Author: Tech

To fully characterize the high-speed link, it is required to perform transmitter (TX) and receiver (Rx) measurements through multiple different paths of the link under test, which brings challenges to the fully automatic test environment. The path width of PCI Express port is generally x1, x4, X8 and x16, which brings challenges to fully automatic TX or RX testing. By including RF switches in the test channel, we can realize multi-channel test without excessively changing the DUT and test equipment cables. In order to minimize the electrical impact of RF switch, ensure that the test is true to the specification requirements or verify the test plan. This paper describes the use of mini circuits RF switch for gen5 (32 GT / s) multi-channel test, provides some overall guidance on setting and automatic test, and puts forward suggestions on the common challenges.

This article will focus on the RF switch configuration required by x16 test. These switch models will support up to 18 channels (the highest PCIe is usually x16) and a lower number of channels. It is recommended to use hard cables to establish fixed connections between different switch assemblies. Hard cables can be obtained from mini circuits. CEM test chart is given earlier in this paper, but these technologies are also applicable to base test.


Figure 2: zt-8sp6t-40 4U / 5u.

Figure 1 shows the ztm2-8sp6t-40 modular switch matrix with eight 40GHz terminated sp6t mechanical switches. This configuration will support up to 18 channels. It is recommended to use phase matched cables to establish a fixed connection between adjacent 40 GHz relays. When the relay is not turned on for the through connection, there will be 50W termination.

Figure 2 shows the zt-8sp6t-40 4U / 5u switch matrix with eight 40GHz terminated sp6t mechanical switches. This configuration will support up to 18 channels. Hard cables are recommended to establish a fixed connection between adjacent 40 GHz relays. The orientation of the switch assembly in this matrix maintains a similar electrical path length between all inputs and outputs. This is particularly attractive for multi-channel RX testing to minimize path to path differences between calibration and testing. When the relay is not turned on for the through connection, there will be 50W termination.

RF switch matrix – gen5 TX test

PCIe gen5 devices (system host or plug-in) will show different transmitter performance in multi-channel ports. All paths are usually verified to fully characterize the link and identify problems such as silicon performance, high near or far end crosstalk, wiring defects, etc. Using RF switch (Fig. 3) in the test setup can realize multi-channel TX verification without engineers or technicians constantly changing the connection. The connection of the 32 GT / s base TX test (see Figure 10) is similar.

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Figure 3: 32 GT / s CEM system transmitter (multiplex).

The system host configuration requires that a conformance test load circuit board (CLB) be inserted into the CEM connector of the DUT, and the RF switch is required to be connected from each path by cable. The plug-in configuration is similar, but the DUT is inserted into the conformance test basic circuit board (CBB). A pair of cables connect the terminated switch matrix back to the 50 GHz oscilloscope. Instruments such as arbitrary waveform generator (AFG) can automatically generate the required 100MHz burst signal to make the DUT cycle through different transmitters to measure various data rates and code types.

Every connection made in the switch setting is very important. Due to insertion loss, it is not recommended to connect more than two relays in series for 32 GT / s TX test. It is recommended to use 1m 2.92mm cable between DUT and RF switch and 0.5m 2.92mm cable between RF switch and oscilloscope input. The differential fast edge of oscilloscope can be used to perform automatic channel to channel delay correction in cooperation with tekexpress software. The matching range of all cables, relays and PCBs in the channel shall fall within + / – 1ps of the positive and negative signal path.

Keeping the 50W (100W differential) connection I / O of the RF switch will minimize the reflection inside the channel, but will introduce partial insertion loss. 32 GT / s signal quality test does not require physical variable ISI circuit board (gen4 test requires), so it is required to embed additional channel and package loss on the oscilloscope. Test fixture characterization (described in Appendix B of the 5.0 PHY test specification) shall be performed, including RF switches. Basically, a filter file with low loss will be selected to realize the worst-case plug-in loss (when testing the system host) or the worst-case system loss (when testing the plug-in). Instead of using expensive VNA, the Tektronix signalcorrect solution can be used to verify channel loss, including RF switching matrix.

The anti embedding technology based on scattering parameter (s parameter) can be used to remove the influence of RF switch insertion loss. Anti embedding leads to increased complexity, but improves accuracy. In addition, the influence of noise amplification must be considered. If phase matched cables are used between relay to relay connections, there is only a small electrical difference from channel to channel. If you think these differences will affect the measurement, you can consider customizing the channel s parameter file. The S-parameter file can be captured by signalcorrect or vector network analyzer (VNA), or the nominal S-parameter file can be provided by Tektronix field project team.

RF switch matrix – gen5 RX test

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Figure 4: 32 GT / s CEM RX test points.

