime. When
the length of the ground plane discontinuity is less than the distance travelled
by the signal during its risetime, then the reflection coefficient can also be
calculated as:
(23)
where: t
pd
= the propagation delay time of the line in ns / in.
t
r
= the risetime of the signal in ns
l = the length of the discontinuity in inches
ρ = the reflection coefficient for 2l
pd
/ t
r
≥ 1
(in this case the value found with the TDR waveshape
with t
r
= 28 ns).
For a discontinuity in the ground plane of 2.5 inches length, a propagation delay
of the line of 0.15 ns / in, and a MECL III gate with 1 ns risetime, the percent
reflected voltage can be calculated. From Figure 11-15B, ρ is found to be 0.36.
Using equation 23
Therefore, the reflection would be 27%. For a MECL 10K series gate with a
risetime of 3.5 ns, the reflection would only be 7.7%, and a MECL 10KH gate
with a rise time of 1.8 ns, the reflection would be 15%.
TDR Example 5
Another measurement was performed to observe the reflections due to the use
of a hybrid divider. The construction of the microstrip board used is shown in
the Figure 11-16. Note that the 50 Ω line branches out into two 100 Ω lines. A
reflection of 4% is observed at point 2 where the junction occurs. Notice that
the resistor exhibits a reflection of - 8%, due to capacitance of the resistor.
1
The HP 54753A and HP 54754A TDR plug-ins’ normalization allows the user
to change the risetime of the measurement system to simulate actual circuit
risetimes.
ρ′
2lt
pd
t
r
------------
ρ•
for
2lt
pd
t
r
------------
1
<
,=
ρ′
20.36
()
2.5 0.15
()
1
------------------------------------------0.27==
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