Abstract:
A high-bandwidth electrical test probe having a probe contact spring of reduced size and characteristic capacitance is presented. The probe includes a contact spring connected at one end to the input port of a probe circuit. The opposite end of the contact spring enters the a probe socket and a predetermined angle of entry. The probe socket has a bore formed therein which is arranged at a non-zero angle relative to the angle of entry of the contact spring into said probe socket bore, thereby guaranteeing electrical contact with the bore. The design allows the use of a very small contact spring, on the order of tens of mils, thereby reducing the parasitic capacitance of the spring and allowing much higher bandwidths than heretofore achievable.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
   This is a divisional of application Ser. No. 10/100,677 filed on Mar. 18, 2002, now U.S. Pat. No. 6,911,811 the entire disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION  
   The present invention pertains generally to electronic test instrumentation, and, more particularly, to a spring and socket assembly for a high bandwidth electronic test probe. 
   The increasing reliance upon computer systems to collect, process, and analyze data has led to the continuous improvement of the system components and associated hardware. New methods for increasing the speed of integrated circuit components while also increasing the functional density and reducing the physical size of integrated circuits are constantly being sought. As a result, it is not uncommon to see integrated circuits running at several GHz with pin spacing on the order of 10 mil apart. 
   In a test environment, electronic test instruments such as oscilloscopes and logic analyzers are often required to measure electrical parameters on device pins or nodes of a circuit. A common tool for collecting measurements in this environment is an electrical test probe. An electrical test probe is used to make a connection between a test point or signal source on a device/circuit under test and a test instrument. An electrical test probe comprises a cable having a connector at one end connectable to the electronic instrument and having a contact device such as a probe pin at the other end of the cable for probing the test point (e.g., a desired device pin or circuit node). Typically, the contact device includes a probe pin connected to probe circuitry which filters a signal seen on the probe pin. The probe pin may be manually springably connectable to the probe circuitry via a spring mechanism. 
   As the speed of integrated circuits increase, the bandwidth required of electrical test probes has exceeded that which can be achieved with prior art probes. As a general rule, in order to achieve accurate measurements, the bandwidth of a test probe should be approximately five times greater than the frequency of the waveform being measured. 
     FIG. 10  is a top view and  FIG. 11  is a cross-sectional side view of a prior art electrical test probe tip  20 . As shown, test probe tip  20  includes circuitry implemented on a printed circuit board  22 . The printed circuit board  22  includes an input port  23  for receiving signals from a contact spring  25 , and an output port  24  for electrical connection to a probe cable  21 . 
   The printed circuit board  22  and probe pin  26  are positioned within a housing  28 . In order to achieve maximum electrical contact, prior art contact spring mechanisms  25  were formed as a flat piece of metal with width d shaped into a hook, as illustrated in  FIGS. 11 and 12 . The width d of such prior art hooks is typically on the order of approximately 100-200 mils wide. Due to the large width d of the contact spring  25 , the contact spring  25  exhibits a large parasitic capacitance C hook  which prevents signals above a certain cutoff frequency f o  from passing. The cutoff frequency of the contact spring  25  is the frequency of the wave when the wavelength λ is twice the width d of the contact spring  25 . At this frequency, λ/2 resonances occur that cause the contact spring  25  to act inductively. Above the cutoff frequency, additional resonances occur regularly. Therefore, the cutoff frequency represents the upper limit of the capacitor&#39;s (i.e., contact spring  25 ) frequency range. As is known in the art, the larger the width d of the contact spring, the greater its parasitic capacitance and inductance and therefore the lower the cutoff frequency of the probe. 
   Accordingly, there exists a need in the industry for a high bandwidth electrical test probe. In particular, a need exists for a probe contact spring of much smaller size and therefore reduced characteristic capacitance that also ensures good electrical contact. 
   In addition, as the node size and the spacing between nodes is reduced, the size of the probe tips must also accordingly be decreased in order to accommodate the required spacing between the nodes under test. Accordingly, there also exists a need in the industry for an electrical test probe that may be rotated to the desired distance without rotating the entire probe assembly. 
