Patent Publication Number: US-6704670-B2

Title: Systems and methods for wideband active probing of devices and circuits in operation

Description:
TECHNICAL FIELD 
     The present invention is generally related to monitoring and testing of integrated circuit devices and, more particularly, is related to systems and methods for wideband active probing of devices and circuits in operation. 
     BACKGROUND OF THE INVENTION 
     Continuing advances in integrated circuit (“IC”) technology are a major cause of the demand for improved systems and methods to monitor and/or test IC devices. For example, chips that are mounted on printed circuit boards (“PCBs”) are being developed with higher component densities and smaller physical dimensions. In turn, the chip packages, i.e. the chip housing and electrical connectors, are being designed in more complex and compact configurations. A ball grid array (“BGA”), a chip package that uses an array of solder balls for the electrical connectors, is a typical example of such complex and compact chip package configurations. Other chip package configurations continue to use pins for electrical connectors, but the pins are smaller and arranged to tighter tolerances, and thus, such configurations are also becoming more complex and compact. 
     IC devices are also being developed that have increased performance characteristics. For example, as IC technology advances, central processing unit (“CPU”) chips, for example that are utilized in computers, are being developed to have increased processing speeds. Furthermore, communication buses that interconnect internal IC devices within a computer system are being developed to support increased speed and bandwidth performance. 
     The increased complexity and compactness of chip package configurations and the increased performance characteristics of chips and other IC devices create challenges to effective and efficient monitoring and/or testing of such devices. For example, to monitor and/or test an IC device, electrical signals are typically obtained from the device and input to monitoring and/or testing equipment, such as an oscilloscope or logic analyzer. A probe is typically connected to such monitoring and/or testing equipment and used to obtain the electrical signals by making physical contact with the electrical connectors or other probe points of the IC device, a process typically referred to as “probing.” Thus, in order to facilitate the effective and efficient monitoring and/or testing of IC devices, the probe must have physical and electrical features which overcome the challenges posed by the increased complexity and compactness of the chip package configurations and the increased performance characteristics of the IC devices. 
     Thus far, various systems and methods have been introduced in an attempt to provide physical and electrical features which overcome the challenges to effective and efficient monitoring and/or testing of IC devices, but shortcomings still persist. For example, active probing systems have been introduced that provide high bandwidth (i.e., high frequency) signal reception and low loading of probe tips (i.e., low current through-flow) by positioning the active electronics circuitry as close as possible to the probe tips. But, this practice results in several disadvantages. First, such active probing systems are typically large and bulky, which makes it difficult or impossible to securely connect the probe tips to the probe points of a PCB mounted IC device (e.g, by soldering) to perform hands-free probing and, further, to place the IC device in operation (e.g., by placing the PCB in an enclosure or card-cage) with the probe tips connected to the probe points to perform “in-situ” (i.e., in actual operation) probing. Second, the cable that connects such active probing systems to monitoring and/or testing equipment is usually bulky and inflexible since it contains power conductors (for power supply to the active electronics circuitry) as well as one or more coaxial cables (for signal transmission), and these undesirable features of the connection cable also makes it difficult to maintain the active probing system in position for probing an IC device. Third, such active probing systems are limited in their probing applications because the probe tips have a fixed relative physical positioning and the electrical characteristics of the probe tips (e.g., the damping resistance) are fixed relative to the probing system and cannot be varied. Finally, the cost of such active probing systems is typically too high to justify soldering probing attachments to them without concern for permanently damaging the probing systems. 
     As another example, differential probes (i.e., probes used for measuring the difference between two signals) have been introduced to provide high frequency differential probing of IC devices. Such differential probes have a fixed spacing between the probe tips which limits the configuration of IC device probe points which can be physically contacted for probing. In an attempt to overcome this shortcoming, “bent-wire” probe tip attachments have been introduced which can replace or modify the fixed-spacing probe tips. These bent-wire probe tip attachments can be attached to existing differential probes and bent to vary the spacing between the attached probe tips in order to contact the intended probe points of an IC device. But, the bent-wire probe tip attachments add undesirable parasitic impedance to the probe tip circuit which reduces the bandwidth (i.e., the high frequency signal reception capability) of the differential probe and, thereby, reduces the capability of the differential probe to accurately obtain signals from high frequency IC devices. Additionally, the positioning of the probe tips of the bent-wire probe tip attachments may undesirably vary during probing of an IC device and thereby result in loss of intended contact with the probe points as well unintended contact with other probe points and/or damaging short-circuit conditions. 
     As yet another example, wideband (i.e., high bandwidth) probes have been introduced to measure voltage signals of IC devices. Such wideband probes typically must contact a probe signal point and a ground probe point of an IC device with a probe tip and a ground contact, respectively, to obtain voltage signals. A fixed position “spring wire” (e.g., an offset bent wire) or “pogo pin” (i.e., a telescopically retracting pin) is typically utilized as a ground contact for these wideband probes. Since the distance between a signal probe point and a ground probe point varies among IC devices, the ground contacts of existing wideband probes are typically bent to allow the probe tips to contact the probe points. There are several disadvantages to utilizing a spring wire as the ground contact in existing wideband probes. First, the length of a practical spring wire is relatively long and, thus, adds undesirable parasitic impedance to the probe circuit, thereby reducing the bandwidth capability of the probe. Second, the positioning of the spring wire ground contact may undesirably vary (or “skate”) during probing of an IC device and thereby result in loss of intended contact with the ground probe point as well as cause unintended contact with other probe points and/or damaging short-circuit conditions. Third, the spacing set between the probe tip and the ground contact by bending the spring wire may vary, even during routine handling of the probe, thus, making the positioning accuracy unreliable for repeated probing without repeated bending adjustments. Similarly, in utilizing a pogo pin as the ground contact in existing wideband probes, the typical practice of bending the pogo pin to facilitate contact with a ground probe point may result in skating, particularly since the bend in the rigid pogo pin typically must be maintained by force applied against the probe points during probing. Because of its rigidity, the pogo pin also does not accurately maintain the adjusted position during repeated probing. 
     Based on the foregoing, it should be appreciated that there is a need for improved systems and methods which address the aforementioned, as well as other, shortcomings of existing systems and methods. 
     SUMMARY OF THE INVENTION 
     The present invention provides systems and methods for wideband active probing of devices and circuits in operation. 
     Briefly described, one embodiment of the system, among others, includes a probe amplifier housing. Probe amplifier circuitry is at least partially contained within the probe amplifier housing. Additionally, the probe amplifier circuitry and the probe housing are configured to be separately arranged and positioned from connected probing and signal monitoring apparatuses. 
     Another embodiment of the system includes a probe amplifier unit that includes probe amplifier circuitry. A first plurality of conductors are electrically connected to the probe amplifier circuitry and a signal monitoring apparatus is electrically connected to the probe amplifier circuitry through the first plurality of conductors. A second plurality of conductors are electrically connected to the probe amplifier circuitry and a probing apparatus is electrically connected to the probe amplifier circuitry through the second plurality of conductors. 
     Yet another embodiment of the system includes means for amplifying a probe signal. Means for power supply and signal transmission and means for signal transmission are each electrically connected to the means for amplifying a probe signal. Means for monitoring signals are electrically connected to the means for amplifying a probe signal via the means for power supply and signal transmission. Additionally, means for probing an electrical circuit are electrically connected to the means for amplifying a probe signal via the means for signal transmission. 
     The present invention can also be viewed as providing methods for wideband active probing of devices and circuits in operation. In this regard, one embodiment of such a method, among others, can be broadly summarized by the following steps: providing a probe amplifier unit that includes probe amplifier circuitry, electrically connecting a first plurality of conductors to the probe amplifier circuitry, electrically connecting a signal monitoring apparatus to the first plurality of conductors, electrically connecting a second plurality of conductors to the probe amplifier circuitry, electrically connecting a probing apparatus to the second plurality of conductors, probing an electrical circuit with the probing apparatus, and monitoring signals with the signal monitoring apparatus. 
     Other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Many aspects of the invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
     FIG. 1A is a perspective view of an embodiment of a probe amplifier unit. 
     FIG. 1B is a perspective view of an embodiment of a probe amplifier unit connected to a differential probe unit. 
     FIG. 1C is a perspective view of an embodiment of a probe amplifier unit connected to a single-end probe unit. 
