[FAQ] TUSB1002A: How to pass USB receiver JTOL compliance with a linear re-driver

One qualification required to certify any product as an official USB product is to pass receiver Jitter tolerance testing, also called RX JTOL for short. RX JTOL tests the ability of a USB port to receive, loopback, and transmit data back through the port to the tester. Before starting an RX JTOL test with a USB3 linear redriver, its recommended to find a passing setting for transmitter compliance first. This will provide a good starting point for the Transmit path EQ settings.

The diagram below shows a simplified setup for an RX JTOL test using the TUSB1002A USB3 re-driver:

The Bit Error Rate Tester (BERT) will start by sending a signal through the RX path to the USB Host. This signal puts the host into loopback mode. Loopback mode causes the USB host to take any signal it receives on the RX path and transmit that same signal onto the TX path. When the host is successfully in loopback mode, the signal path will reflect the diagram below:

In order to pass the RX JTOL test, it is important to configure the right EQ settings on the USB3 re-driver. A dual-channel re-driver like the TUSB1002A will have separate EQ values for the TX and RX path. In our example diagram, CH1_EQ controls the TX path while CH2_EQ controls the RX path.

The first step is to find a good EQ setting for the TX path. To do this, it is recommended to run USB transmitter compliance on the port first. Once a passing EQ setting is found, that same setting should be used for the TX path when performing RX JTOL testing. Now it is necessary to find the correct RX path EQ setting:

Note that a linear re-driver is only able to compensate on the receiver side of the re-driver. In practice, this means the TUSB1002A will only compensate the insertion loss between the USB connector and the TUSB1002A on the receiving path. The rest of the RX path needs to be compensated by the USB host. The USB host EQ may need to be tuned to get the best setting and pass RX JTOL:z

Typically, this will result in a low CH2_EQ value if the TUSB1002A is near the USB connector. It is crucial to make sure the USB host is able to compensate for the loss between the TUSB1002A and the USB host when designing a USB3 system.

Once the BERT is able to receive a clean signal on its input the correct CH2_EQ setting has been found. If the BERT cannot pass RX JTOL no matter what EQ is set on the TUSB1002A, then it may be necessary to change the EQ value on the USB host.