   SUMMARY OF THE INVENTION  
   Accordingly, it is an object of the invention to achieve a high bandwidth probe. 
   It is also an object of the invention to employ a probe contact spring of much smaller size and therefore reduced characteristic capacitance that also ensures good electrical contact. 
   It is an object of the invention to provide a test probe that may be rotated to the desired distance without rotating the entire probe assembly. 
   The present invention achieves these and other advantageous objectives, with a high-bandwidth electrical test probe having a probe contact spring of reduced size and characteristic capacitance. The probe includes a contact spring connected at one end to the input port of a probe circuit. The opposite end of the contact spring enters the probe socket at a predetermined angle of entry. The probe socket has a bore formed therein which is arranged at a non-zero angle relative to the angle of entry of the contact spring into said probe socket bore, thereby guaranteeing electrical contact with the bore. The design allows the use of a very small contact spring, on the order of tens of mils, thereby reducing the parasitic capacitance of the spring and allowing much higher bandwidths than heretofore achievable. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS  
     A more complete appreciation of this invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
       FIG. 1  is a block diagram of a conventional test setup; 
       FIG. 2  is a top view of an electrical test probe implemented in accordance with the invention; 
       FIG. 3  a cross-sectional view of the electrical test probe assembly implemented in accordance with the invention, with the contact spring positioned not in electrical contact with the probe socket; 
       FIG. 4  is a cross-sectional view of the probe socket used in the electrical test probe assembly of  FIG. 3 ; 
       FIG. 5  is a cross-sectional view of the nose cone used in the electrical test probe assembly of  FIG. 3 ; 
       FIG. 6  is a cross-sectional view of the probe pin used in the electrical test probe assembly of  FIG. 3 ; 
       FIG. 7A  a cross-sectional view of the electrical test probe assembly of  FIG. 3 , with the contact spring positioned in electrical contact with the probe socket; 
       FIG. 7B  an alternative embodiment of a cross-sectional view of an electrical test probe assembly in accordance with the invention, with the contact spring positioned in electrical contact with the probe socket; 
       FIG. 8  is a coaxial view of a cross-section of the test probe assembly of  FIG. 7B ; 
       FIG. 9  is a coaxial view of a cross-section of the test probe assembly of  FIG. 7  with the nose cone rotated 90° from the position of the nose cone in  FIG. 8 . 
       FIG. 10  is a perspective view of a prior art electrical test probe; 
       FIG. 11  is a side view of the prior art contact spring used in the electrical test probe of  FIG. 10 ; and 
       FIG. 12  is a top view of the prior art contact spring of  FIG. 11 . 
   

   DETAILED DESCRIPTION OF THE INVENTION  
   Turning now to the drawings,  FIG. 1  illustrates a test setup environment  2  for measuring a signal on a test point  5  of an electronic circuit under test  4 . 
   The test setup environment  2  includes an electronic instrument  6  (e.g., an oscilloscope, spectrum analyzer, or logic analyzer) connected to a test probe  10 . The test probe  10  comprises a probe tip  12  which may be place in electrical contact with the test point  5  of the circuit under test  4 . The probe tip typically comprises circuitry (internal to the probe tip  12 ) for filtering, conditioning, and amplifying the signal seen on the test point prior to passing it on to the test instrument over a probe cable  14 . 
     FIG. 2  shows a top view of an electrical test probe  100  implemented in accordance with the invention. As illustrated, electrical test probe assembly  101  comprises an electrical cable  102  connected to a connector  105  (e.g., a BNC connector) at one end  103  and to a probe assembly  100  at the opposite end  104  of the cable  102 . 