     FIG. 1D is a perspective view of an embodiment of a probe amplifier unit connected to a differential probe tip unit. 
     FIG. 1E is a perspective view of an embodiment of a probe amplifier unit connected to a single-end probe tip unit. 
     FIG. 2A is a plan view of an embodiment of the differential probe unit depicted in FIG.  1 B. 
     FIG. 2B is a cross-sectional view of an embodiment of the differential probe unit depicted in FIG.  1 B. 
     FIG. 2C is an end view of an embodiment of the differential probe unit depicted in FIG.  1 B. 
     FIG. 2D is a side view of an embodiment of the differential probe unit of FIG. 1B depicted with the probe tips contacting solder points on a printed circuit board. 
     FIG. 2E is an end view of an embodiment of the differential probe unit of FIG. 1B with the probe tips positioned at a minimal spacing. 
     FIG. 2F is an end view of an embodiment of the differential probe unit of FIG. 1B with the probe tips positioned at a middle spacing. 
     FIG. 2G is an end view of an embodiment of the differential probe unit of FIG. 1B with the probe tips positioned at a maximal spacing. 
     FIG. 3A is a plan view of an embodiment of the single-end probe unit depicted in FIG.  1 C. 
     FIG. 3B is a cross-sectional view of an embodiment of the single-end probe unit depicted in FIG.  1 C. 
     FIG. 3C is an end view of an embodiment of the single-end probe unit depicted in FIG.  1 C. 
     FIG. 3D is a side view of an embodiment of the single-end probe unit of FIG. 1C depicted with the probe tip and ground tip contacting solder points on a printed circuit board. 
     FIG. 3E is an end view of an embodiment of the single-end probe unit of FIG. 1C with the ground tip positioned at a minimal spacing to the probe tip. 
     FIG. 3F is an end view of an embodiment of the single-end probe unit of FIG. 1C with the ground tip positioned at a middle spacing to the probe tip. 
     FIG. 3G is an end view of an embodiment of the single-end probe unit of FIG. 1C with the ground tip positioned at a maximal spacing from the probe tip. 
     FIG. 4 is a perspective view of an embodiment of the differential probe tip unit depicted in FIG.  1 D. 
     FIG. 5 is a perspective view of an embodiment of the single-end probe tip unit depicted in FIG.  1 E. 
     FIG. 6 depicts exemplary probe tip units that may be implemented in various embodiments the differential and/or single-end probe tip units of FIGS. 1D and 1E. 
    
    
     DETAILED DESCRIPTION 
     Having summarized the invention above, reference is now made in detail to the figures which depict exemplary embodiments of the invention. Referring to FIG. 1A, a perspective view of an embodiment of a probe amplifier unit  102  is shown. The probe amplifier unit  102  includes an enclosure, housing, or other structure that encloses probe amplifier circuitry (not shown). Typically, such probe amplifier circuitry supports a wideband active probing system that provides high bandwidth signal reception and low loading of connected probe tips. For example, the probe amplifier circuitry housed within the probe amplifier unit  102  may provide a probing bandwidth of at least 3 GHz. 
     The probe amplifier unit  102  may be constructed of various materials. For example, the probe amplifier unit  102  may be constructed of various types of plastic material. Other materials may be used to construct the probe amplifier unit  102  within the scope of the invention. Such other materials preferably should not interfere with the operation or use of the probe amplifier unit  102 . 
     A power supply/signal transmission cable  104  is typically connected to the probe amplifier unit  102 . The power supply/signal transmission cable  104  typically includes cable insulation or sheathing that covers various conductors. For example, the power supply/signal transmission cable  104  may contain a coaxial conductor  105 , such as a coaxial cable, for transmission of signals. Additionally, the power supply/signal transmission cable  104  may contain one or more power conductors  106 , such as insulated copper conductors that are solid or stranded in configuration. Typically, the power conductors  106  contained within the power supply/signal transmission cable  104  are configured to provide DC electrical power to the probe amplifier unit  102 . Thus, for example, the power conductors  106  may include a positive conductor, a negative conductor, and a ground conductor. However, the power conductors  106  may also include additional conductors for the purpose of providing electrical power to the probe amplifier unit  102 . The power supply/signal transmission cable  104  may additionally include other conductors for various purposes such as, for example, to transmit control signals to/from the probe amplifier unit  102 . It is noted that in some embodiments (not depicted), the foregoing various conductors may not be contained within a cable or within just a single cable. Further, the power supply/signal transmission cable  104  and/or the various conductors contained therein may include other types of cables or conductors that serve the aforementioned purposes. 
     The power supply/signal transmission cable  104  is usually connected to the probe amplifier unit  102  by a power supply/signal transmission cable connector  108 . The power supply/signal transmission cable connector  108  may include one or more connectors and/or connections to the probe amplifier unit  102  as depicted. For example, the coaxial conductor  105  of the power supply/signal transmission cable  104  may be connected to the probe amplifier circuitry of the probe amplifier unit  102  by a coaxial connector  109 . As another example, the power conductors  106  of the power supply/signal transmission cable  104  may be connected to the probe amplifier circuitry of the probe amplifier unit  102  by solder terminal connections (not shown). Various other types of connectors and/or connections may be implemented to connect the various conductors of the power supply/signal transmission cable  104  to the probe amplifier unit  102  within the scope of the invention. 
     The power supply/signal transmission cable connector  108  may also include a strain relief device  118 . Such a strain relief device  118  functions to prevent excessive strain from being placed on the various conductors, connectors, and/or connections at the connection point of the power supply/signal transmission cable connector  108  to the probe amplifier unit  102 . For example, if the probe amplifier unit  102  is handled, strain may be transferred from the probe amplifier unit  102  to the strain relief device  118  to the sheathing of the power supply/signal transmission cable  104 . In some embodiments, the power supply/signal transmission cable connector  108  may also function as a strain relief device. Various types and configurations of strain relief devices may be implemented within the scope of the invention. 
     As indicated in FIG. 1A, the far end of the power supply/signal transmission cable  104  is typically connected to monitoring and/or testing equipment (not depicted). For example, the power supply/signal transmission cable  104  may be connected to an oscilloscope or logic analyzer. The far end of the power supply/signal transmission cable  104  may also be connected to other types of equipment to facilitate the function of the probe amplifier unit  102 . 
     One or more probe cables, examples of which are indicated by reference numerals  112  and  113 , may also be connected to the probe amplifier unit  102 . Typically, these probe cables  112 ,  113  convey signals received by a probe unit (not shown) to the probe amplifier unit  102 . The probe cables  112 ,  113  may include cable insulation or sheathing that contains one or more types of conductors. Typically, the probe cables  112 ,  113  are insulated coaxial conductors (e.g., coaxial cables) that are adapted to convey signals to facilitate wideband probing. The probe cables  112 ,  113  may be substantially flexible to facilitate handling of the probe amplifier unit  102  without causing substantial movement of a connected probe unit. The probe cables  112 ,  113  may include other types of conductors that serve to facilitate wideband probing or other functions. 
     As depicted in FIG. 1A, the probe cables  112 ,  113  are typically connected to the probe amplifier unit  102  by probe cable connectors  114 ,  115 , respectively. The probe cable connectors  114 ,  115  may include one or more connectors and/or connections to the probe amplifier unit  102 . For example, one or more of the probe cable connectors  114 ,  115  may be coaxial connectors. Typically, the probe cable connectors  114 ,  115  provide a connection of the probe cables  112 ,  113  to the probe amplifier circuitry contained within the probe amplifier unit  102 . Various other types of connectors and/or connections may be implemented to connect the probe cables  112 ,  113  to the probe amplifier unit  102  within the scope of the invention. 
     The probe cable connectors  114 ,  115  may also include strain relief devices, for example, as depicted. Such strain relief devices prevent excessive strain from being placed on the various conductors, connectors, and/or connections at the connection points of the probe cables  112 ,  113  to the probe amplifier unit  102 . In some embodiments, the probe cable connectors  114 ,  115  may also function as strain relief devices, as depicted for example in FIG.  1 A. Various other types and configurations of strain relief devices may also be implemented within the scope of the invention. 
     The probe cables  112 ,  113  are typically connected to a probe unit (not depicted). In this regard, FIGS. 1B-1E depict the probe amplifier unit  102  connected to various probe units by one or more probe cables  112 ,  113 . 
