Measuring Input Protection Components’ Effects With a TDR

My previous post used a Tektronix 1502 to examine discontinuities in cables. This post examines the discontinuity introduced by a “nominally invisible” protection diode on a PCB; it is clearly visible with the TDR, but probably won’t affect the final application.

The application is a piece of test equipment with a high-speed front end where the input voltage must not go outside the ±5V PSU rails. There are several ways of protecting high-speed inputs:

  • TVS diodes are used to protect many types of interfaces (USB, HDMI, etc) from static damage. They have the advantage of very low capacitance (down to 0.1pF), but are asymmetric and aren’t designed to pass significant current, so they have a poorly defined clamp voltage
  • zener diodes have high capacitance, are asymmetric and the clamping voltage is poorly defined
  • Schottky diodes tied to the rails can have low capacitance (~1pF) when reverse biassed, and can handle significant current (10s of mA).

This post examines how the pair of BAS70 Schottky diodes affect a signal on a 50Ω transmission line. The BAS70’s diodes are each ~1.75pF with zero reverse bias and about half that with 5V reverse bias.

The PCB has been repurposed from an earlier project. It is a 5cm long 50Ω coplanar waveguide. Four have been crudely connected to make a 20cm line, with an edge mounted SMA connector at one end and a 49.9Ω 0603 resistor at the other. In the picture below, one vertical division corresponds to about 5Ω. The three peaks on the left are the connectors between the cable and the PCB, and the three equal peaks on the right are the joins between the four boards at 5cm intervals.

It can be seen that the coplanar waveguide is indeed approximately 50Ω, but the joins are inductive, as indicated by a positive peak. The horizontal timebase is calibrated for a polythene dielectric, not FR4, hence the “~5cm/div”. A crude Spice simulation of the line shows similar peaks, and tends to indicate that the TDR’s bandwidth is barely adequate.

When the BAS70 dual diode is added, it adds a capacitance across the line, as shown in the diagrams below

As expected, increasing the bias noticeably reduces the capacitance and therefore reduces the discontinuity. The correspondance between the Spice simulation and the measurements is acceptable, given the approximations.

Hence the Tekronix 1502 “Cable Tester” can be used to examine discontinuities on a PCB.

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