Abstract:
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.

Description:
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 
   As the value and use of information continues to increase, individuals and businesses seek additional ways to process and store information. One option available to users is information handling systems. An information handling system generally processes, compiles, stores, and/or communicates information or data for business, personal, or other purposes thereby allowing users to take advantage of the value of the information. Because technology and information handling needs and requirements vary between different users or applications, information handling systems may also vary regarding what information is handled, how the information is handled, how much information is processed, stored, or communicated, and how quickly and efficiently the information may be processed, stored, or communicated. The variations in information handling systems allow for information handling systems to be general or configured for a specific user or specific use such as financial transaction processing, airline reservations, enterprise data storage, or global communications. In addition, information handling systems may include a variety of hardware and software components that may be configured to process, store, and communicate information and may include one or more computer systems, data storage systems, and networking systems. 
   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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be better understood, and its numerous objects, features and advantages made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference number throughout the several figures designates a like or similar element. 
       FIG. 1  depicts a block diagram of a test adapter configured to measure emissions associated with a USB cable interfacing an information handling system and a hard disk drive peripheral; 
       FIG. 2  depicts a block diagram of a test adapter configured to inject noise at a cable interfacing an information handling system and keyboard peripheral to measure the effectiveness of shielding of the keyboard and cable; 
       FIG. 3  depicts a circuit diagram of a dual-input resistive divider probe for interfacing with an information handling system external cable; and 
       FIG. 4  depicts a circuit diagram of a test adapter for measuring the electromagnetic compatibility of an information handling system and peripheral. 
   

   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 to  FIG. 1 , a block diagram depicts a test adapter  10  configured to measure emissions associated with a USB cable  12  interfacing an information handling system  14  and a hard disk drive peripheral  16 . Information handling system  14  communicates information with hard disk drive  16  through USB cable  12  using standardized USB differential signaling. In alternative embodiments, alternative peripherals can interface with information handling system  14  with alternative cables  12 . For example, displays, optical disc drives, mice, keyboards, storage networks or other types of peripherals are interfaced with test adapter  10  using 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 adapter  10  interfaces with the signals  22  communicated across cable  12 , including ground and power signals, and provides a test connector  18  for each signal  22  so that a test instrument, such as a spectrum analyzer  20 , can read and analyze the signals  22 . As depicted by  FIG. 2 , analysis of signals  22  from a peripheral  16  to information handling system  14  helps to determine the source of EMI associated with combined operations of peripheral  16  and information handling system  14 , such as a common mode component. 
   Referring now to  FIG. 2 , a block diagram depicts a test adapter  10  configured to inject noise at a cable  12  interfacing an information handling system  14  and keyboard peripheral  24  to measure the effectiveness of shielding of the keyboard and cable. A noise generator  26  generates emissions of a desired character and injects the noise through a test connector  18  and adapter  10  into cable  12 . Test adapter  10 , which is shielded to reduce inadvertent emissions, sends noise signals  22  through cable  12  to peripheral  24 . A test instrument, such as spectrum analyzer  20 , measures emissions from peripheral  24  to analyze the effectiveness of shielding for peripheral  24 . 
   Referring now to  FIG. 3 , a circuit diagram depicts a dual-input resistive divider probe  28  for interfacing with an information handling system external cable. Resistor divider probe  28  has a first input V 1   30  and second input V 2   32  that each interface with a wire of a differential wire pair. Isolation resistors  34  are high-value resistors in parallel that interface with a test connector  18 , such as a coaxial cable. For example, the resistance R s  of isolation resistors  34  is 400 ohms to support connection to a 50 ohm coaxial cable and a 50 ohm test instrument  20  with division ration of approximately 1/10. A small capacitance  36  is added in parallel with isolation resistors  34  to compensate for input capacitance of test instrument  20 . As set forth in the equation: 
               V   out         V   1     +     V   2         =         Z   out         Z   1     +     2   ·     Z   out           =       R   out           R   s     ⁡     (       1   +     S   ·     C   out     ·     R   out           1   +     S   ·     C   S     ·     R   S           )       +     2   ·     Z   out                   
When R s  of isolation resistors  34  times the capacitance C s  of capacitors  36  is substantially equal to the resistance R OUT  and capacitance C OUT  associated with test instrument  20 , frequency dependence for test signals falls out. Removal of the frequency dependence by insertion of capacitors  36  in parallel with isolation resistors  34  allow 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 probe  28  can be given correction factors for frequency domain measurements. In one embodiment, capacitance  36  is added by wrapping copper tape or other conductive materials over each isolation resistor  34 .
 
   Referring now to  FIG. 4 , a circuit diagram depicts a test adapter  10  for measuring the electromagnetic compatibility of an information handling system and peripheral. In the example embodiment of  FIG. 4 , cable  12  can be a differential serial link having a differential pair of wires  38  or a unidirectional link having a single ended wire  40 . One or more power or ground wires  42  provide cable  12  with power and a ground reference. Test adapter  10  has a shielded housing and cable shields  44  that prevent entry or escape of EMI from test adapter  10 . For example, test adapter  10  has opposing USB ports  44  so 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 adapter  10 &#39;s shielded enclosure, a resistor divider probe  28  interfaces an RF connector  18  with differential pair wires  38  while single probes  46  interface single ended wire  40  and power/ground wires  42  with connectors  18 . In order to read signals received from a peripheral, a test instrument is connected to the appropriate connector  18 , and in order to provide noise to a peripheral, a noise generator is instead connected to the connector  18 . A common mode choke  48  disposed along differential pair wires  38  support testing of EMI with isolation of the common mode component of the signal sent along cable  12 . 
   Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.