     FIG. 1B depicts the probe amplifier unit  102  connected to a differential probe unit  200  by the probe cables  112 ,  113 . FIG. 1C depicts the probe amplifier unit  102  connected to a single-end probe unit  300  by the probe cable  112 . FIG. 1D depicts the probe amplifier unit  102  connected to a differential probe tip unit  400  by the probe cables  112 ,  113 . And, FIG. 1E depicts the probe amplifier unit  102  connected to a single-end probe tip unit  500  by the probe cable  112 . The foregoing probe units will be discussed in further detail below. 
     As discussed above with respect to FIG. 1A, the probe amplifier unit  102  may facilitate wideband active probing by interfacing a probe unit to monitoring and/or testing equipment. In that regard, the implementation of the probe amplifier unit  102  provides physical and electrical features not provided by existing systems and methods. These features overcome challenges posed by the increased complexity and compactness of IC chip package configurations and the increased performance characteristics of IC devices. For example, the implementation of the probe amplifier unit  102  provides high bandwidth signal reception (e.g., 3 GHz or greater) and low loading of the probe tips of a connected probe unit while allowing the probe amplifier circuitry to be positioned remotely from the probe unit. The implementation of the probe amplifier unit  102  also provides other beneficial physical and electrical features that are not provided by existing systems and methods. These additional features are discussed hereafter. 
     The implementation of the probe amplifier unit  102  to facilitate wideband active probing of IC devices provides the capability to securely connect a probe tip unit (e.g., the probe tip units  400 ,  500  depicted in FIGS. 1D,  1 E) to an IC device. This capability provides such benefits as hands-free probing and minimized parasitic input impedance at the connection to the probe points. This capability to securely connect a probe tip unit is available for several reasons. One, the probe tip unit does not need to include the probe amplifier circuitry since that circuitry is remotely contained in the probe amplifier unit  102 . As a result, the probe tip unit is sufficiently small and lightweight to facilitate secure connections of the probe tip unit to the probe points of an IC device (e.g., by soldering the probe tips of the probe tip unit to the probe points of an IC device on a PCB). Two, the probe tip unit is typically connected to the probe amplifier unit  102  by one or more probe cables  112 ,  113 . As discussed previously, the probe cables  112 ,  113  are typically substantially flexible such that a probe tip unit that is secured to an IC device is not physically disturbed by movement of the probe amplifier unit  102  or other connected components. 
     The implementation of the probe amplifier unit  102  to facilitate wideband active probing of IC devices also provides the capability to perform probing of an IC device while in operation. That is, a probe tip unit can be secured (e.g., by soldering) to the probe points of an IC device and the IC device can thereafter be probed while it is in operation. For example, a PCB can be placed in its usual enclosure or card-cage and probed while in operation. This is possible because of the relatively small size and light weight of the probe tip unit, since it typically does not include the probe amplifier circuitry, and because of the flexibility of the probe cables  112 ,  113 , which in this implementation do not include multiple types of conductors (e.g., signal and power) that tend to make a cable inflexible and bulky. 
     The implementation of the probe amplifier unit  102  for wideband active probing also provides the capability to efficiently and inexpensively modify electrical characteristics of the wideband active probing system such as the damping resistance of the probe tips. This is because various probe units and probe tip units (e.g., the probe units  200 ,  300  and probe tip units  400 ,  500  depicted in FIGS. 1B,  1 C,  1 D, and  1 E, respectively, and variations thereof) can be connected to the probe amplifier unit  102  to modify the electrical characteristics of the overall probing system (i.e., the probe unit or probe tip unit, probe amplifier unit, and connecting conductors). This capability to interchange probe tip units connected to the probe amplifier unit  102  also provides the convenient and efficient capability to pre-secure probe tip units to various IC devices and then connect the probe amplifier unit  102  to any one of the probe tip units while the IC device is in operation. This is convenient and efficient, for example, when several IC devices are tested with one available active probing system. 
     Finally, the implementation of the probe amplifier unit  102  for wideband active probing provides an inexpensive capability to perform wideband active probing on a variety of IC devices. Since the probe amplifier circuitry is contained in the probe amplifier unit  102  and not in the probe units, the probe units are less costly and, thus, various probe units can be obtained to conform to various physical and electrical probing constraints at a lower overall expense than obtaining multiple existing probe units that each include probe amplifier circuitry. Additionally, probe tip units (e.g., the probe tip units  400 ,  500 ) used with the probe amplifier unit  102  can be secured to probe points without the concern of possibly damaging more costly existing probe tip units that include probe amplifier circuitry. 
     It should be apparent from the foregoing discussion that the implementation of the probe amplifier unit  102  for wideband active probing provides beneficial physical and electrical features not provided by existing systems and methods. These features overcome the challenges posed by the increased complexity and compactness of IC chip package configurations and the increased performance characteristics of IC devices. Having discussed several embodiments of the probe amplifier unit  102  as well as several benefits provided by the implementation of the probe amplifier unit  102  for wideband active probing, discussion is now focused in more detail on several embodiments of probe units and probe tip units that can be implemented, for example, with the probe amplifier unit  102  for wideband active probing. 
     FIGS. 2A-2G show various views of embodiments of a differential probe unit  200  of the present invention. As is known, a differential probe unit typically obtains two signals that are then manipulated (for example by probe amplifier circuitry) to obtain a differential signal (i.e., the difference between the signals). The differential probe unit  200  includes a probe unit housing  202  which typically contains various components of the differential probe unit  200 . The probe unit housing  202  may be constructed of various materials. For example, the probe unit housing  202  may be constructed of various types of plastic. Other materials may be used to construct the probe unit housing  202  that, preferably, do not interfere with the operation or use of the differential probe unit  200 . 
     The differential probe unit  200  also includes probe barrels  204 ,  205 . The probe barrels  204 ,  205  are typically cylindrical in shape and have an interior volume. Typically, the probe barrels  204 ,  205  are constructed of an electrically conductive material, for example a metal or alloy, such that they function as electrical conductors. In some embodiments, various known probe barrel designs may be implemented for the probe barrels  204 ,  205  within the scope of the invention. 
     As depicted for example in FIGS. 2A-2B, the probe barrels  204 ,  205  typically extend partially outside of the probe unit housing  202  such that a portion of the probe barrels  204 ,  205  are at least partially surrounded by and/or contained within the probe unit housing  202 . The length of the portion of the probe barrels  204 ,  205  that extends outside of the probe unit housing  202  may vary to facilitate various probing applications. The probe barrels  204 ,  205  each typically extend out of the probe unit housing  202  through openings in the probe unit housing  202  as depicted, for example, in FIG.  2 C. Further, the probe barrels  204 ,  205  may rest on portions of the internal structure (not shown) of the probe unit housing  202  in order to maintain the probe barrels  204 ,  205  in a substantially consistent position when the differential probe unit  200  is handled or used for probing. 
     Probe barrel nose cones  206 ,  207  (also referred to as “probe barrel end caps”) extend from the ends of the probe barrels  204 ,  205  that extend outside the probe unit housing  202 . The probe barrel nose cones  206 ,  207  are typically substantially conical in shape, although other shapes may be implemented, and typically have an interior volume. Typically, the probe barrel nose cones  206 ,  207  are constructed of an electrically non-conductive material, for example a plastic or rubber, such that they function as electrical insulators. In some embodiments, various known probe barrel nose cone designs may be implemented for the probe barrel nose cones  206 ,  207  within the scope of the invention. 
     The longitudinal axes of the probe barrel nose cones  206 ,  207  extend from the probe barrels  204 ,  205  at offset angles a 1  and a 2 , respectively, from the longitudinal axes of the probe barrels  204 ,  205 , as depicted for example in FIG.  2 A. Typically, the offset angles a 1 , a 2  are each at least 15° and at most 25°. However, the offset angle a 1 , a 2  may be other independent values within the scope of the invention. The benefits of this offset angle feature of the probe barrel nose cones  206 ,  207  will be discussed below. 
