HP 54753A User Manual page 259

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.
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:
where:
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
to change the risetime of the measurement system to simulate actual circuit
risetimes.
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements
ρ'
t = the propagation delay time of the line in ns / in.
pd
t = the risetime of the signal in ns
r
l = the length of the discontinuity in inches
ρ = the reflection coefficient for 2l
(in this case the value found with the TDR waveshape
with t
(
2 0.36
ρ'
= ----------------------------------------- -
The HP 54753A and HP 54754A TDR plug-ins' normalization allows the user
Transmission Line Theory Applied to Digital Systems
1
This can be determined by scaling the value
2lt 2lt

----------- - ρ • <
pd pd
----------- -
= , for

t t
r r
pd
= 28 ns).
r
)2.5 0.15 ( )
= 0.27
1

1

≥ 1
/ t
r
(23)
11-31