     FIG. 3  shows a cross-sectional view of the electrical test probe assembly  101  implemented in accordance with the invention. As illustrated therein, the electrical test probe assembly  101  comprises probe circuitry  124  implemented on a printed circuit board  120  for filtering, conditioning, and amplifying a signal detected by a probe tip  110 , discussed hereinafter. Preferably, all such circuitry is implemented within an integrated circuit  124  on the printed circuit board  120 . The printed circuit board  120  includes an input port  125  for receiving signals detected by the probe tip  110 , and an output port  126  for electrical connection to the electrical cable  102 . 
   The printed circuit board  120  is positioned within a housing  112 . In the illustrative embodiment, the housing  112  is a cylindrical barrel  114  with a coaxial bore  115  formed therein. The probe cable  102  enters one end  117  of the cylindrical barrel  114  and is electrically connected to the output port  126  of the printed circuit board  120 . A contact spring  130  is electrically connected to the input port  125  of the printed circuit board  120 , and exits the opposite end  118  of the cylindrical barrel  114 . 
   The barrel  114  is connected to a probe tip  110 , which includes a nose cone  140 , probe socket  150 , and probe pin  160 . The nose cone  140  is configured to house the probe socket  150  and probe pin  160 . The contact spring  130  exiting the barrel  114  is electrically connectable to the probe pin  160  within the probe socket  150 . 
   In a preferred embodiment, the contact spring  130  is compressibly connectable to the probe socket  150  via a compression spring  180  (also called a z-compliance spring) housed in the bore shaft  115  of the barrel. Decompression tab  184  is attached to the printed circuit board  120  and are slidable along the coaxial axis of the bore shaft  115  of the barrel. The decompression tab  184  protrudes to the exterior of the barrel  112  and slides parallel to the axis of the coaxial bore  102  of the barrel  112 . In the fully released position, the compression spring projects the printed circuit board  120  along the coaxial axis of the housing in the direction of the probe tip  110 , exerting sufficient force against the printed circuit board  120  to ensure that the contact spring  130  is fully inserted in electrical contact with the probe socket  150 , as described hereafter. Electrical contact between the probe pin  160  and printed circuit board  120  may be broken by manually positioning the tab  184  in the direction opposite the probe tip  110 , thereby compressing the spring  180  to cause the contact spring to exit the probe socket  150  and lose electrical contact therewith. 
   The compression spring operates to position the contact spring  130  either in or not in electrical contact with the probe pin  160  by compressing the printed circuit board  120  in the barrel  114  either in the direction of, or in the opposite direction of, the probe tip  110  relative the barrel  114 . 
   With further reference to  FIG. 4 , in the preferred embodiment, the probe socket  150  comprises a substantially cylindrical barrel structure  151  with a coaxial bore  152  formed therethrough. In the preferred embodiment, the coaxial bore  152  of the probe socket  150 , hereinafter “probe socket bore  152 ”, comprises a conical probe socket bore section  153 , a first equi-diameter probe socket bore section  154 , and a second equi-diameter probe socket bore section  155 . The conical probe socket bore section  153  forms a bore having a diameter gradually decreasing from a maximum diameter opening at one end  156  of the probe socket  150  to a minimum non-zero diameter opening into the first equi-diameter probe socket bore section  154 . 
   Preferably, in order to ensure maximum electrical contact, the diameter of the first equi-diameter probe socket bore section  154  substantially matches, or is only slightly greater than, the diameter of the contact spring wire  130 . In the preferred embodiment, the diameter of the first equi-diameter probe socket bore section  154  is approximately 10 mils. The first equi-diameter probe socket bore section  154  opens at one end into the conical probe socket bore section  153  and opens at the other end into the second equi-diameter probe socket bore section  155 . 
   The diameter of the second equi-diameter probe socket bore section  155  preferably substantially matches, or is only slightly greater than, the diameter of the plug end  164  of the probe pin  160  to ensure maximum electrical contact between the probe socket bore section  155  and the probe pin  160 . In the preferred embodiment, the diameter of the second equi-diameter probe socket bore section  155  is approximately 20 mils. 
   A probe pin plug  164  of a probe pin  160  is fitted into the second equi-diameter probe socket bore section  155  on the socket end  157  of the probe socket  150 . In the preferred embodiment, the probe pin plug  164  is approximately 20 mils. 
