System and method for information handling system peripheral EMC test

Electromagnetic compatibility of an information handling system and peripheral for achieving defined electromagnetic interference constraints is tested through a test adapter that interfaces with an external cable connecting the information handling system and peripheral. Signals from the external cable are passed through an isolation resistor and parallel capacitance to compensate for input capacitance of a signal tester. A common mode choke isolates a common mode component of the signal at the external cable. A noise generator applies a signal through the test adapter to the external cable to allow measurement of signals emitted from the peripheral due to injected noise.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to the field of information handling system peripheral testing, and more particularly to a system and method for information handling system peripheral EMC test.

2. Description of the Related Art

Information handling systems are growing consistently more powerful due, in part, to improvements in the components used to build information handling systems. For example, central processing units (CPUs) have seen greater numbers of features packed into a given area and have seen increases in operating speeds. Hard disk drives are built to rotate at greater speeds and retrieve and store information at greater rates. Random access memory (RAM) stores more information in a given area and stores and retrieves information with increased speed, such as with double data rate protocols. Information is communicated between components with serial links that have high frequency clock signals. Serial links have also substantially improved and simplified peripheral interfaces with information handling systems through external cables, such as interfaces with displays, mice, keyboards, printers and mass storage devices. As an example, the Universal Serial Bus (USB) is widely accepted through the industry to interface a wide variety of external peripherals with information handling systems.

One difficulty with the improved performance of information handling system components is that the greater operating speeds tend to produce greater amounts of electromagnetic interference (EMI). For example, current signaling schemes associated with high speed differential links operate at 3 GB/s for SAS and 4 GB/s for FC with future signaling schemes projected to operate at 10/GB/s or higher. Information handling system manufacturers are required by regulatory agencies to meet defined standards that limit the amount of EMI from a system. To meet regulatory standards, manufacturers design component layout and chassis shielding so that worst case operating conditions will not exceed defined EMI limits. The systems are typically tested by reading EMI with a spectrum analyzer located outside of the chassis. However, EMI measurements sometimes vary from predicted levels when information handling systems use peripherals interfaced through an external cable due to a common-mode component. The electromagnetic compatibility (EMC) of an information handling system with various peripherals is often difficult to predict since the EMC measurement sometimes exceeds the sum of the expected individual EMIs. For example, an information handling system interfaced by a USB cable with an external hard drive might exceed EMI requirements even though the information handling system and hard drive meet individual EMI requirements. Isolating the source of the excess EMI, such as hard drive operations versus the system data traffic or crosstalk onto external signals, is difficult.

SUMMARY OF THE INVENTION

Therefore a need has arisen for a system and method which tests information handling systems and peripherals for EMC, such as EMI related to a common-mode component.

In accordance with the present invention, a system and method are provided which substantially reduce the disadvantages and problems associated with previous methods and systems for testing the compatibility of information handling systems and peripherals for desired EMI constraints. A resistor divider probe interfaces with a differential wire pair to isolate and test a common mode component of signals sent between an information handling system and peripheral. Capacitance in parallel with the resistors of the probe compensate for the input capacitance of a test instrument so that testing bandwidth extends to multiple frequencies.

More specifically, a test adapter interfaces with a cable connecting an information handling system and a peripheral. The test adapter interfaces with the wires of the cable, such as a differential wire pair, a single ended wire or power/ground wires, so that signals sent through each wire is available at a connector for access by a test instrument, such as a spectrum analyzer. The interfaces between the connectors and the cable wires are made with a probe that has a capacitance in parallel with a resistance to extend the bandwidth of signals measurable by the test instrument. A resistor divider probe interfaces with a differential wire pair of a serial link to measure EMI associated with signals sent by the serial link to an information handling system. A common mode choke is disposed along the differential pair to isolate the common mode component of the signal. In addition to reading signals sent from a peripheral to an information handling system, the test adapter connectors allow insertion of noise to the cable so that the effectiveness of shielding of the peripheral can be tested.

The present invention provides a number of important technical advantages. One example of an important technical advantage is that information handling systems and peripherals are tested for EMC in an accurate and repeatable manner. A common mode component associated with interaction between an information handling system and peripheral is tested over bandwidths that extend through multiple frequency ranges, such as are used to communicate through a differential pair of a serial link. EMI measurements are available by reading signals across a cable with the information handling system and peripheral operating or by injecting noise through the cable to measure noise emitted from the peripheral. Repeatable results allow comparison of system performance against know measurements of emissions or immunity to determine acceptable levels of common-mode emissions and/or shielding.

