We’re all familiar with the acronyms for high-speed data protocols found in computers and many consumer products. PCIe, DDR, SATA, HDMI, USB… they all need a connector assembly, routing, pinout, and mounting style that can provide high-rate data streams. These connectors are carefully designed and simulated to ensure they provide the desired bandwidth and signal integrity for high-speed and high-frequency signals. The integration of these connectors into an electronic assembly is not a simplistic design task.
In this article, we will look at some of the important design approaches that are used to evaluate connector selection in an electronic system. We’ll mostly look at the case of digital systems, where high bandwidth is required to ensure data transfer through the connector, but the same ideas apply to RF systems. The key in both areas is to ensure signal integrity as a signal is transferred from a transmitting PCB, across an interface to a mating connector, and along a cable or receiving PCB.
Selecting High-Speed Connectors
Before you start looking at signal integrity in connectors, make sure you design your interconnects properly to ensure you hit target impedances, especially in differential channels. In high-speed PCB design, most of the routing difficulty will be dominated by designing and routing differential pairs for standard interfaces. Make sure you know the relevant bandwidth for your signaling standard (Nyquist frequency) as this will be used to evaluate a connector’s ability to transfer a signal towards a cable or receiver.
Types of High-Speed Connectors
Standardized: Connectors intended for use in high-speed interconnects will have a set of specifications they need to meet to ensure they can efficiently transfer signals to a receiver. For example, if a connector intends to comply with a specific signaling standard (USB, DDR, etc.), then it must comply with a set of signal integrity metrics. These are defined in the standards document (provided by IEEE, JEDEC, or another organization).
Non-standard: Other connectors are non-standard, meaning they are not meant to be used for a specific signaling standard or interface. However, if you compare their specifications against the interface requirements, you can determine whether a given connector will work with a particular high-speed interface. Some datasheets will also list conformance, despite the fact that the connectors are non-standard. This is common in areas like high-reliability embedded products, where a non-standard connector is selected for its mechanical specifications.
This non-standard connector will be compatible with Ethernet operating at high Gbps data rates.
The important specifications to examine in a datasheet are shown below. These are broken into two areas: high-speed and high-frequency.
Targets standard value for high-speed protocols (normally 50 Ohms)
Takes specific value (50 or 75 Ohms)
- Lower speed: both
- Higher speed: SMD
- Can be edge connector or vertical
- May be panel mounted
- Generally SMD, can be edge connector or vertical
- May be panel mounted
Specified based on data rate and signaling format (e.g., NRZ vs. PAM4)
Based on excitation of specific modes in the connector body
Can be high if multiple data streams are being consolidated
Shielded with one signal connection (e.g., coax cable)
Other specifications like construction materials and mechanical characteristics are still important and will be the main determinants of reliability. However, the above specifications are where you would start to ensure signal integrity. When working at high data rates, and especially
Evaluating Signal Integrity
When selecting a high-speed connector, it’s important to evaluate the connector with a test coupon and the intended transmission line geometry to ensure signal integrity. The signal integrity issues that can arise in connector selection are not always obvious just from looking at a datasheet, or by testing the connector in isolation. A test board + connector configuration should be used to ensure signal integrity, specifically by evaluating reflections and distortion along the length of the link.
There are two measurements that can be used to examine the performance and signal integrity of a high-speed connector:
- S-parameters (return loss and insertion loss)
- An eye diagram (for digital) or a power spectrum measurement (for RF)
Eye diagram: This is the standard method for visualizing losses, crosstalk, intersymbol interference (ISI),crosstalk, and bit error rates. Some oscilloscopes can be used to sample digital data and directly overlay on top of the time-domain trace.
Example eye diagram collected with an oscilloscope.
After examining the above measurement and comparing with signaling standard specifications, it’s possible to evaluate whether the channel passes compliance. A corresponding measurement in the frequency domain is an S-parameter measurement, which aims to spot signal integrity problems for the entire channel or the individual connector.
S-parameters: The S-parameters in the link with the connector can be used to get an estimate of bandwidth in the channel. When a large dip is seen in the insertion loss (or large return loss) within the required bandwidth, it indicates the bandwidth limit in the channel. S-parameters for the connector (if desired) can be compared to the S-parameters without the connector. The connector structure (which includes its signal launch and mounting pads/pins) can then be determined with a de-embedding procedure.
Example S-parameter spectrum collected with a VNA.
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