   At the opposite end  156  of the probe socket  150  proximate to the printed circuit board  120  of the test probe  100 , the contact spring  130  fits through the conical probe socket bore section  153  and into the first equi-diameter probe socket bore section  154 . In the preferred embodiment, the diameter of the contact spring wire  130  is approximately 10 mils. 
   Referring now to  FIG. 5  in conjunction with  FIGS. 3 and 4 , in the preferred embodiment, a nose cone  140  houses the probe socket  150 . A nose cone bore  142  formed within the nose cone  140  comprises includes a conical nose cone bore section  143  opening into and arranged at a different angle relative to a first and second nose cone bore section  144  and  145 . The first nose cone bore section  144  substantially conforms to the exterior shape and size of the probe socket  150 . Preferably, the exterior of the probe socket  150  includes a recess  158  on its exterior, and the first nose cone bore section  144  of the nose cone bore  142  includes a mating tab  148  that substantially fits within the exterior probe socket recess  158 . 
   The exterior probe socket recess  158  on the exterior of the probe socket  150  and the mating tab  148  on the interior wall of the first nose cone bore section  144  together form a snap lock. The snap lock operates to lock the probe socket  150  into place when it is inserted fully into the first nose cone bore section  144  of the nose cone bore  142 . In this regard, the probe socket  150  and/or mating tab  148  of the first nose cone bore section  144  is made of a sufficiently flexible material to provide a sufficient amount of give to allow the non-recessed exterior portion of the probe socket  150  to pass over the tab  148  as the probe socket  150  is inserted into the first nose cone bore section  144 . Once the probe socket  150  is inserted far enough that the tab  148  passes into the recess  158  on the probe socket  150 , the probe socket  150  is locked securely in place. In the preferred embodiment, the nose cone  140  is formed as a molded plastic part. The molded plastic provides sufficient flexibility to allow insertion of the probe socket  150  into the first nose cone bore section  144  but is sufficiently inflexible such that the probe socket  150  is not easily removable once the tab passes into the recess of the probe socket  150 . 
   The second nose cone bore section  145  houses a probe pin  160 . The probe pin  160  comprises a probe pin shaft  163  with a probe pin plug  164  situated at one end of the probe pin shaft  163 , and a probe pin head  162  situated at the opposite end of the probe pin shaft  162 . The probe pin head  162  is preferably conical in shape with a point at one end which operates as the electrical contact tip  161 . The second nose cone bore section  145  substantially conforms to the exterior shape and size of the probe pin shaft  163  and a substantial portion of the probe pin head  162 . As described previously, the probe pin  160  comprises a probe pin plug  164  at one end that fits securely into one end of the probe socket  150 . Preferably, the probe pin shaft  163  and probe pin head  162  fit snugly into the second nose cone bore section  145  of the nose cone bore  142  to further assist in holding the probe pin  160  securely in place. The probe pin head  162  preferably extends slightly outside of the nose cone bore  142  to allow the contact tip  161  to make electrical contact with pads, nodes, or pins on the device/board under test. 
   One end of the first nose cone bore section  144  of the nose cone bore  142  opens coaxially into the second nose cone bore section  145  of the nose cone bore  142 . 
   The other end of the first nose cone bore section  144  of the nose cone bore  142  opens into the conical nose cone bore section  143  of the nose cone bore  142  where the diameter of the conical nose cone bore section  143  is the smallest. The axis of the first and second sections coincides, and hence are coaxial. The axis A-A′ of the conical nose cone bore section  143  is arranged at an angle, θ, with respect to the coaxial axis B-B′ of the first and second nose cone bore sections  144  and  145 . In the preferred embodiment, the angle θ is an obtuse angle, where 90°&lt;θ&lt;180°. 
