Abstract:
Calibration standards for accurate high frequency or wide bandwidth calibration measurements. A “short” or “reflect” standard is formed in a printed circuit board from a conductive coating on a generally planar surface. The conductive coating connects a signal trace to one or more ground planes. The generally planar surface is at least as wide as the signal trace and is preferably several times wider than the signal trace to provide a short standard with properties uniform over a wide frequency range. The short standard is incorporated into a printed circuit upon which a device under test is to be mounted. Connections to the short standard are made through components equivalent to components used to connect a device under test. When a through and line standard are added to the same board, the test board contains all the standards needed for a TRL calibration.

Description:
BACKGROUND OF INVENTION 
     1. Field of Invention 
     This application relates generally to high frequency measurements and more specifically to calibration standards. 
     2. Discussion of Related Art 
     In many applications it is desirable to measure the performance characteristics of electronic components. A network analyzer is a piece of test equipment often used for this purpose. 
     For the measurements made by a network analyzer to accurately reflect the performance of an electronic component, the network analyzer must be calibrated. Calibration is often achieved by attaching devices of known electrical properties, called standards, to the terminals of the network analyzer. By comparing the measurements made by the network analyzer on the known standards to the actual value of the standards, errors in the measurement process can be identified. These errors can be used to compute an adjustment that is applied to each measurement made with the network analyzer. 
     There are two popular methods for calibrating a network analyzer. One is called the SOLT (Short Open Load Through) method. This approach involves alternatively connecting a calibration standard to each of the terminals that represents a short to ground, an open circuit and a load of known impedance, often a fifty ohm load. In addition, the two ports of the network analyzer are connected together to make the “through” measurement. Taking measurements on these combination of reference standards is sufficient to allow computation of correction factors needed to calibrate the network analyzer. 
     These measurements are taken at multiple frequencies over the operating range of the network analyzer so that correction values are available for all the frequencies in the operating range. In practice, it is difficult to provide a standard that presents a fifty ohm load across the range of frequencies at which the network analyzer might operate. 
     The TRL (Through Reflect Line) calibration approach is often used for wide bandwidth or high frequency measurements because it does not require a load standard. The TRL calibration method requires three calibration standards. A “through” standard can be similar to one used in the SOLT approach. Likewise, the “reflect” standard can be the same as the “short” standard used as in the SOLT approach. The line standard is similar to the through standard, but an additional length of line is placed between the two ports of the network analyzer. The characteristics of the line standard depend on the length of line added and the frequency at which measurements are made. Accordingly, multiple line standards are often provided, with each line used to make calibration measurements over a specific range of frequencies (i.e., a specific bandwidth). Measurements taken on the through, reflect and one or more line standards allow computation of correction factors to calibrate the network analyzer. 
     Calibration in this fashion can compensate for any linear errors in the measurement equipment that are introduced in the signal path that is present when the calibration standards are connected to the network analyzer. The end point in the signal path that is used to connect to the calibration standards is sometimes called the “calibration plane.” If additional connectors or circuitry is added beyond the calibration plane while making measurements on a device under test, any errors introduced by these added components are not removed by the calibration process. 
     To remove errors introduced in the signal path between the calibration plane and the device under test, a process sometimes referred to as “de-embedding” is used. De-embedding involves developing a mathematical model of the components added between the calibration plane and the device under test. Correction factors that compensate out the effect of any such added components are then mathematically computed from the model. These correction factors are applied to any measurements taken by the network analyzer to “de-embed,” or mathematically remove, any effect of the components between the calibration plane and the device under test. 
     An alternative approach that combines calibration and de-embedding in one step is called “calibration through the use of equivalent fixtures.” In this approach, calibration standards are mounted in fixtures that are as similar as possible to the fixture used to mount the device under test. When the network analyzer is connected to these standards for calibration measurements, the signal path to the calibration standards has the same properties as the signal path to the device under test. Correction factors computed from measurements made with these calibration standards should also remove any errors introduced in the signal path to the device under test. The calibration plane is effectively at the device under test. 