     Probe tips  208 ,  209  extend partially out of the ends of the probe barrel nose cones  206 ,  207 , respectively, as depicted in FIGS. 2A-2B. Thus, a portion of each probe tip  208  and  209  is at least partially surrounded by a respective probe barrel nose cone  206  and  207 . The probe tips  208 ,  209  are typically constructed of a substantially conductive material, for example a metal or alloy, such that they function as electrical conductors. The probe tips  208 ,  209  are typically cylindrical in shape, although other shapes may be implemented within the scope of the invention. Further, the probe tips  208 ,  209  may have ends with various shapes, for example pointed, in order to facilitate making and/or maintaining physical electrical contact with probe points on a device under test. The probe tips  208 ,  209  may include impedance elements (not shown) to facilitate wideband active probing. In some embodiments, various known probe tip designs may be implemented for the probe tips  208 ,  209 . 
     The probe tips  208 ,  209  extend out of the probe barrel nose cones  206 ,  207  substantially in line with the longitudinal axes of the probe barrel nose cones  206 ,  207 , respectively. Thus, the longitudinal axes of the probe tips  208 ,  209  are offset from the longitudinal axes of the probe barrels  204 ,  205 , respectively, at substantially the offset angles a 1  and a 2 , which as discussed above, typically are each at least 15° and at most 25°. Further, as discussed above, the offset angles a 1 , a 2  may be other independent values. The benefits of this offset feature of the probe tips  208 ,  209  will be discussed below. 
     Probe cables  212 ,  213  typically extend from within the interior of the probe unit housing  202  to outside of the probe unit housing  202 . As indicated for example in FIGS. 2A-2B, the far ends of the probe cables  212 ,  213  typically connect to a probe amplifier (such as the probe amplifier unit  102  depicted in FIG.  1 A and discussed above). Further, as depicted for example in FIG. 2A, the local ends of the probe cables  212 ,  213  connect to the probe barrels  204 ,  205 , respectively. The probe cables  212 ,  213  may connect to the probe barrels  204 ,  205  in various ways within the scope of the invention, including various known ways of connecting cables to probe barrels. The connection of the probe cables  212 ,  213  to the probe barrels  204 ,  205  will be discussed further below. 
     The probe cables  212 ,  213  may include insulation or sheathing that contains one or more types of conductors. Typically, the probe cables  212 ,  213  are insulated coaxial conductors (e.g., coaxial cables) that are adapted to convey signals to facilitate wideband probing. In some embodiments, the probe cables  212 ,  213  may be substantially flexible to facilitate handling of a connected probe amplifier unit without causing substantial movement of the differential probe unit  200 . The probe cables  212 ,  213  may include other types of conductors that serve to facilitate wideband active probing or other functions, within the scope of the invention. 
     One or more strain relief devices (not shown) may extend from the probe unit housing  202  and attach to the probe cables  212 ,  213 . Such strain relief devices serve to prevent excessive strain from being placed on the connections of the probe cables  212 ,  213  to the probe barrels  204 ,  205  and/or other components of the differential probe unit  200 . In some embodiments, various known types of strain relief devices may be implemented within the scope of the invention. Further, in some embodiments, one or more strain relief devices (not depicted) may be implemented alternatively at the connection of the probe cables  212 ,  213  to the probe barrels  204 ,  205 . 
     The differential probe unit  200  may also include one or more slots  214 ,  215 , as depicted for example in FIGS. 2A and 2C. These slots  214 ,  215  define openings to the interior of the probe unit housing  202  such that the probe barrels  204 ,  205  are at least partially exposed. The openings defined by the slots  214 ,  215  may have various shapes, for example a substantially rectangular shape as depicted in FIG.  2 C. 
     One or more positioning elements  216 ,  217  may extend out of the slots  214 ,  215 , respectively. The positioning elements  216 ,  217  are attached to the probe barrels  204 ,  205 , respectively, such that movement of the positioning elements  216 ,  217  causes movement of the probe barrels  204 ,  205 . The positioning elements  216 ,  217  may, for example, be implemented as levers that are attached to the probe barrels  204 ,  205 . But, other types of components that may serve to transfer movement to the probe barrels  204 ,  205  through the slots  214 ,  215  may be implemented within the scope of the invention. Typically, the positioning elements  216 ,  217  are constructed of a non-conductive material such as a plastic. Additionally, other methods may be implemented within the scope of the invention to cause the movement of the probe barrels  204 ,  205 . For example, internal gears, levers, and/or other such components in mechanical communication with external knobs, slide switches, and/or other such components (not depicted) may be implemented to cause the movement of the probe barrels  204 ,  205  within the scope of the invention. 
     One or more elastic compressible elements  220 ,  221  may also be included in the differential probe unit  200 . For example, as depicted in FIGS. 2A-2B, elastic compressible elements  220 ,  221  may engage between the probe barrels  204 ,  205 , respectively, and a portion of the probe unit housing  202 . The elastic compressible elements  220 ,  221  may be implemented, for example, as normally decompressed or lightly compressed helical springs. In such an implementation, the elastic compressible elements  220 ,  221  may at least partially surround a portion of the probe cables  212 ,  213  within the probe unit housing  202 , as depicted for example in FIGS. 2A-2B. 
     The differential probe unit  200  may also include a ground connector  224 , as depicted for example in FIGS. 2A and 2C. The ground connector  224  is typically constructed of a conductive material, such as a metal or alloy, such that it functions as an electrical conductor. The ground connector  224  is typically connected to or in substantial contact with the probe barrels  204 ,  205  such that the probe barrels  204 ,  205  and the ground connector  224  are in electrical communication and maintain substantially the same electrical potential. The ground connector  224  may also be implemented at other locations of the differential probe unit and in other forms than that depicted in FIGS. 2A and 2C. 
     The combination of the probe cables  212 ,  213 , the probe barrels  204 ,  205 , the probe barrel nose cones  206 ,  207 , and the probe tips  208 ,  209 , respectively, typically serve to sense and convey signals, that are obtained for example from an IC device, to a probe amplifier and subsequently to monitoring and/or testing equipment when the differential probe unit  200  is used to perform probing. In that regard typically at least one conductor of the probe cables  212 ,  213  is electrically connected to the probe barrels  204 ,  205 , respectively, for example a ground conductor. Further, typically at least one other conductor of the probe cables  212 ,  213  is electrically connected to the probe tips  208 ,  209 , respectively, for example a signal conductor. These connections may be made directly to the probe tips  208 ,  209  or through various circuitry (not shown) internal to the probe barrels  204 ,  205  and/or the probe barrel nose cones  206 ,  207 . The probe tips  208 ,  209  extend out of the probe barrel nose cones  206 ,  207 , respectively, and are held in a substantially fixed position by at least the probe barrel nose cones  206 ,  207 . Further, the probe barrel nose cones  206 ,  207  typically electrically isolate the probe tips  208 ,  209  from the probe barrels  204 ,  205 , respectively. Thus, when the probe tips  208 ,  209  are brought into contact with probe points of an IC device, for example, signals are obtained by the probe tips  208 ,  209  that may be transmitted to a probe amplifier by one or more conductors of the probe cables  212 ,  213  that are electrically connected to the probe tips  208 ,  209 , respectively. 
     The combined structures of the probe barrels  204 ,  205 , probe barrel nose cones  206 ,  207 , and probe tips  208 ,  209 , respectively, which will be collectively referred to hereafter as “probe assemblies”, may be capable of several position adjustments. The position adjustment capabilities of the probe assemblies are possible without temporarily or permanently forcing the deformation of components of the probe assemblies. For example, the probe assemblies can be adjusted to various positions without bending or stretching components of the probe assemblies to conform to the positions of probe points. Further, the probe assemblies are capable of position adjustments without degrading the overall electrical features of the differential probe unit  200 , such as high bandwidth performance. 
