Abstract:
A system, apparatus and method for testing and measuring high frequency signals on a trace is described. In one embodiment of the invention, a footprint is manufactured on a trace to allow the testing of a signal while reducing the amount of distortion caused by prior art structures and methods. The footprint is designed to reduce stub effects and capacitance on a signal being communicated on the trace.

Description:
BACKGROUND 
     A. Technical Field 
     This invention relates to signal monitoring, and more particularly, to testing high frequency signals on a trace. 
     B. Background of the Invention 
     The importance of integrated circuitry is well known. Technological advancements have led to continual reduction in the size of integrated components and circuitry. The electronic devices employing integrated circuitry have not only seen a reduction in their size but also an improvement in signal processing efficiency. One reason for the improvement of signal processing is the use of higher frequency signals that are able to communicate large amounts of data within an integrated circuit. 
     As integrated circuits have become smaller and signal frequencies within the circuits have increased, the ability to effectively test and measure signals and components within the circuits has become more difficult. For example, it may be difficult for an engineer to locate a failure in an integrated circuit because of the problems in tapping or extracting an electrical signal internal to the integrated circuit. The ever-increasing signal frequencies on IC traces have made it difficult to effectively tap and monitor these internal IC signals. Electromagnetic interferences, including signal reflection and distortion, oftentimes render a tapped signal unusable for monitoring purposes. Furthermore, footprints on a trace that are designed to allow testing of a signal may reduce the performance of the trace by effectively creating a stub on the trace and/or adding unwanted capacitance. 
     The existing methods of monitoring signals typically include the use of an oscilloscope to probe a signal on a particular trace.  FIG. 1  illustrates an exemplary signal monitoring method using an oscilloscope. As shown therein, a driver  102  sends a signal to a receiver  104  through a signal trace  106 , which acts as a medium through which the signal travels. A metallic end of a scope probe  108  is brought in contact with the signal trace  106  resulting in a portion of the signal to be diverted onto the probe and sent to the oscilloscope. The signal traversing between the driver and the receiver can thus be tested. 
     The probe  108  may distort the signal because its metallic contact may function as a stub resulting in added dispersion and reflection to the signal being monitored. If sufficiently high frequency signals are being monitored, this dispersion and reflection on the signal may render an oscilloscope reading of the signal to be imprecise or unusable. 
     A sub-miniature A (“SMA”) connector may be used to more effectively measure a high speed signal. As illustrated in  FIG. 2 , “zero ohm” resistors R 1   210  and R 2   212  may be placed on the trace to allow connection of the SMA connector. It is important to note that a zero ohm resistor may in fact have a small amount of resistance associated with it. During the normal operation of a driver  202  and a receiver  204 , the resistor R 1   210  is placed in the footprint area specified, while the resistor R 2   212  is physically removed. To observe the signal on the PCB trace  206  the resistor R 1   210  is then removed and R 2   212  placed in the footprint area specified. The signal is directed towards SMA connector  208  to be observed on the scope. 
     This method reduces the reflections but still adds stub and extra capacitance that are caused by the surface mount footprint on the trace. This stub and capacitance may lead to inaccurate results and distort the signal received by the receiver  204 . At specified frequency of operation, which tends to be more than 1 Gbps, the accuracy of the observed results greatly affects the troubleshooting procedure. The extra trace on board in mentioned operating conditions adds to the inaccuracy. 
     There is a need of a method designed to allow observance of high frequency signals on trace without adding reflections and dispersions into the original signal. 
     SUMMARY OF THE INVENTION 
     The present invention provides a system, apparatus and method for measuring and testing high frequency signals on a trace. In one embodiment of the invention, a footprint is manufactured on a trace to allow the testing of a signal while reducing the amount of distortion caused by prior art structures and methods. In one embodiment, the footprint allows surface mount resistors to be connected to define a particular path for the signal. A first path is provided in which the signal is communicated on the trace between IC components, such as a driver and a receiver. A second path is provided in which the signal is communicated from the trace to a testing or measurement device, such as an oscilloscope. 