       FIG. 1  shows an example of a test set-up that might be used to make performance measurements on a device under test, such as an electrical connector  110 . The device under test is connected to a printed circuit board  100 . Coaxial connectors such as connector  120  are attached to the board. Traces  130  within printed circuit board  100  connect the co-axial connectors  120  to connector  110 . 
     To measure the performance of electrical connector  110 , a network analyzer can be connected to coaxial connectors such as  120 . In use, cables running from a network analyzer are connected to the co-axial connectors such as  120 . 
     Before making measurements, calibration standards can be connected to the ends of those same cables. To make calibration measurements using an equivalent fixture approach, reference standards are included on test board  100 . These reference standards are connected through traces to coaxial connectors that match, as closely as possible, the signal paths to the device under test. For example, coaxial connectors  140  and  142  are shown joined together by trace  144 . Trace  144  might serve as a “through” reference standard and will be twice the length of trace  130 . Trace  144  introduces errors that approximate the errors introduced when a signal traverses one of the traces  130  from coaxial connector  120  to a connector  110  and returns back to the network analyzer through another trace. Longer traces similarly configured can provide a “line” standard used for a TRL calibration. 
     A short or reflect standard might also be incorporated onto printed circuit board  100 .  FIG. 1  shows a coaxial connector  146  coupled through trace  148  to via hole  150 . If via hole  150  is plated through, it will short trace  148  to the ground planes within board  100 . Accordingly, it will act as a short circuit to the signal applied through coaxial connector  146 . 
       FIG. 1  shows traces such as  130  as lines on the surface of printed circuit board  100 . Art work on the surface of a printed circuit board, such as  100 , is often used to make a visually perceptible representation of a trace within the circuit board. However, the actual electrical signals are carried on thin conductive strips, or traces, embedded within the board  100 . These traces run between parallel ground planes within circuit board  100 , forming stripline transmission lines to carry the signals. The artwork depicted on the surface of the board is intended as an aid in illustrating the underlying structure. 
     We have recognized that use of a plated through hole such as via  150  does not act as a true short circuit. In particular, we have noted that the plated through hole creates a frequency dependent reactive load, particularly at high frequencies. The problem is most noticeable at frequencies greater than 3 GHz. 
     SUMMARY OF INVENTION 
     In one aspect, the invention relates to a printed circuit board having a calibration standards. The printed circuit board includes a ground plane and a signal trace running parallel to the ground plane, a generally planar surface having a width wider than the width of the trace; and a conductive coating disposed on the generally planar surface that makes a direct electrical connection between the trace and the ground plane. 
     In another aspect, the invention relates to a test fixture for a device under test. The fixture has a position for mounting a device under test, a first separable connector and at least one signal connection path between the position for mounting a device under test and the first separable connector. The board includes a first calibration standard that includes a conductive, generally planar surface; a second separable connector; and a first signal trace electronically connecting the second separable connector and the conductive generally planar surface. A ground plane, parallel to the first signal trace, is electrically connected to the conductive generally planar surface. Also included is a second calibration standard that has a third separable connector; a fourth separable connector; and a second signal trace electrically connected between the third and the fourth separable connectors and parallel to the ground plane, the second signal trace being at least twice as long as the signal connection path. 
     In yet another aspect, the invention relates to a method of making calibrated measurements on a component mounted to a substrate using a network analyzer. According to the method, the network analyzer is connected to a plurality calibration standards on the substrate through separable connectors, at least one of the calibration standards being a reflect standard including a trace, a ground plane and a generally planar conducting member wider than the trace and disposed on a surface of the substrate, the conducting member connecting the ground plane and the trace. Measurements are made on the plurality of calibration standards with the network analyzer. The network analyzer is connected to the component through separable connectors, and the measurements of the component are adjusted based on measurements made of the calibration standards. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. In the drawings: 
         FIG. 1  is a sketch showing a prior art test assembly; 
         FIG. 2  is a sketch of a test assembly incorporating the invention; 
         FIG. 3A  is a cross section of a portion of the test board in  FIG. 2 ; 
         FIG. 3B  is a cross section of a portion of the test board of  FIG. 3A ; 
         FIG. 4  is a sketch of an alternative embodiment of a test board incorporating the invention; and 
         FIG. 5  is a flow chart illustrating the process for calibrating a network analyzer using a test board incorporating the invention. 