     Each probe assembly is capable of retracting into the probe unit housing  202  along the longitudinal axis of the probe barrels  204 ,  205 , respectively, as indicated in FIGS. 2A and 2D. The elastic compressible elements  220 ,  221 , which may engage the probe barrels  204 ,  205 , typically maintain the probe assemblies partially extended out of the probe unit housing  202 . However, if pressure is applied to, for example, the probe tip  209 , the respective probe assembly is able to retract partially into the probe unit housing  202 , as depicted for example in FIG.  2 D. Further, when the pressure on the probe tip  208  is removed, the elastic compressible element  221  may cause the respective probe assembly to re-extend out of the probe unit housing  202  to its typical partially extended position. The extent of retraction and extension of the probe assemblies may be limited, for example, by portions of the internal structure of the probe unit housing  202 . This capability of the probe assemblies to reciprocate (i.e., move in/out of the probe unit housing  202 ) allows the probe tips  208 ,  209  to make effective contact with probe points that are at different heights from a common surface, for example two solder points on a PCB as depicted in FIG. 2D, when the differential probe unit  200  is utilized for probing. The probe cables  212 ,  213  connected to the probe assemblies are arranged within the probe unit housing  202  to facilitate the reciprocating movement of the probe assemblies. For example, if the probe cables  212 ,  213  are attached to a strain relief device that extends from the probe unit housing  202  (as described above), the strain relief device may allow sufficient movement of the probe cables  212 ,  213  in accordance with the reciprocating movement of the probe assemblies to avoid placing substantial strain on the probe cables  212 ,  213 . As another example, a sufficient portion of the probe cables  212 ,  213  may be stored within the probe unit housing  202  to facilitate the reciprocating movement of the probe assemblies without placing substantial strain on the probe cables  212 ,  213 . 
     Each probe assembly (i.e., probe barrel  204 ,  205 , probe barrel nose cone  206 ,  207 , and probe tip  208 ,  209 , respectively) may also be capable of rotational movement about the longitudinal axis of the probe barrels  204 ,  205 , respectively, as indicated for example in FIGS. 2E-2G. As described above, the internal structure of the probe unit housing  202  is adapted to maintain the probe barrels  204 ,  205 , and thus the probe assemblies, in a substantially consistent position when the differential probe unit  200  is handled or used for probing. In this regard, the internal structure of the probe unit housing  202  is adapted to maintain the probe assemblies in at least a substantially consistent rotational position with respect to the longitudinal axes of the probe barrels  204 ,  205 . Each probe assembly can be rotated about the longitudinal axis of its respective probe barrel  204 ,  205  by, for example, applying rotational force to the assembly. Once the probe assembly is rotated to a particular position, the internal structure of the probe unit housing  202  causes it to maintain that rotational position during handling or probing with the differential probe unit  200  due to, for example, the frictional engagement of the probe barrels  204 ,  205  and the internal structure of the probe unit housing  202 . The probe cables  212 ,  213  connected to the probe assemblies are arranged within the probe unit housing  202  to facilitate the rotational movement of the probe assemblies. For example, if the probe cables  212 ,  213  are attached to a strain relief device that extends from the probe unit housing  202  (as described above), the strain relief device may allow sufficient movement of the probe cables  212 ,  213  in accordance with the rotational movement of the probe assemblies to avoid placing substantial strain on the probe cables  212 ,  213 . As another example, a sufficient portion of the probe cables  212 ,  213  may be stored within the probe unit housing  202  to facilitate the rotational movement of the probe assemblies without placing substantial strain on the probe cables  212 ,  213 . 
     As described above, the differential probe unit  200  may include slots  214 ,  215  through which positioning elements  216 ,  217  extend, and positioning elements  216 ,  217  may be attached to the probe barrels  204 ,  205 , respectively. In this regard, the positioning elements  216 ,  217  may be an integral part of the probe assemblies. Thus, when force is applied to one of the positioning elements  216 ,  217 , it causes the respective probe assembly to rotate accordingly. As depicted for example in FIGS. 2E and 2G, the rotational movement of each probe assembly may be limited when the positioning elements  216 ,  217  engage the sides of the slots  214 ,  215 , respectively. For example, with respect to FIG. 2E, if the positioning element  217  is moved substantially upward, the respective probe assembly will be rotated until the positioning element  217  engages the top edge of the slot  215 . Thus, when the differential probe unit  200  is used for probing, each probe assembly can be independently rotated to a desired position by moving the positioning elements  216 ,  217 . Furthermore, the probe assemblies will remain in the desired position (as described above) when the positioning elements  216 ,  217  are no longer moved. 
     The rotational capability of the probe assemblies and the offset axis configuration of the probe barrel nose cones  206 ,  207  and probe tips  208 ,  209  from the probe barrels  204 ,  205  respectively, as described above, provides the benefit of a variable spacing capability between the probe tips  208 ,  209 . This variable spacing capability of the probe tips  208 ,  209  facilitates probing of various configurations of probe points, for example of an IC device mounted on a PCB, without bending or stretching components of the probe assemblies or forcing the components to conform to probe point spacing by temporarily deforming them. Further, the variable spacing capability of the probe tips  208 ,  209  does not degrade the overall electrical features of the differential probe unit  200 , such as high bandwidth performance. 
     FIGS. 2E-2G show exemplary depictions of the variable spacing capability of the probe tips  208 ,  209 . As depicted in FIG. 2E, when the positioning elements  216 ,  217  are moved substantially upward until they engage the top edge of the slots  214 ,  215 , the probe assemblies may be rotated such that the probe tips  208 ,  209  are positioned at a minimal spacing D 1 . This spacing D 1  of the probe tips occurs because the probe assemblies are rotated such that the probe tips  208 ,  209  are tilted inward due to their offset longitudinal axes from the longitudinal axes of the probe barrels  204 ,  205 , respectively, as discussed above. Similarly, FIG. 2F depicts the positioning levers  216 ,  217  moved to a position approximately midway between the top and bottom edges of the slots  214 ,  215 , and accordingly, the probe assemblies may be rotated such that the probe tips  208 ,  209  are positioned at a spacing D 2  which is greater than spacing D 1 . Finally, in FIG. 2G, the positioning elements are moved substantially downward to engage the bottom edge of the slots  214 ,  215  causing the probe assemblies to rotate such that the probe tips  208 ,  209  may be positioned at a maximal spacing D 3  which is greater than spacing D 2 . At this position, the probe tips  208 ,  209  are tilted outward due to their offset longitudinal axes from the longitudinal axes of the probe barrels  204 ,  205 , respectively, as discussed above. 
     It is noted for clarity that the foregoing descriptions with respect to FIGS. 2E-2G are exemplary of the variable spacing capabilities of the probe tips  208 ,  209 , but are not exclusive. For example, depending on the embodiment of the differential probe unit  200 , the positioning and spacing of the probe tips  208 ,  209  with respect to the positions of the positioning elements  216 ,  217  may vary from what is depicted in FIGS. 2E-2G and described in the foregoing. 
     Discussion is now focused on another group of probe unit embodiments of the present invention. FIGS. 3A-3G depict various views of embodiments of a single-end probe unit  300  of the present invention. As is known, a single-end probe unit, in contrast to a differential probe unit, typically obtains a signal with respect to ground, such as a voltage signal from an IC device. The single-end probe unit  300  includes a probe unit housing  302  which typically contains various components. The probe unit housing  302  is typically substantially cylindrical and has an interior volume. Further, the probe unit housing  302  is typically constructed of an electrically non-conductive material, for example a plastic or rubber, such that it functions as an electrical insulator. In some embodiments, various known probe housing designs may be implemented for the probe unit housing  302  within the scope of the invention. 
     The single-end probe unit  300  further includes a probe barrel  304 . The probe barrel  304  is typically cylindrical in shape and has an interior volume. Typically, the probe barrel  304  is constructed of an electrically conductive material, for example a metal or alloy, such that it functions as an electrical conductor. In some embodiments, various known probe barrel designs may be implemented for the probe barrel  304 . 
     As depicted in FIGS. 3A-3B, the probe barrel  304  typically extends partially outside of the probe unit housing  302  such that a portion of the probe barrel  304  is at least partially surrounded by and/or contained within the probe unit housing  302 . The length of the portion of the probe barrel  304  that extends outside of the probe unit housing  302  may vary to facilitate various probing applications. The probe barrel  304  typically extends out of the probe unit housing  302  through an opening in the probe unit housing  302  as depicted in FIG.  3 C. Further, the probe barrel  304  may rest on portions of the internal structure (not shown) of the probe unit housing  302  in order to maintain the probe barrel  304  in a substantially consistent position when the single-end probe unit  300  is handled or used for probing. 
