the propagation velocity. If the measurement is perfect, the line will intercept the y-axis at
zero. Otherwise, the actual y-axis intercept gives the measurement error.
11.7.3. TDT Method
Improved accuracy of propagation delay measurements can be completed with the TDR
used in TDT mode (TDT means that the signal is also observed at the far end of the line). In
this method, probes are placed at each end of the test structure. A pulse is injected into one
end and captured at the other end. This approach has less edge-rate degradation than the
simpler methods already mentioned, which results in improved accuracy. The measurement
is completed by injecting the pulse on one end of the test coupon with a 50-
probe and
capturing the signal at both the launch point and the open end with a low-capacitance high-
bandwidth high-impedance probe (10× or 20× ), as illustrated in
Figure 11.24
. The
advantage of this technique over the TDR is that the captured signal has propagated only
once down the coupon when it is captured, which will allow less time for the edge rate to be
degraded by losses or reflections.
Figure 11.24: Example of a basic TDT measurement setup for evaluating propagation
velocity of a microstrip trace.
The measurements are completed by connecting the 50-
probe to a sampling head with the
TDR/TDT mode "ON." The high-impedance probes should be connected to separate
channels and with the head function's TDR/TDT setting mode to "OFF." Measure the delay
between the transmitted and received signals (using the high-impedance probes at the driver
and receiver). This is the propagation delay of the trace [
McCall, 1999
].
All methods of measuring propagation velocity are highly subjective, due to where the
measurement point along the response is taken. The ideal measurement point is that at
which the response just begins to rise. This would eliminate the error due to different edge-
rate degradation if measurement were taken at, for example, the 50% point. However, in
most circumstances the incident preshoot and foot characteristics can make it quite difficult
to determine exactly when the response is rising [
McCall, 1999
].
Summary :
The measurement is completed by injecting the pulse on one end of the test coupon with a 50- probe and capturing the signal at both the launch point and the open end with a low-capacitance high- bandwidth high-impedance probe (10× or 20× ), as illustrated in Figure 11.24 . All methods of measuring propagation velocity are highly subjective, due to where the measurement point along the response is taken.
Tags :
measurement,end,propagation,tdt,point,captured,elocity,highimpedance,probes,probe,response,delay,signal