The PCIe gen5 device (system or plug-in) receiver is tested using finely calibrated pressure eye diagram signals. This “worst case” signal is established in the reference plane (no channel) through multiple calibration steps, and the “worst case” channel used must be 34 DB ~ 37 DB @ 16 GHz. This section will discuss how to use the terminated RF switch in this signal calibration during RX test, and then test the DUT through multiple links.

Calibration of amplitude, TX equalization, random jitter and sinusoidal jitter at TP3 test point requires direct connection between Anritsu mp1900a Bert PPG and Tektronix 50 GHz oscilloscope. It is recommended to use 1m 2.92mm cable (e.g. Tektronix product No.: pmcable1m) to complete this connection. Figure 5 shows the TP3 calibration connection, and the RF switch is not included in this step. Since the RF switch introduces some electrical channel differences, it is recommended not to include this effect before the TP3 reference plane.

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Figure 5: 32 GT / s TP3 pressure eye diagram (basic and CEM).

In tp2p, differential mode interference (DMI), common mode interference (CMI) and the last pressure eye diagram are used to calibrate the crosstalk term. This test point comes after TP2 (the physical channel between Bert and oscilloscope), but tp2p includes the effects of package embedding, RX equalization and clock recovery. Figure 6 adds an RF switch to the TP2 calibration, where the switch is introduced behind the test fixture (basic or CEM). At this point, the engineer must determine whether a single TP2 calibration is required (recommended for zt-8sp6t-40 4U / 5u) or more than two TP2 calibrations are required (it is best to consider the different electrical path lengths of ztm2-8sp6t-40). It is not recommended to cascade more than two relays for 32 GT / s pressure eye calibration.

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Table 6: 32 GT / s TP2 pressure eye diagram.

It is recommended to use 1m 2.92mm cable between Bert and RF switch and a shorter 0.5m 2.92mm cable between RF switch and oscilloscope. The differential fast edge of oscilloscope can be used to perform automatic channel to channel delay correction in cooperation with tekexpress software. The matching range of all cables, relays and PCBs in the channel shall fall within + / – 1ps of the positive and negative signal path.

PCI Express gen5: automatic multiplex test

Keeping the 50W (100W differential) connection I / O of the RF switch will minimize the reflection inside the channel, but will introduce partial insertion loss. 32 GT / s signal quality test does not require physical variable ISI circuit board (gen4 test requires), so it is required to embed additional channel and package loss on the oscilloscope. Test fixture characterization shall be performed, including RF switches. Basically, a filter file with low loss will be selected to realize the worst-case plug-in loss (when testing the system host) or the worst-case system loss (when testing the plug-in). Instead of using expensive VNA, the Tektronix signalcorrect solution can be used to verify channel loss, including RF switching matrix.

The anti embedding technology based on scattering parameter (s parameter) can be used to remove the influence of RF switch insertion loss. Anti embedding leads to increased complexity, but improves accuracy. In addition, the influence of noise amplification must be considered. If phase matched cables are used between relay to relay connections, there is only a small electrical difference from channel to channel. If you think these differences will affect the measurement, you can consider customizing the channel s parameter file. The S-parameter file can be captured by signalcorrect or vector network analyzer (VNA), or the nominal S-parameter file can be provided by Tektronix field project team.

Using the calibrated 32 GT / s pressure eye diagram signal in multiple channels for receiver test requires two RF switch matrices, as shown in Figure 7. When the link is X8 or lower, a single RF switch matrix can be considered. Signals from the Anritsu mp1900a PPG must be distributed to all PCIe paths. The device will be in loopback mode, so the digitized signal will be returned through the TX pin and the switch will be returned to the single input of the Bert error detector. Many systems supporting 32 GT / s will show a high loss return channel to the bit error detector and require external re driver to equalize the signal so that it can be detected by the test equipment.

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Figure 7: Rx Leq test of 32 GT / s system (multi-channel).

It is recommended to use 1m 2.92mm cable between Bert and RF switch and a shorter 0.5m 2.92mm cable between RF switch and oscilloscope. The shortest 2.92mm cable shall be considered between the DUT TX and the bit error detector.

How to establish communication with mini circuits

Tekexpress TX automation software provides built-in control function to control Mini circuits switch matrix during automatic TX test. Users develop their own automation software to control RF switch during TX or RX test. How to connect: you can communicate with mini circuits switch in two ways: USB uses DLL (Dynamic Link Library); Ethernet HTTP request.

Basic Calibration and test diagram

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Figure 8: 32 GT / s basic root or non root TX (multiplex).

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Figure 9: 32 GT / s basic RX test points.

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Figure 10 32 GT / s TP2 pressure eye diagram.

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Figure 11: Rx Leq test of 32 GT / s system (multi-channel).

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