Controlling Crosstalk In Connector Pinouts
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As noted in many previous articles, controlling crosstalk is one of the key goals in any PCB design. In most instances, when we talk about crosstalk, it’s in reference to the unwanted interaction of the electromagnetic field traveling on one transmission line with a neighboring transmission line. But crosstalk can also occur in the connector pin out. It is especially of concern in today’s high-speed designs where there is no isolation between the receive and drive signals going into a connector. This article will describe this type of crosstalk, the types of disruptions it causes, wherein the design cycle it needs to be factored in and how it can be successfully controlled.
It’s useful to review what crosstalk is and how it occurs. The term crosstalk is often used interchangeably with coupling and, as noted above, it is the unwanted interaction between signals or traces that are traveling in parallel. In this context, the parallel travel can be in the same signal layer or in an adjacent layer. This unwanted interaction takes the form of energy coupled into the neighboring line, and it is a disruptive noise signal. This signal impacts any operation but becomes particularly critical when it occurs in those designs that are characterized as “high-speed.”
As noted above, the crosstalk that is of interest relative to connector pinouts is that which occurs when there is no isolation between the receive and drive signals going into a connector. The situation that occurs with high speed serial links is a result of the amplitude of the transmit signals tending to be much higher than the receive the signals. In the case of some SERDES, the difference can be as high as 38 dB, which in round numbers is a ratio of the driver being 100 while that of the receiver is five.
Note: SERDES is an acronym for “Serializer” and “Deserializer,” and it is a device commonly used in high speed communications to compensate for limited inputs and outputs. SERDES converts data between parallel interfaces and serial interfaces, using one or more differential lines to transmit data from point A to point B. In high-speed serial links, the receive signals are quite sensitive to crosstalk, and it doesn’t take much to make them fail.
There are basically two types of connectors: right-angle connectors and connectors with round cases.
Right-angle connectors are used to connect a daughter board to a backplane. These connectors tend to have rows of pins that are isolated by baffles. Here, the isolation is achieved by putting the receive signals in one row of the pins and transmit signals in the other so that the baffle physically separates them. This right-angle connector technology has been around for quite a long time, so it’s readily available to the industry.
As we have moved to data rates of 32 Gb/S and above, it’s necessary to go one step further and use connectors that have differential pairs surrounded by a shield such as twinax. Twinaxial cable, aka as twinax, is a type of cable similar to the common coaxial copper cable, but it has two inner conductors instead of one. Initially, it’s primary use was in IBM’s IBM3x and AS/400 computer systems. With today’s high-speed products, it has become the defacto “go-to” solution for differential signaling.
It’s interesting to note that while twinax provides automatic crosstalk protection, this was not the reason it was developed. Initially, twinax was created to address EMI problems based on the following:
- Wires sitting in space are really good antennas. At the time that twinax was developed, the industry was experiencing 15-16 Gb/S disruptive noise radiating out of the connector that was leading to EMI failures.
So, while the control of crosstalk was not the impetus for twinax technology, it became the best solution for addressing it. In addition, it’s much easier to decide how to pin out a connector when twinax is used.
The other type of connector, which is primarily used by the aerospace industry, is a round connector. Round connectors have an array of pins spaced next to each other with no baffles in between. The challenge becomes how to protect drive and receive signals when there is no means for isolation. The “workaround” is to select a connector that has far more pins than what is necessary for the product being developed. The result is that drive signals are on one side of the connector, and the receive signals are on the other side with the pins between them serving as spacers.
At What Point in The Design Process is The Connector Type Determined?
Determining what type of connector is needed for the product being designed has to happen at the very start of the design process.
Lee Ritchey, Founder and President of Speeding Edge notes, “When a product such as a server is being developed, the first step in the product architecture process should be to go into a room and draw an outline of the backplane and figure out how the design will be pinned out so that is both routable and won’t have a crosstalk problem. It’s pretty much square one of the design process when this takes place. If it’s not done then, you can easily end up with crosstalk problems. In addition, if it’s not done then, there is no fix downstream in the process. That’s why this determination has to be made from the get/go.”
Ritchey continues, “Obviously, if you use twinax, there are cost considerations. Because the twinax connector is more complex, it is going to cost more. ‘Standard’ connectors have pins that are not shielded. The pins are simply stamped out of a sheet of metal, and then they are stuck in the connector housing. With twinax, the pins are encapsulated in dielectric, and then they are shielded. It’s hard to envision how this is done because it is so complex.”
He concludes, “Selecting twinax as your connector technology for a given design is definitely more expensive. But, if the difference is between having a product that works without crosstalk issues versus one that does not, the cost of twinax becomes far less of a concern.”
Controlling the crosstalk that can occur in connector pins is a primary issue of concern for today’s high speed designs. This issue can be readily addressed by selecting the right type of connector at the beginning of the design process.
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