     A probe barrel nose cone  306  (also referred to as a “probe barrel end cap”) extends from the end of the probe barrel  304  that extends outside the probe unit housing  302 . The probe barrel nose cone  306  is typically substantially conical in shape, although other shapes may be implemented, and typically has an interior volume. Typically, the probe barrel nose cone  306  is constructed of an electrically non-conductive material, for example a plastic or rubber, such that it functions as an electrical insulator. In some embodiments, various known probe barrel nose cone designs may be implemented for the probe barrel nose cone  306 . 
     The longitudinal axis of the probe barrel nose cone  306  extends from the probe barrel  304  at an offset angle a 3  from the longitudinal axis of the probe barrel  304 , as depicted for example in FIG.  3 A. Typically, the offset angle a 3  is at least 15° and at most 25°. However, the offset angle a 3  may be other values within the scope of the invention. The benefits of this offset feature will be discussed below. 
     A probe tip  308  extends partially out of the end of the probe barrel nose cone  306  as depicted in FIGS. 3A-3B. Thus, a portion of the probe tip  308  is at least partially surrounded by the probe barrel nose cone  306 . The probe tip  308  is typically cylindrically shaped, although other shapes may be implemented within the scope of the invention. The probe tip  308  is typically constructed of a substantially conductive material, for example a metal or alloy, such that it functions as an electrical conductor. The probe tip  308  may include one or more resistive elements (not shown) to facilitate wideband active probing. Further, the end of the probe tip  308  may have various shapes, for example pointed, to facilitate making and/or maintaining contact with a probe point. In some embodiments, various known probe tip designs may be implemented for the probe tip  308 . 
     The probe tip  308  extends out of the probe barrel nose cone  306  substantially in line with the longitudinal axes of the probe barrel nose cone  306 . Thus, the longitudinal axis of the probe tip  308  is offset from the longitudinal axis of the probe barrel  304  at substantially the offset angle a 3 , which as discussed above, typically is at least 15° and at most 25°. Further, as discussed above, the offset angle a 3  may be other values. The benefits of this offset feature of the probe tip  308  will be discussed below. 
     The single-end probe unit  300  also includes a ground tip unit  314  that is attached to the probe barrel  304 . The ground tip unit  314  has a ground tip connector  315  which is typically substantially annular shaped. The ground tip connector  315  at least partially surrounds a portion of the probe barrel  304  that extends from the probe unit housing  302 . The ground tip connector  315  is adapted to maintain the ground tip unit  314  in a substantially consistent position when the single-end probe unit  300  is handled or used for probing. 
     A ground tip receptacle  316  is connected to the ground tip connector  315 . The ground tip receptacle  316  is typically cylindrical in shape and has an interior volume. A ground tip  317  extends from inside of the ground tip receptacle  316 . The ground tip  317  is typically cylindrical in shape, although other shapes may be implemented. The end of the ground tip  317  may have various shapes, for example pointed, to facilitate making and/or maintaining contact with a probe point. The ground tip  317  extends partially outside of the interior of the ground tip receptacle  316  such that a portion of the ground tip  317  is at least partially surrounded by and/or contained within the ground tip receptacle  316 . The end of the ground tip  317  that is partially surrounded by and/or contained within the ground tip receptacle  316  and the internal structure of the ground tip receptacle  316  are typically adapted such that the ground tip  317  is retained at least partially within the ground tip receptacle  316 . For example, as depicted in FIG. 3B, the end of the ground tip  317  may be flared to a diameter that is wider than the opening in the ground tip receptacle  316  through which the ground tip  317  extends such that the flared end engages the internal structure of the ground tip receptacle  316 . 
     As depicted in FIG. 3B, the ground tip unit  314  may also include an elastic compressible element  320  disposed within the ground tip receptacle  316 . The elastic compressible element  320  typically engages the end of the ground tip  317  and a portion of the internal structure of the ground tip receptacle  316 . The elastic compressible element  320  may be implemented, for example, as a normally decompressed or lightly compressed helical spring. 
     The components of the ground tip unit  314 , i.e. the ground tip connector  315 , the ground tip receptacle  316 , the ground tip  317 , and the elastic compressible element  320 , are typically constructed of a substantially conductive material, for example a metal or alloy, such that they function as electrical conductors. Further, these components of the ground tip unit  314  are typically in electrical communication. 
     The longitudinal axes of the ground tip receptacle  316  and the ground tip  317  are typically substantially in-line. Further, the ground tip unit  314  is typically attached to the probe barrel  304  such that the longitudinal axes of the ground tip receptacle  316  and the ground tip  317  are typically substantially parallel to the longitudinal axis of the probe barrel  304 . 
     A probe cable  312  typically extends from within the interior of the probe unit housing  302  to outside of the probe unit housing  302 . As indicated for example in FIGS. 3A-3B, the far end of the probe cable  312  typically connects to a probe amplifier (such as the probe amplifier unit  102  depicted in FIG.  1 A and discussed above). Further, as depicted for example in FIG. 3A, the local end of the probe cable  312  connects to the probe barrel  304 . The probe cable  312  may connect to the probe barrel  304  in various ways within the scope of the invention, including various known ways of connecting a cable to a probe barrel. The connection of the probe cable  312  to the probe barrel  304  will be discussed further below. 
     The probe cable  312  may include insulation or sheathing that contains one or more types of conductors. Typically, the probe cable  312  is an insulated coaxial conductor (e.g., a coaxial cable) that is adapted to convey signals to facilitate wideband probing. In some embodiments, the probe cable  312  may be substantially flexible to facilitate handling of a connected probe amplifier unit without causing substantial movement of the single-end probe unit  300 . The probe cable  312  may include other types of conductors that serve to facilitate wideband active probing or other functions, within the scope of the invention. 
     A strain relief device  318  may extend from the probe unit housing  302  and attach to the probe cable  312 , as depicted for example in FIGS. 3A-3B. The strain relief device  318  serves to prevent excessive strain from being placed on the connection of the probe cable  312  to the probe barrel  304  and/or other components of the single-end probe unit  300 . In some embodiments, various known types of strain relief devices may be implemented. Further, in some embodiments, a strain relief device (not depicted) may be implemented alternatively at the connection of the probe cable  312  to the probe barrel  304 . 
     The combination of the probe cable  312 , the probe barrel  304 , the probe barrel nose cone  306 , the probe tip  308 , and the ground tip unit  314  typically serve to convey signals, that are obtained for example from an IC device, to a probe amplifier and subsequently to monitoring and/or testing equipment when the single-end probe unit  300  is utilized to perform probing. In that regard, typically at least one conductor of the probe cable  312  is electrically connected to the probe barrel  304 , for example a ground conductor. The ground tip unit  314  is typically connected in electrical communication with the probe barrel  304  by the ground tip connector  315  such that the ground tip  317  is in electrical communication with the probe barrel  304 . 
     Typically, at least one other conductor of the probe cable  312  is electrically connected to the probe tip  308 , for example a signal conductor. This connection may be made directly to the probe tip  308  or through various circuitry (not shown) internal to the probe barrel  304  and/or the probe barrel nose cone  306 . The probe tip  308  extends out of the probe barrel nose cone  306  and is held in a substantially fixed position by at least the probe barrel nose cone  306 . Further, the probe barrel nose cone  306  typically electrically isolates the probe tip  308  from the probe barrel  304 . Thus, when the probe tip  308  and ground tip  317  are brought into contact with probe points of an IC device, for example, signals are obtained by the probe tip  308  which may be transmitted to a probe amplifier by one or more conductors of the probe cable  312  that are electrically connected to the probe tip  308 . 
     The ground tip unit  314  may be capable of several position adjustments. The position adjustment capabilities of the ground tip unit  314 , which are described hereafter, are possible without temporarily or permanently forcing the deformation of components of the ground tip unit  314 . For example, the ground tip unit  314  can be adjusted to various positions without bending or stretching the components of the ground tip unit  314  to, for example, conform to the positions of probe points. Further, the ground tip unit  314  is capable of position adjustments without degrading the overall electrical features of the single-end probe unit  300 , such as high bandwidth performance. 