DETAILED DESCRIPTION

A test adapter tests for electromagnetic compatibility of an information handling system and peripheral by interfacing with the cable connecting the information handling system and peripheral to detect electromagnetic interference associated with the cable or to inject noise into the cable to test shielding of the peripheral. For purposes of this disclosure, an information handling system may include any instrumentality or aggregate of instrumentalities operable to compute, classify, process, transmit, receive, retrieve, originate, switch, store, display, manifest, detect, record, reproduce, handle, or utilize any form of information, intelligence, or data for business, scientific, control, or other purposes. For example, an information handling system may be a personal computer, a network storage device, or any other suitable device and may vary in size, shape, performance, functionality, and price. The information handling system may include random access memory (RAM), one or more processing resources such as a central processing unit (CPU) or hardware or software control logic, ROM, and/or other types of nonvolatile memory. Additional components of the information handling system may include one or more disk drives, one or more network ports for communicating with external devices as well as various input and output (I/O) devices, such as a keyboard, a mouse, and a video display. The information handling system may also include one or more buses operable to transmit communications between the various hardware components.

Referring now toFIG. 1, a block diagram depicts a test adapter10configured to measure emissions associated with a USB cable12interfacing an information handling system14and a hard disk drive peripheral16. Information handling system14communicates information with hard disk drive16through USB cable12using standardized USB differential signaling. In alternative embodiments, alternative peripherals can interface with information handling system14with alternative cables12. For example, displays, optical disc drives, mice, keyboards, storage networks or other types of peripherals are interfaced with test adapter10using Firewire, SATA, SAS, PCI Express, LVDS, TMDS or other types of serial differential link cables as well as unidirectional link cables, such as a video cable that sends RGB information. Test adapter10interfaces with the signals22communicated across cable12, including ground and power signals, and provides a test connector18for each signal22so that a test instrument, such as a spectrum analyzer20, can read and analyze the signals22. As depicted byFIG. 2, analysis of signals22from a peripheral16to information handling system14helps to determine the source of EMI associated with combined operations of peripheral16and information handling system14, such as a common mode component.

Referring now toFIG. 2, a block diagram depicts a test adapter10configured to inject noise at a cable12interfacing an information handling system14and keyboard peripheral24to measure the effectiveness of shielding of the keyboard and cable. A noise generator26generates emissions of a desired character and injects the noise through a test connector18and adapter10into cable12. Test adapter10, which is shielded to reduce inadvertent emissions, sends noise signals22through cable12to peripheral24. A test instrument, such as spectrum analyzer20, measures emissions from peripheral24to analyze the effectiveness of shielding for peripheral24.

Referring now toFIG. 3, a circuit diagram depicts a dual-input resistive divider probe28for interfacing with an information handling system external cable. Resistor divider probe28has a first input V130and second input V232that each interface with a wire of a differential wire pair. Isolation resistors34are high-value resistors in parallel that interface with a test connector18, such as a coaxial cable. For example, the resistance Rsof isolation resistors34is 400 ohms to support connection to a 50 ohm coaxial cable and a 50 ohm test instrument20with division ration of approximately 1/10. A small capacitance36is added in parallel with isolation resistors34to compensate for input capacitance of test instrument20. As set forth in the equation:

VoutV1+V2=ZoutZ1+2·Zout=RoutRs⁡(1+S·Cout·Rout1+S·CS·RS)+2·Zout
When Rsof isolation resistors34times the capacitance Csof capacitors36is substantially equal to the resistance ROUTand capacitance COUTassociated with test instrument20, frequency dependence for test signals falls out. Removal of the frequency dependence by insertion of capacitors36in parallel with isolation resistors34allow the bandwidth subject to testing to extend to multiple GHz, such as the frequency ranges associated with high speed serial links. In addition to flat frequency measurements, resistor divider probe28can be given correction factors for frequency domain measurements. In one embodiment, capacitance36is added by wrapping copper tape or other conductive materials over each isolation resistor34.

Referring now toFIG. 4, a circuit diagram depicts a test adapter10for measuring the electromagnetic compatibility of an information handling system and peripheral. In the example embodiment ofFIG. 4, cable12can be a differential serial link having a differential pair of wires38or a unidirectional link having a single ended wire40. One or more power or ground wires42provide cable12with power and a ground reference. Test adapter10has a shielded housing and cable shields44that prevent entry or escape of EMI from test adapter10. For example, test adapter10has opposing USB ports44so that a USB cable from an information handling system connects to one port and a cable from a peripheral connects to another port. Within test adapter10's shielded enclosure, a resistor divider probe28interfaces an RF connector18with differential pair wires38while single probes46interface single ended wire40and power/ground wires42with connectors18. In order to read signals received from a peripheral, a test instrument is connected to the appropriate connector18, and in order to provide noise to a peripheral, a noise generator is instead connected to the connector18. A common mode choke48disposed along differential pair wires38support testing of EMI with isolation of the common mode component of the signal sent along cable12.