     
    
    
     DETAILED DESCRIPTION 
     This invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” “containing,” “involving,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
       FIG. 2  shows a test board  200  incorporating a device under test, here shown as connector  110 . Coaxial connectors such as  120  are connected to the device under test through traces, such as  130  within printed circuit board  200 . Test board  200  also includes calibration standards, which are preferably connected through paths within circuit board  200  that have electrical properties that are equivalent to the paths through which signals are coupled to connector  110 . 
     A “through” standard is created by trace  244  connecting coaxial connectors  240  and  242 . A line standard is made by trace  262  connecting coaxial connectors  260  and  264 . 
     A short, or “reflect,” standard is created by a slot  250  connected through trace  248  to coaxial connector  246 . We have discovered that forming a short with a slot such as  250  provides a calibration standard that is very uniform as a function of frequency. 
       FIG. 3A  shows in cross section the portion of printed circuit board  200  containing the short calibration standard. Trace  248  is shown within circuit board  200 . Trace  248  is between and parallel with ground planes  316  and  320 . Via hole  310  connects trace  248  to a coaxial connector such as  246  ( FIG. 2 ). 
     Coaxial connector  246  is not shown in  FIG. 3A . However, coaxial connector  246  maybe connected to printed circuit board  200  such as is shown in U.S. Pat. No. 6,639,154, to Cartier et al., filed Oct. 28, 2003, entitled: APPARATUS FOR FORMING A CONNECTION BETWEEN A CIRCUIT BOARD AND A CONNECTOR, HAVING A SIGNAL LAUNCH, U.S. Pat. No. 6,452,379, to Cartier, U.S., filed Sep. 17, 2002, entitled: METHODS AND APPARATUS FOR CONNECTING TO A SIGNAL LAUNCH, or U.S. Pat. No. 6,717,398, to Cartier, filed Apr. 6, 2004, entitled: SIGNAL LAUNCH CONNECTING TECHNIQUES, all of which are hereby incorporated by reference in their entirety. 
     Via hole  310  is plated with a conductive coating  312 . As is known in the art, this conductive coating can be a metal. As shown in  FIG. 3A , trace  248  extends to via  310  and therefore is in electrical contact with conductive coating  312 . Ground planes  316  and  320  do not extend to via  310 . Ground plane clearance, such as  318 , is provided to avoid shorting the ground planes  316  and  320  to trace  248  near via  310 . 
     Slot  250  also has a conductive coating  314 . Preferably, coating  314  may be formed at the same time that vias such as  310  are coated. Trace  248  extends to slot  250 . Likewise, ground planes  316  and  320  also extend to slot  250 . In this way, metal coating  314  shorts trace  248  to ground planes  316  and  320  near slot  250 . 
     Turning to  FIG. 3B , slot  250  is shown in cross section from the perspective illustrated as B in  FIG. 3A .  FIG. 3B  indicates that trace  248  has a width W. We have discovered that the electrical properties that slot  250  presents to trace  248  is more uniform as a function of frequency if the wall of slot  250  carrying conductive coating  314  is substantially planar in the vicinity of trace  248 . Preferably, the planar regions of metal coating  314  extend a distance D on either side of trace  248 . Preferably, the distance D will be at least equal to the width W of trace  248 . More preferably, the distance D will equal or exceed twice the width W of trace  248 . 