     The ground tip  317  may be capable of retracting into the ground tip receptacle  316 , as indicated for example in FIGS. 3A,  3 B and  3 D. The elastic compressible element  320 , which may engage the ground tip  317 , typically maintains the ground tip  317  partially extended out of the ground tip receptacle  316 . However, if pressure is applied to the ground tip  317 , for example by contact with a probe point, the ground tip  317  is able to retract partially into the ground tip receptacle  316 , as depicted for example in FIG.  3 D. Further, when the pressure on the ground tip  317  is removed, the elastic compressible element  320 , which typically engages the ground tip  317  and a portion of the internal structure of the ground tip receptacle  316 , causes the ground tip  317  to re-extend out of the ground tip receptacle  316  to its typical partially extended position. The extent of retraction and extension of the ground tip  317  may be limited by, for example, portions of the internal structure of the ground tip receptacle  316 , as discussed above. The capability of the ground tip  317  to reciprocate (i.e., move in/out of the ground tip receptacle  316 ) allows the probe tip  308  and ground tip  317  to make effective contact with probe points (e.g., a signal probe point and a ground probe point) that are at different heights from a common surface. For example, the probe tip  308  and ground tip  317  are able to make effective contact with two solder points at different heights on a PCB, as depicted for example in FIG. 3D, when the single-end probe unit  300  is utilized for probing. The probe tip  317  and the probe tip receptacle  316  are adapted such that the ground tip  317  maintains electrical communication with the probe barrel  304  during the reciprocating movement of the probe tip  317  in and out of the probe tip receptacle  316 . The elastic compressible element  320  may also serve to maintain the ground tip  317  in electrical communication with the probe barrel  304  in cooperation with the probe tip  317  and the probe tip receptacle  316 . 
     The ground unit  314  may also be capable of rotational movement about the longitudinal axis of the probe barrel  304 , as indicated for example in FIGS. 3E-3G. As described above, the ground tip connector  315  is adapted to maintain the ground tip unit  314  in a substantially consistent position when the single-end probe unit  300  is handled or used for probing. In this regard, the ground tip connector  315  is adapted to maintain the ground tip unit  314  in a substantially consistent rotational position with respect to the longitudinal axis of the probe barrel  304 . The ground tip unit  314  can be rotated about the longitudinal axis of the probe barrel  304  by, for example, applying a rotational force to it. Once the ground tip unit  314  is rotated to a particular position, the ground tip connector  315  causes the ground tip unit  314  to maintain that rotational position during handling or probing with the single-end probe unit  300  due to, for example, the frictional engagement of the ground tip connector  315  and the probe barrel  304 . The ground tip connector  315  is adapted such that the ground tip  317  maintains electrical communication with the probe barrel  304  during the rotational movement of the probe tip unit  314  about the longitudinal axis of the probe barrel  304 . 
     The rotational capability of the ground tip unit  314  and the offset axis configuration of the probe barrel nose cone  306  and probe tip  308  from the probe barrel  304 , as described above, provides the benefit of a variable spacing capability between the probe tip  308  and the ground tip  317 . This variable spacing capability between the probe tip  308  and the ground tip  317  facilitates probing of various configurations of probe points, for example of an IC device mounted on a PCB, without bending or stretching components of the single-end probe unit  300  or forcing the components to conform to probe point spacing by temporarily deforming them. Further, the variable spacing capability between the probe tip  308  and the ground tip  317  does not degrade the overall electrical features of the single-end probe unit  300 , such as high bandwidth performance. 
     FIGS. 3E-3G show exemplary depictions of the variable spacing capability between the probe tip  308  and the ground tip  317 . As depicted in FIG. 3E, when the ground tip unit  314  is rotated to a first position about the longitudinal axis of the probe barrel  304 , the probe tip  308  and the ground tip  317  are positioned at a minimal spacing S 1 . This spacing S 1  between the probe tip  308  and the ground tip  317  occurs because the ground tip unit  314  is rotated such that the probe tip  308  is tilted toward the ground tip  317  due to the offset longitudinal axis of the probe tip  308  from the longitudinal axis of the probe barrel  304 , as discussed above. Similarly, FIG. 3F depicts the ground tip unit  314  rotated to a second position about the longitudinal axis of the probe barrel  304  such that the probe tip  308  and the ground tip  317  are positioned at a spacing S 2 , which is greater than spacing S 1 . Finally, in FIG. 3G, the ground tip unit  314  is rotated to a third position about the longitudinal axis of the probe barrel  304  such that the probe tip  308  and the ground tip  317  are positioned at a maximal spacing S 3 , which is greater than spacing S 2 . At this position, the ground tip unit  314  is rotated such that the probe tip  308  is tilted away from the ground tip  317  due to the offset longitudinal axis of the probe tip  308  from the longitudinal axis of the probe barrel  304 , as discussed above. 
     Having discussed several embodiments of probe units above, discussion is now focused on several embodiments of probe tip units of the present invention. In that regard, FIG. 4 depicts a differential probe tip unit  400  of the present invention. As is known, a differential probe tip unit typically obtains two signals, from an IC device for example, that are then manipulated (for example by probe amplifier circuitry) to obtain a differential signal (i.e., the difference between the signals). 
     The differential probe tip unit  400  includes a probe unit housing  402  which may contain various probe tip unit circuitry (not shown) ranging from conductor traces to various resistive, capacitive, and/or other electronic elements. The probe unit housing  402  may be constructed of various materials, such as various types of plastic. Other materials may be used to construct the probe unit housing  402 , preferably that do not interfere with the operation or use of the differential probe tip unit  400 , within the scope of the invention. 
     The differential probe tip unit  400  includes probe tip units  406 ,  407  which are connected to the probe tip unit circuitry at the probe connection points  404 ,  405 , respectively. The probe tip units  406 ,  407  include probe tips  408 ,  409 , respectively. The probe tip units  406 ,  407  may also include probe impedance elements  410 ,  411 , respectively, to facilitate wideband active probing using the differential probe tip unit  400 . The probe connection points  404 ,  405  may be implemented by various types of electrical connections, for example solder connections or compression terminal connections, which are all within the scope of the invention. 
     The components of the probe tip units  406 ,  407  are typically substantially electrically conductive such that the probe tips  408 ,  409  are in electrical communication with the probe connection points  404 ,  405 , respectively. For example, the probe tips  408 ,  409  are typically constructed of electrically conductive material, such as a metal or alloy. The probe tips  408 ,  409  may have various shapes, for example cylindrical, to facilitate their connections to probe points of, for example, an IC device. 
     Probe cables  412 ,  413  typically extend from within the interior of the probe unit housing  402  to outside of the probe unit housing  402 . As indicated for example in FIG. 4, the far ends of the probe cables  412 ,  413  typically connect to a probe amplifier (such as the probe amplifier unit  102  depicted in FIG.  1 A and discussed above). The local ends of the probe cables  412 ,  413  typically connect to the probe tip unit circuitry (not shown) within the probe unit housing  402 . The probe cables  412 ,  413  may connect to the probe tip unit circuitry in various ways within the scope of the invention, for example by solder connections or compression terminal connections. 
     The probe cables  412 ,  413  may include cable insulation or sheathing that contains one or more types of conductors. Typically, the probe cables  412 ,  413  are insulated coaxial conductors (i.e., coaxial cables) that are adapted to convey signals to facilitate wideband probing. The probe cables  412 ,  413  are typically substantially flexible to facilitate handling of a connected probe amplifier unit without causing substantial movement of the differential probe tip unit  400 . The probe cables  412 ,  413  may include other types of conductors that serve to facilitate wideband active probing or other functions, within the scope of the invention. 
     One or more strain relief devices  418  may extend from the probe unit housing  402  and attach to the probe cables  412 ,  413 . Such strain relief devices  418  serve to prevent excessive strain from being placed on the connections of the probe cables  412 ,  413  to the probe tip unit circuitry and/or other components of the differential probe tip unit  400 . In some embodiments, various known types of strain relief devices may be implemented within the scope of the present invention. 
     The combination of the probe cables  412 ,  413  and the probe tip units  406 ,  407 , respectively, typically serve to convey signals, that are obtained for example from an IC device, to a probe amplifier and subsequently to monitoring and/or testing equipment when the differential probe tip unit  400  is utilized to perform probing. In that regard, typically at least one conductor of the probe cables  412 ,  413  is electrically connected to the probe tip units  406 ,  407 , respectively, for example a signal conductor. These connections may be made directly to the probe tip units  406 ,  407  or through various probe tip unit circuitry (not shown) internal to the probe unit housing  402 . Further, typically at least one other conductor of the probe cables  412 ,  413  is electrically connected to the ground circuitry of the probe tip unit circuitry (not shown), for example a ground conductor. In this regard, when the probe tips  408 ,  409  are brought into contact with probe points of an IC device, signals are obtained by the probe tip units  406 ,  407  which may be transmitted to a probe amplifier by one or more conductors of the probe cables  412 ,  413  that are electrically connected to the probe tip units  406 ,  407 , respectively. 