     In one embodiment of the invention, the footprint is manufactured by partially overlaying a first surface mount area with a second surface mount area. The first and second surface mount areas may be positioned at a 90 degree angle from each other. This footprint design minimizes stub effects and capacitance generated by prior art testing connections. 
     A user is able to define the path a signal travels by removing or inserting surface mount components on the footprint. In particular, the footprint may operate in two modes depending on where surface mount components are located on the footprint. These surface mount components are components that conduct electrical current such as zero ohm resistors and alternating current coupling capacitors. 
     In one embodiment, the footprint is provided to test single ended signaling. In another embodiment, the footprint is provided to test differential signaling. Various testing devices may be used in connection with the footprint including an oscilloscope with SMA connector(s). 
     Other objects, features and advantages of the invention will be apparent from the drawings, and from the detailed description that follows below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will be made to embodiments of the invention, examples of which may be illustrated in the accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in the context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments. 
         FIG. 1  shows a block diagram of a system for directing signal for monitoring using scope probe. 
         FIG. 2  is a block diagram of a system-allowing signal monitoring present in prior art. 
         FIG. 3  is a block diagram of the system as designed by the present invention to allow monitoring of signal using single ended signaling according to one embodiment of the invention. 
         FIG. 4  is a diagram representing the footprints design as suggested by the present invention using differential signaling according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A system, apparatus and method for observing a high frequency signal on a trace is described. The system may operate in multiple modes including a first mode in which a signal is communicated between a driver and a host, and a second mode in which the signal is communicated to a measurement device, such as an oscilloscope. A footprint that is located on a trace allows for the switching between modes while minimizing the amount of distortion on the signal during either of the modes of operation. 
     The invention described herein is explained using specific exemplary details for better understanding. However, the invention disclosed can be worked on by a person skilled in the art without the use of these specific details. The implementations of the invention can be embodied into a multiple types of printed circuit boards. The block diagrams shown are only exemplary implementation as per the rules dictated by the invention. Also, the connections between various components may not necessarily be direct. The components may not necessarily be on same board or plane but may be connected using a backplane. Further, the signal routing in between can be subjected to encoding, re-formatting or modifications. 
     References in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, characteristic, or function described in connection with the embodiment is included in at lest one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment. 
     A. System Overview 
       FIG. 3  illustrates a signal measurement footprint located on a trace according to one embodiment of the invention. The signal being observed is the signal traversing between the driver  302  and the receiver  304  on a common board. The components can alternatively be on different boards connected using a backplane or other connecting means. The signal travels from driver to the receiver via trace  306 , such as a PCB trace. 
     According to one embodiment of the invention, the system has two modes of operation. During a first mode of operation, the signal is communicated from a driver  302  to a receiver  304  via the trace  306 . Comparatively, during a second mode of operation, the signal is communicated from the driver  302  to a connector  308 , such as an SMA connector, so that it may be measured by a measuring device. The connector  308  may be defined to have specific impedance value, aligning it with the input of the scope. For example, an SMA connector may be used having an impedance value be 50 ohms that matches an impedance value of an oscilloscope. 
     A footprint is provided on the trace to allow a user to switch between the two modes. The footprint comprises a first area  310   a  on which a surface mount component may be positioned to enable current to flow between the driver  302  and the receiver  304 . The footprint comprises a second area  310   b  that is partially overlaid on the first area  310   a  and on which a surface mount component may be positioned to enable current to flow from the driver  302  to the connector  308 . It is important to note that the driver  302  and receiver  304  are exemplary components and the present invention is applicable to any two components between which a signal travels. 
     A user is able to effectively switch the mode of the footprint by removing or inserting a surface mount component on the first or second areas of the footprint. The surface mount components may be numerous different components that conduct an electrical signal including a zero ohm resistor and alternating current coupling capacitor. 
