Introduction

When using the multi-lead probe, users directly attach it to exposed traces or pins on the target PCB to allow an analyzer to capture, record, decode, display and monitor the PCIe traffic on the system under test.

An important consideration when using the multi-lead probe is the election of resistor values for the probe tip, to ensure reliable PCIe traffic capture as well as minimal disruption to the system under test

Overview

Consider the following simplified implementation of a PCI Express channel consisting of a transmitter (Tx) and a receiver (Rx) with a differential pair and DC blocking capacitors as the transmission medium.

There are three potential probing points on the channel:

  1. BGA pins on the TX or Rx package,
  2. The via/trace on the outer layer of the PCB,
  3. The pins of DC blocking capacitor.

The signal amplitude is attenuated the further it is from the transmitter. The probe takes a fraction of the signal energy to be processed by the analyzer. The effectiveness of the probe varies depending on the transmitter’s signal characteristics such as voltage swing and deemphasis level, the receiver input sensitivity and the position of the tapping point between transmitter and receiver.

Tapping at the receiver input pins (A) with too large of a resistor value will not likely provide sufficient energy for the probe to reliably capture the data. Tapping at the pins of the DC-caps (C) near the transmitter with too small of a resistor value may cause excess noise due to large signal swing and potentially the presence of strong de-emphasis.

Selecting an Optimum Resistor Value

As a general rule for selecting the value of tapping resistor we suggest that if the tapping point is close to the transmitter (as in case C), a high resistor value (e.g., 470Ω) is the optimum value. On the other hand, probing at or close to the receiver’s input pins (point A) may require a low value (e.g., 250Ω), and probing at some point (B) between (A) and (C) will require a resistor in the range from 250 Ω to 470Ω. If the system is a mobile device and the trace lengths between transmitter and receiver are small, the system may operate in low-power mode and so a smaller value of resistor will be needed even when placed near the transmitter (point C).

Here are some general guidelines:

  1. Is the probing point closer to the transmitter or receiver? Approximate the distance from the receiver as 0%(A) to 100%(C) and call this dp%.
  2. If the eye diagram measurement from which the eye height and width information is available at the tapping point, use the graph below to select the best value. PCIe Gen2 requires a minimum of 60mV differential peak-to-peak and 60ps eye width at the probing point.
  3. If the amplitude at the probing point is not available, an estimate can be made using the distance from the receiver of the probing point as a percentage of the track length from transmitter to receiver, as follows:
    1. RTAP = 270 + (200 * dp%)
  4. . Select the probe tip value closest to the calculated value.

Remember the value does not have to be exact and other factors may influence the optimum selection. The standard values of resistors offered by Teledyne LeCroy cover the vast majority of users.