   To assemble the probe tip  110  of the electrical test probe assembly  101 , the probe socket  150  is inserted into the first nose cone bore section  144  of the nose cone bore  142 . In the preferred embodiment, the probe socket  150  is inserted through the conical nose cone bore section  143  of the nose cone bore  142  and into the first nose cone bore section  144  of the nose cone bore  142 . The probe socket  150  is then further inserted until the tab  148  on the interior wall of the first nose cone bore section  144  of the nose cone bore  142  snaps into the exterior recess  158  of the probe socket  150 , thereby locking the probe socket  150  in place within the first nose cone bore section  144  of the nose cone bore  142 . 
   The contact tip of the contact spring wire  130  is then inserted into the conical nose cone bore section  143  of the nose cone bore  142 . 
     FIG. 7A  illustrates the positioning of the contact spring  130  in accordance with a preferred embodiment of the invention. As illustrated in the exploded portion  190   a  of  FIG. 7A , the contact spring enters the conical nose bore section  143  along an axis B-B′ that hits the lower wall of the conical probe socket bore section  153 . Axis B-B′ is arranged at an angle θ with respect to the axis of the first and second equi-diameter probe socket bore sections  154  and  155 . 
   As shown in  FIG. 7A , because of the offset angle θ between the angle of incidence of the contact spring  130  and the axis A-A′ of the probe socket bore  142 , the contact spring is guaranteed a first point of contact  171  at the bending point of the contact spring along the wall of the conical probe socket bore section  153 . As the wire is further inserted into the nose cone  140 , the contact tip of the contact spring wire  130  is forced along the wall of the conical probe socket bore section  153  and into the first equi-diameter probe socket bore section  154 . Upon further insertion, the contact tip of the contact spring wire  130  eventually hits the far wall of the first equi-diameter probe socket bore section  154 , ensuring a second guaranteed point  172  of electrical contact. Further insertion of the contact spring  130  forces the slides the contact spring tip further into the first probe socket bore section  154 . 
     FIG. 7B  shows an alternative embodiment of the probe socket assembly  101  wherein the contact spring  130  enters the first probe socket bore section  154  directly, providing a single guaranteed point of contact  173  at the bending point of the contact spring wire. 
   It will be appreciated that the offset angle θ between the axis of the probe socket bore  152  relative the angle of incidence of the contact spring  130  when the probe socket  150  is inserted into the probe socket  150  thus guarantees electrical contact between the contact spring  130  and probe pin  160 . In particular, because the axes are offset, the contact spring wire  130  must bend under insertion force in at least one place  171 ,  172 ,  173  in order to further insert into the first equi-diameter section  154  of the probe socket  150 . 
   As discussed previously, the second equi-diameter probe socket bore section  155  of the probe socket  150  substantially matches the diameter of the probe pin plug  164  such that the probe pin  160  plug fits snugly in place and in electrical contact within the probe socket  150 . Accordingly, because the contact spring  130  is guaranteed to make electrical contact with the probe socket  150 , as discussed above, the contact spring  130  is also guaranteed to make electrical contact with the probe pin  160 . 
   It will be appreciated from the above detailed description that the contact spring  130  of the electrical test probe assembly  101  is axially independent of the rotational axis of the nose cone  140 . The nose cone  140  may thus be rotated to any angle without requiring the contact spring  130  to also rotate.  FIG. 8  shows a cross-sectional view of the electrical test probe assembly  101  when the contact spring is inserted in the probe socket of the probe tip assembly  101 . As illustrated, the printed circuit board  120  is rotationally at 0° in this example, and the nose cone is positioned arbitrarily at a 45° offset relative to the position of the printed circuit board  120 .  FIG. 9  illustrates a cross-sectional view of the electrical test probe assembly  101  of the invention where the nose cone  140  has been rotated away from the 0° point by another 90°. As shown, the printed circuit board  120  remains at 0° relative the 0° point, while the nose cone is now positioned at 135° relative the 0° point. To accommodate the different position of the nose cone, the contact spring  130  merely bends in a different direction. The contact spring  130  and printed circuit board  120  have not axially rotated. 
   Although this preferred embodiment of the present invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.