     B. Footprint Design 
     The invention minimizes the distortion of a signal using a specific footprint located on a trace. Referring again to  FIG. 3 , a first surface area  310   a  and a second overlaid surface area  310   b  create a footprint that allows a user to switch between modes in order to drive a receiver  304  or test the signal going to the receiver  304 . In one of the embodiment of the invention, the first and second surface areas  310   a ,  310   b  are positioned at 90 degrees or 180 degrees relative to each other. The angle between the two footprints may be changed depending on the design of the trace. However, the preferred angular displacement between the first and second surface areas  310   a ,  310   b  is 90° and 180°. Such an arrangement provides overlapped resistor/footprint area, which minimizes any stubs and capacitance that might have been caused by the footprint. 
     In another of the embodiment of the invention, differential signaling measurement is provided by multiple footprints on a trace.  FIG. 4  illustrates exemplary footprints that provide differential measurement a signal on a trace. The footprints  406 ,  408  allow for differential signals to be measured using a connector(s) that interfaces with the footprints. The footprints  406 ,  408  comprise overlaid surface areas  406   a ,  406   b  and  408   a ,  408   b  respectively, minimizing any capacitance or stubs caused by the footprints. 
     C. Modes of Operation 
     As described above, one embodiment of the invention provides for a trace signal measurement system having two modes of operation. A first mode operates in which a signal is communicated on a trace without any measurement and a second mode operates in which the signal is communicated to a measuring device, such as an oscilloscope. A footprint is provided on the trace that allows a user to switch between modes. 
     a) First Mode of Operation 
     Referring to  FIG. 3 , a surface mount component is connected on a first surface area  310   a  of the footprint  310  and a second surface area  310   b  on the footprint  310  is open during operation in the first mode. This surface mount component may be a variety of different components that transmit electrical current including a zero ohm resistor or capacitor. Accordingly, current is allowed to flow between the driver  302  and the receiver  304  while prevented from flowing through the second surface area  310   b . The footprint  310  minimizes or removes a stub and capacitance effects resulting from the gap at the second surface area  310   b.    
       FIG. 4  illustrates a footprint or footprints that enable differential testing of a signal according to one embodiment of the invention. In this particular example, a first footprint  406  and a second footprint  408  are located on a trace. The first footprint  406  has a first surface area  406   a  and a second surface area  406   b  similar to the footprint described above. The second footprint  408  has also a first surface area  408   a  and a second surface area  408   b.    
     A user may switch to a first mode, in which the signal travels between two components coupled by the trace, by inserting surface mount components at surface areas  406   a  and  406   b . Accordingly current is allowed to flow through the surface areas  406   a  and  408   a  with minimal distortion caused by the design of the footprints  406  and  408 . Current is not able to flow through the surface areas  406   b  and  408   b  because of the gap left by the removal of surface mount components. The differential mode signaling may improves electromagnetic compatibility and may be more suited to the high frequency applications. 
     b) Second Mode of Operation 
     A second mode of operation is provided that allows a signal on a trace to be measured and otherwise tested. Referring to  FIG. 3 , a surface mount component is connected at surface area  310   b  on footprint  310  while a gap at  310   a  is created by removing a surface mount component. Once again, these surface mount components may be a variety of different components that transmit electrical current including a zero ohm resistors or capacitors. Attaching a component on footprint area  310   b  allows the signal from the driver  302  to be directed to a connector  308 , such as an SMA connector, on which the signal is provided to a measuring device. 
     Referring to  FIG. 4 , a second mode of operation is shown relative to a differential signaling model. The second mode of operation is initiated by removing the surface mount components at  406   a  and  408   b  and inserting surface mount components at  408   a  and  408   b . In one embodiment, surface mount capacitors are used because they will transmit high frequency signals and filter lower frequency signals. A connector or connectors are coupled to transmit the signal to a measurement device, such as an oscilloscope. 
     During the second mode of operation, the signal being tapped may be transmitted on a uniform trace, having a minimum excess trace structure that may function as a stub or provide unwanted capacitance. 
     The present invention thus provides a method for high accuracy and minimum distortion monitoring of a signal as is needed in high frequency measurements. The method can be used during testing, troubleshooting and validation procedures to ensure proper working of the electronic devices. 
     While the present invention has been described with reference to certain exemplary embodiments, those skilled in the art will recognize that various modifications may be provided. Accordingly, the scope of the invention is to be limited only by the following claims.