Forget ICT--Use MDI Testing with 10GBASE-T PHY

One of the bottlenecks in Ethernet switch production, however, is the need to perform Media-Dependent Interface (MDI) tests. Typically, manufacturers use in-circuit tests (ICTs) to verify the functionality of connectors, transformers, resistors, and other components in a module path. To ensure proper performance, ICT must be performed at several points along the path, and the switch manufacturer must write ICT test programs for each product. This becomes a particularly cumbersome procedure in high-density switch production.

Currently, it is possible to perform MDI testing using the 10GBASE-T PHY itself. Silicon manufacturers are enabling the PHY's transmit and receive functionality to transmit a pattern and predict the path performance based on what it receives in return. This new capability eliminates the need to use ICT or to write ICT test plans, and it reduces the process of component verification to a few seconds per module. In fact, almost all of the available 10GBASE-T products in production have the MDI test hardware and software included in the PHY, including multi-port network interface controllers (NICs) and 16, 24 and 48 port 10GBASE-T switches available from top tier networking companies. This enables equipment manufacturers to increase final yield while decreasing test time and delivering a much higher quality product.

Why have an alternative to ICT?
Every network equipment manufacturer wants to meet specifications with every product and to ensure reliability and performance. This requires testing for component-dependent and component-independent problems. ICT has been a useful testing tool for many years, but with the emergence of such higher-speed protocols as 10GBASE-T it presents several challenges, and most switch and NIC makers avoid using ICT in the MDI path when testing 10GBASE-T products.

To perform ICT, manufacturers must design probe points into the circuit board. This can create a less-than-optimum layout that can have several negative consequences:

• Extra complexity--adding probe points adds to layout complexity and creates a series of sensitive RF areas in the circuit. Since RF is very sensitive, it takes longer to complete the designs and the RF points add capacitance to the circuit. In fact, some manufacturers deliberately stay away from adding probe points because it can have too great an impact on the circuit's performance.
• Reduced bandwidth--because adding probe points increases capacitance, it reduces bandwidth.
• Worse insertion loss and return loss--the antenna stubs used for probes create discontinuities that increase the potential for return loss problems. A circuit trace should have the fewest possible return loss issues, but antenna stubs can cause reflections that work against this goal. While return loss is a much more significant issue, adding probe points can also impact insertion loss under certain conditions because the added probe points can increase the effective capacitance of some components.

Ultimately, these impacts combine to reduce the product's overall performance.

Another drawback to ICT is that it is limited in scope. Traditional ICT probes only test performance between two nodes on a circuit board. The manufacturer ends up performing many discrete, node-to-node tests that produce static values, which increases manufacturing time and costs. Moreover, ICT doesn't always provide a clear view of the circuit's overall performance--it verifies that certain nodes are performing to specifications, but not necessarily that the end-to-end circuit performs as it should.

Due to these issues, many vendors previously skipped ICT and instead performed functional testing at a later stage of the manufacturing process. Unfortunately, this leads to less efficient manufacturing because performance issues that could have been detected at the board stage are not discovered until much later.

Obviously, it is much better to discover performance issues as early as possible in the manufacturing process, rather than investing a lot of time and material in manufacturing only to find out that the product does not meet specifications. What manufacturers need is a way to perform early-stage testing that does not result in ICT issues.

Enabling MDI Testing in PHY
10GBASE-T PHYs incorporate advanced analog and digital signal processing, so that the PHY itself is uniquely capable of performing test and measurement functions during manufacturing. By initiating the use of MDI testing in the PHY new ground in enhancing 10GBASE-T system performance and reducing overall system costs has been broken.

Under software control, the 10GBASE-T PHY can operate as an oscilloscope, a spectrum analyzer, and a network analyzer. This presents opportunities for a different kind of early-stage circuit testing. However, while some basic signal processing capabilities are present in every 10GBASE-T PHY, the PHY manufacturer must implement features and design software that allows manufacturers to perform MDI testing. MDI tests are faster and more accurate than ICT because they test an entire circuit rather than performance between two discrete nodes on a board. The following are sample MDI tests showing various circuit characteristics.

For example, a circuit with a missing capacitor on Lane 2 (Figure 1) would cause a detrimental change in return loss which can easily be detected by the MDI test. In addition, failures such as open/shorted inductors, capacitors, and resistors are common problems in manufacturing assembly that will be detected by the MDI tests.