Figure 11.11: TDR pulse characteristics that affect measurement resolution.
Rise Time.
The most prominent TDR characteristic that affects resolution is the pulse rise time. As
discussed previously, the response is based on the reflection. The general rule of thumb is
that the reflection from two narrowly spaced discontinuities will be indistinguishable if they
are separated by less than half the rise time. Subsequently, the TDR can only accurately
resolve structures that are electrically long compared to the rise time.
(11.5)
System rise (T
rise
) time is characterized by the fall or rise time of the reflected edge from an
ideal short or open at the probe tip.
Faster rise times usually provide more versatility for analyzing small electrical parasitics but
are also harder to interpret correctly. Most systems designs may operate at a much slower
edge rate than the TDR output. Subsequently, the peaks and dips of the impedance
discontinues measured with a TDR will not necessarily represent those seen in the system
because the slower edge rate will tend to mask small variations. For example, consider two
microstrip PCB traces from two different boards that are connected together with a typical
inductive connector. The connector is soldered to the main board, while the second board
fits into the connector similar to a PCI or ISA card. The ISA or PCI board has pads called
edge fingers that are exposed and fit in the connector to make electrical contact. To
characterize the connector between the two boards, the overall lumped electrical model
must include the connector, via, and the edge finger pad capacitance as illustrated in
Figure
11.12
.
Figure 11.13
shows the respective TDR response of the connector for different edge
rates. Notice how the effect of the inductive impedance discontinuity decreases with slow
edge rates. The effect of edge rates on reflections is explained in
Section 2.4.3.
Summary :
(11.5) System rise (T rise ) time is characterized by the fall or rise time of the reflected edge from an ideal short or open at the probe tip. To characterize the connector between the two boards, the overall lumped electrical model must include the connector, via, and the edge finger pad capacitance as illustrated in Figure 11.12 .
Tags :
rise,edge,connector,time,tdr,two,rates,figure,electrical,board,boards,pulse,isa