Measuring Connectors

John Lin writes:

I would like to replace one connector type with a different, less expensive model. How do I prove the two connectors have the same electrical characteristics? Also, how will the power and ground-pin assignments within the connector affect its performance?

Three basic measurements will do the job. All three measurements use a mated pair of connectors hand-soldered to a solid ground plane on each side (Figure 1).

A simple test setup can characterize signal transmission, crosstalk, and EMI.

Figure 1—A simple test setup can characterize signal transmission, crosstalk, and EMI. (Courtesy EDN)

On either side of the connector, ground all the pins that you will use for power or ground connections. Leave the other pins unconnected but accessible to your test equipment.

First, test signal fidelity using a stand-alone signal generator to transmit a digital signal through the connector. Use the voltage and rise time that will be present in the finished system. Load, or terminate, the signal on the far side of the connector as you would normally so that you get realistic currents through the connector as well as realistic voltages. See if the signal looks OK after it passes through. This test is the easiest for a connector to pass.

If your signal generator has a 50-Ω output, and the coaxial cable is 50-Ω, the connector under test will react as if a 50-Ω source is driving it. If you want to simulate a source impedance other than 50 Ω, use an impedance-matching pad. The most common difficulty associated with this test is a failure to appreciate the importance of keeping the hand-soldered connections extremely short.

The signal-fidelity test tells you whether the connector impedance matches your transmission-line impedance. If it's a good match, the signal will shoot through unscathed. If it's a poor match, the initial signal edge may come through degraded, and you may also see subsequent residual reflections, depending on how you terminate the line.

Terminate both ends of the victim connection with impedances that approximate the actual impedances that your system uses. For example, if low impedance sources drive your signal, then ground the source side of each victim connection as if a low impedance source is holding it in a zero state.

Make several measurements, and plot crosstalk versus distance between aggressor and victim. Use this plot to sum up the worst-case aggregate crosstalk.

The third test measures one form of EMI. Using the crosstalk measurement setup, tie a 6-ft wire onto the solid ground plane on one side of the connector. Stretch the wire horizontally across your (preferably wooden!) lab bench. Next, tie another 6-ft wire onto the solid ground plane on the other side of the connector. Stretch this wire horizontally in the other direction. You've just made a dipole transmitter.

Using a calibrated antenna and a sensitive spectrum analyzer, have an EMI engineer plot the received signal power as a function of frequency. This measurement lets you compare the ground-transfer impedance of various connector styles. When you pump high-frequency signal currents through a connector, the currents return to their sources through the ground (or power) pins of the connector. The returning signal currents passing through the ground-transfer impedance of the connector create tiny voltage shifts between the ground on one side of the connector and the ground on the other, driving the dipole antenna. These same tiny ground shifts also drive many common EMI failure mechanisms, which is why this test is a good way to measure EMI-shielding effectiveness.

Changing the number of power and ground pins in your layout will affect all three measurements. For open-pin-field connectors, EMI changes inversely in proportion to the number of power and ground pins. Aggregate crosstalk changes inversely with the square of the number of power and ground pins. Signal fidelity improves when the configuration of power and ground pins immediately surrounding the signal pathway matches the correct trace impedance.