Introduction

PCI Express® 6.0 achieves its 64-GT/s data rate, double that of PCIe® 5.0, by moving from non-return-to-zero (NRZ) signaling to four-level pulse-amplitude-modulation (PAM4) signaling. This results in the need for more complex algorithms for voltage and timing measurements.

Transmitter Equalization Coefficients and Presets Measurement

Transmitter equalization measurements are performed on the new PAM4 Compliance Pattern signal using the AC method that was first introduced in PCIe 5.0.

During the measurement (Figure 1), the device under test (DUT) transmits the Compliance Pattern with the corresponding TX equalization coefficients. An oscilloscope captures this equalized Compliance Pattern, and post-processing software extracts the equalized pulse-response waveform. The DUT also transmits the Compliance Pattern with no TX equalization, which the oscilloscope captures, and the post-processing software applies TX equalization coefficients to construct an equalized pulse-response waveform. This process results in TX preset coefficients that represent the best-fit TX equalization coefficients to minimize the mean square error between the measured equalized pulse-response waveform and the reconstructed equalized pulse-response waveform.

Figure 1. Methodology for measuring Tx equalization coefficients and presets from the PCI Express Base Specification Revision 6.0, 16 December 2021, Figure 8-7 on page 1326. Courtesy of PCI-SIG.

The 11 presets in PCIe 6.0 (Figure 2) are now annotated as Q0 through Q10, and their preshoots are proportionally higher compared to the PCIe 5.0 preshoots—in particular, presets 5 (Q4) through 9 (Q8).

Figure 2. Tx Preset Ratios and Corresponding Coefficient Values for 64.0 GT/s from the PCI Express Base Specification Revision 6.0, 16 December 2021, Table 8-2 page on 1325. Courtesy of PCI-SIG.

Figure 3 shows an eye diagram of the Compliance Pattern using preset Q1 transmitter equalization coefficients, with the grid to the right of the eye diagram showing the pulse response for Q1 in magenta. The pulse response for Q0 without equalization, in yellow, is used as a reference to find Q1's best-fit transmitter equalization coefficients for C-2, C-1, C0and C+1.

Figure 3. Transmitter equalization test results for preset Q1.

Using SDAIII Software to Measure Transmitter Equalization

The SDAIII software provides capabilities to measure Tx EQ for NRZ signals. With the addition of the SDAIII-PAMx option, you can also test Tx EQ for PAM3 and PAM4 modulated signals to confirm that the preset values measured on the DUT conform to those in the specification. Following are the steps for testing Tx EQ for PCIe 6.0 signals.

Note: For a valid measurement, all waveforms must be measured on the same day using the exact same equipment.

Preparation

1. Configure the DUT to output the Compliance Pattern signal using preset Q0 with no equalization.

2. Set the oscilloscope set to acquire using:

  • DBI for 65 GHz bandwidth, 80 GS/s
  • Differential input on C2B and C3B
  • 50% Edge trigger on C2B

3. In the SDAIII software:

  • On the SDA dialog, Enable SDA and select Modulation PAM4.
  • On the Signal dialog, select Standard PCIe 6.0.
Measuring Presets

1. Make a Single acquisition of the unequalized DUT output signal.

2. Open the Pulse and TxEQ dialog and Select Show Pulse Response to view the pulse of the acquired signal.

3. Select Enable TxEQ to display the Reference Pulse controls.

4. Optionally, adjust the number of UIs to Show. The number of post-cursors is equal to the Total minus the number of Pre cursors.

The horizontal scale of the pulse response you can Show in UIs or Time.

5. Check Show Ref Response (so you can see the reference pulse after it is captured), then click Save Pulse Response as Ref to save the Q0 waveform as the reference pulse (PulseRef).

You'll now see two, "identical" waveforms on the display—one the Pulse in green and the other the saved PulseRef in yellow. The “LED” indicator next to the Reference Pulse label turns from red to green, indicating you have a valid reference waveform stored.

6. Configure the DUT to output the Compliance Pattern using the next preset to be tested (e.g., Q5) and acquire the signal using the same settings as you did for the Q0 signal.

Once the acquisition is processed, you should now see the signal with the new preset as the green Pulse waveform. Measurements for that preset appear on the TxEQ table.

7. Review the on screen table of TxEQ measurements and compare the values to Figure 2. Each should fall within the limits set forth in Table 8-2 of the standard.

8. Repeat Steps 6 and 7 for each preset you wish to measure, keeping the Q0 waveform as the PulseRef to which each is compared.

Changing the Reference Pulse

You must change the saved PulseRef waveform for measurements made on different days or using different equipment. In that case, click Delete Pulse Response Ref and repeat the entire procedure above to save a new reference.

Measuring Custom Presets

The Tx EQ measurement tools can be used to test signals with custom presets following the same procedure as described above, saving the first acquisition with no equalization as the PulseRef. In this case, enter the TotalCursors to Measure on the Tx Eq dialog. The number of Post cursors is equal to the Total minus the number of Pre cursors entered.

Generating Step-Response Waveforms

Optionally, you can view the Pulse and the PulseRef signals as step-response waveforms by selecting the checkboxes Show Step Response and Show Ref Step. The step-response waveforms are color-coded green and yellow the same as the pulse-response waveforms.