       FIG. 4  shows an alternative embodiment of a test board incorporating an improved short reference standard. Test board  400  includes a through standard  440  and a line standard  460 , which can be the same as the reference standards shown in connection with  FIG. 2 . Reflect standard  450  has similar performance characteristics as the reflect standard shown in  FIG. 2 . 
     Coaxial connector  452  couples a signal to trace  454 . Trace  454  is terminated in a generally planar conducting member. In the embodiment of  FIG. 4 , the generally planar conducting member  456  is formed by depositing a conductive material along an edge  412  of printed circuit board  400 . 
     Planar conducting member  456  can be formed in any convenient manner. It might, for example, be formed by depositing metal during the manufacture of printed circuit board  400 . Alternatively, insulative portions of circuit board  400  near edge  412  might be etched or otherwise removed to expose the ends of trace  454  and ground planes  316  and  320 . Thereafter, conductive member  456  can be applied in any convenient means, such as depositing metal or applying a conductive overlay. As with the embodiment shown in  FIG. 3B , it is desirable that the conductive member such as  456  extend a distance D beyond both edges of the trace  454 . Preferably, the distance D is grater than or equal to twice the width of trace  454 . 
     A test board as described above may be used as part of a process of making calibrated measurements on a component mounted to the test board or other substrate using a network analyzer.  FIG. 5  is a flow chart of such a process. At block  510 , the network analyzer is connected to a plurality calibration standards on the test board. This connection may be made through separable connectors. At block  512 , measurements are made on the plurality of calibration standards with the network analyzer. These measurements may be made in any desired operating range. However, the invention facilitates high frequency measurements, allowing measurements to be made at frequencies in excess of 3 GHz. 
     At block  514 , the measurements are used to compute a calibration. The calibration matrix may be computed according to a process as has been used in the prior art. But, any suitable process, whether now known or hereafter developed, may be used. 
     At block  516 , the network analyzer is connected to the component on which measurements are to be made. In the example illustrated, the component is an electrical connector. The connection may be made through separable connectors. At block  518 , measurements are made on the connector or other component being measured. At block  520 , the measurements of the component are adjusted based on measurements made of the calibration standards. This correction may be made using a computation as has been used in the prior art, buy any suitable process, whether now known or hereafter developed, may be used. 
     Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. For example,  FIG. 2  shows that slot  250  presents a generally planar surface to the end of trace  248 . However, the wall of slot  250  facing trace  248  has a slight amount of curvature to it. While better performance can be achieved by keeping the wall of slot  250  planar for as great a distance as possible beyond the edges of trace  248 , significant advantage still can be obtained without a perfectly flat surface. Preferably, though, the walls of slot  250  will have a generally planar portion extending beyond the edge of trace  248  for at least the width of the trace. Further, the wall of slot  250  will preferably not curve out of a plane by more than five percent of the width of the generally planar portion. 
     The embodiments shown above illustrate a generally planar conductive member in the region of a conductive trace. Other configurations for a conductive member might be employed. Preferably, the conductive member will be symmetrical around the signal trace for at least a distance D in all directions. The distance D is preferably as long as the width of the trace and more preferably at least 2 times the width of the trace. Further, it is preferable that the conductive member be normal to the trace where they intersect. 
     As another variation, it is possible that the planar surface could be formed as part of a larger structure that is not planar. 
       FIG. 2  shows slot  250  formed having a major axis and a minor axis with the major axis being substantially longer than the minor axis. Such a configuration is preferred because it provides a compact structure on printed circuit board  200 . However, it should be appreciated that a substantially planar portion could be formed from the arc of a circular hole of sufficient radius. However, for a circular hole to provide a generally planar portion, the radius of the hole preferably would be sufficiently large that the generally planar portion extending at least the width of the trace on either side of the trace spanned an arc of the circle of less than 30 degrees and preferably less than 20 degrees. 
     Circuit boards are shown to have stripline transmission lines. Circuit boards could also be made with microstrip transmission lines. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.