A high-bandwidth probing plan: TDR measurements are useful for characterizing signal channels. But bandwidth limits are determined in the first 0.100".

THE TIME DOMAIN reflectometer (TDR) is the de facto standard for characterizing the impedance profile and transmission properties of a fabricated circuit board trace. The ultimate limits to the bandwidth of a TDR measurement are set in the first 0.1002 Careful design and optimization of the signal launch into the trace call dramatically improve the bandwidth of the measurement.

On the front of the TDR instrument is a 3.5 into connector, compatible with an SMA. Usually, a precision, low-loss, 50f2 cable is connected to the TDR and an SMA connector is soldered into the board. In the transition from the coax geometry to the stripline or microstrip in a board through the SMA connection, there is an impedance discontinuity. The quality of this connection strongly influences the size of the discontinuity and the bandwidth of the connection.

The larger the discontinuity, the more high-frequency components that reflect to the source and the fewer that get launched into the transmission line. If characterizing a path for 5 Gbps signals, the connection method may limit the measured system performance. To increase the bandwidth of the characterization, think about the design of the launch before building the board.

A key ingredient in DfT for high-bandwidth characterization is the use of a pad and via design that is transparent to the signal. This usually means using a small-diameter via without a through-hole connection, and optimizing the clearance holes in the planes.

FIGURE 1 shows the TDR response for different connection designs. The green curve is the TDR response, with a roughly 35 psec rise time, for a conventional through-hole SMA connection to a bottom trace. On this scale, one division is a reflection coefficient of 10% and corresponds to an impedance change of about 10 [ohm]. At this rise time, the impedance discontinuity is over 18 [ohm], and is predominately capacitive.

[FIGURE 1 OMITTED]

Some believe that avoiding vias will also avoid the impedance discontinuity. But just as many problems can be generated with an SMA edge-coupled launch attached directly to a surface trace. The gold curve shows the measured TDR response of an edge-coupled launch using an SMA. The impedance discontinuity is over 18 [ohm], at this rise time, and is inductive.

One way to avoid this problem is to use microprobes and design the surface pads for probing. The key feature is to use a copper fill, shorted to all adjacent ground vias. In Figure 1, the gray vias have been shorted to the copper fill. With this configuration, every signal via can be probed.

The red TDR curve shows the response of a microprobe launch into a 50 [ohm] stripline which has been optimized. The impedance discontinuity, at this rise time, is less than 5 [ohm] and is inductive.

Finally, it is possible to use an SMA connection to a circuit board trace, if it is optimized. The blue curve shows such a connection. Its impedance discontinuity, less than 5 [ohm], compares to a microprobe launch. The specific via design is proprietary.

So if you want to characterize channels at the highest bandwidth, before designing the board consider how the lines will be probed.

DR. ERIC BOGATIN is CTO of Synergetix (synergetix.com). He can be reached at [e-mail]. He is speaking this month at Printed Circuit Design Conference West (pcbwest.com)