     The differential probe tip unit  400  is typically adapted to facilitate hands-free and in-operation probing. In that regard, the differential probe tip unit  400  is typically adapted to be small-sized and lightweight such that the probe tip units  406 ,  407  can be securely connected to probe points of, for example, an IC device that is mounted on a PCB. The flexibility of the probe cables  412 ,  413 , as discussed above, also facilitates such hands-free probing by minimizing physical disturbance of the differential probe tip unit  400  when connected equipment, such as a probe amplifier, are handled or repositioned. Typically, the probe tip units  406 ,  407  are secured by soldering the probe tips  408 ,  409  to the probe points, although other types of connections may be implemented within the scope of the invention. Preferably, connections are made in a manner, such as by soldering, to minimize the parasitic input impedance to the probe tip units  406 ,  407  at the connections to the probe points. 
     The small size and light weight of the differential probe tip unit  400 , as well as the flexibility of the probe cables  412 ,  413 , also facilitates in-operation probing. Thus, the probe tip units  406 ,  407  can be secured to probe points on a PCB which is placed in normal operation by inserting it into an enclosure or card-cage. This capability facilitates more effective monitoring and/or testing to, for example, troubleshoot a problem component or PCB circuitry. Further, this capability facilitates connecting several differential probe units  400  to various probe points so that efficient in-operation probing can be conducted using a single probe amplifier that is connected to each of the differential probe units  400  as needed. 
     Electrical characteristics of a wideband active probing system, such as the damping resistance of the probe tips, can be efficiently and inexpensively modified by utilizing various embodiments of the differential probe tip unit  400  that have different electrical characteristics. The electrical characteristics of the differential probe tip unit  400  may be varied by, for example, modifying the internal probe tip unit circuitry and/or the probe tips. In that regard, various configurations of probe tip units  406 ,  407  may be implemented with the differential probe tip unit  400 . 
     FIG. 6 shows several configurations of probe tip units, among others, that may be implemented with the differential probe tip unit  400 . Solder-on probe tip units  601  and plug-on probe tip units  602  are two exemplary configurations of probe tip units shown. There are also SMT (surface-mount technology) grabber probe tip units  603  and wedge probe tip units  604 . Many other configurations of probe tip units can be implemented with the differential probe tip unit  400  within the scope of the invention. 
     Since the probe tip units  406 ,  407  are connected to the differential probe tip unit  400  at the connection points  404 ,  405 , the probe tip units  406 ,  407  can be replaced efficiently and inexpensively if, for example, there is a need to modify the electrical characteristics of the probing system or an existing probe tip unit is damaged. In that regard, the probe tip units  406 ,  407  can be replaced with other probe tip units, such those depicted in FIG. 6 by, for example, de-soldering the existing probe tip units  406 ,  407  from the connection points  404 ,  405  and soldering replacement probe tip units to the connection points  404 ,  405 . Alternatively, several differential probe tip units  400  can be configured with various probe tip units  406 ,  407  to be implemented as needed for probing. 
     FIG. 5 depicts a single-end probe tip unit  500  of the present invention. As is known, a single-end probe tip unit, in contrast to a differential probe tip unit, typically obtains a signal with respect to ground, such as a voltage from an IC device. The single-end probe tip unit  500  includes a probe unit housing  502  which may contain various probe tip unit circuitry (not shown) ranging from conductor traces to various resistive, capacitive, and/or other electronic elements. The probe unit housing  502  may be constructed of various materials, such as various types of plastic. Other materials may be used to construct the probe unit housing  502 , preferably that do not interfere with the operation or use of the single-end probe tip unit  500 , within the scope of the invention. 
     The single-end probe tip unit  500  includes probe tip units  506 ,  507  which are connected to the probe tip unit circuitry at the probe connection points  504 ,  505 , respectively. The probe tip units  506 ,  507  include probe tips  508 ,  509 , respectively. The probe tip units  506 ,  507  may also include probe impedance elements  510 ,  511 , respectively, to facilitate wideband active probing using the single-end probe tip unit  500 . The probe connection points  504 ,  505  may be implemented by various types of electrical connections, for example solder connections or compression terminal connections, and are all within the scope of the invention. 
     The components of the probe tip units  506 ,  507  are typically substantially electrically conductive such that the probe tips  508 ,  509  are in electrical communication with the probe connection points  504 ,  505 , respectively. For example, the probe tips  508 ,  509  are typically constructed of electrically conductive material, such as a metal or alloy. The probe tips  508 ,  509  may have various shapes, for example cylindrical, to facilitate connections to probe points of, for example, an IC device. 
     A probe cable  512  typically extends from within the interior of the probe unit housing  502  to outside of the probe unit housing  502 . As indicated for example in FIG. 5, the far end of the probe cable  512  typically connects to a probe amplifier (such as the probe amplifier unit  102  depicted in FIG.  1 A and discussed above). The local end of the probe cable  512  typically connects to the probe tip unit circuitry (not shown) within the probe unit housing  502 . The probe cable  512  may connect to the probe tip unit circuitry in various ways within the scope of the invention, for example by solder connections or compression terminal connections. 
     The probe cable  512  may include cable insulation or sheathing that contains one or more types of conductors. Typically, the probe cable  512  is an insulated coaxial conductor (e.g., a coaxial cable) that is adapted to convey signals to facilitate wideband probing. The probe cable  512  is typically substantially flexible to facilitate handling of a connected probe amplifier unit without causing substantial movement of the single-end probe tip unit  500 . The probe cable  512  may include other types of conductors that serve to facilitate wideband active probing or other functions, within the scope of the invention. 
     A strain relief device  518  may extend from the probe unit housing  502  and attach to the probe cable  512 . Such a strain relief device  518  serves to prevent excessive strain from being placed on the connections of the probe cable  512  to the probe tip unit circuitry and/or other components of the single-end probe tip unit  500 . In some embodiments, various known types of strain relief devices may be implemented within the scope of the present invention. 
     The combination of the probe cable  512  and the probe tip units  506 ,  507  typically serve to convey signals, that are obtained for example from an IC device, to a probe amplifier and subsequently to monitoring and/or testing equipment when the single-end probe tip unit  500  is utilized to perform probing. In that regard, typically at least one conductor of the probe cable  512  is electrically connected to the probe tip unit  506 , for example a signal conductor. Further, typically at least one other conductor of the probe cable  512  is electrically connected to the probe tip unit  507 , for example a ground conductor. These connections may be made directly to the probe tip units  506 ,  507  or through various probe tip unit circuitry (not shown) internal to the probe unit housing  502 . In this regard, when the probe tips  508 ,  509  are brought into contact with probe points of an IC device, for example, signals are obtained by the probe tip units  506 ,  507  which may be transmitted to a probe amplifier by one or more conductors of the probe cable  512  that are electrically connected to the probe tip units  506 ,  507 , respectively. 
     The single-end probe tip unit  500  is typically adapted to facilitate hands-free and in-operation probing. In that regard, the single-end probe tip unit  500  is typically adapted to be small-sized and lightweight such that the probe tip units  506 ,  507  can be securely connected to probe points of, for example, an IC device that is mounted on a PCB. The flexibility of the probe cable  512 , as discussed above, also facilitates such hands-free probing by minimizing physical disturbance of the single-end probe tip unit  500  when connected equipment, such as a probe amplifier, are handled or repositioned. Typically, the probe tip units  506 ,  507  are secured by soldering the probe tips  508 ,  509  to the probe points, although other types of connections may be implemented within the scope of the invention. Preferably, connections are made in a manner, such as by soldering, to minimize the parasitic input impedance to the probe tip units  506 ,  507  at the connections to the probe points. 
     The small size and light weight of the single-end probe tip unit  500 , as well as the flexibility of the probe cable  512 , also facilitates in-operation probing. Thus, the probe tip units  506 ,  507  can be secured to probe points on a PCB which is placed in normal operation by inserting it into an enclosure or card-cage. This capability facilitates more effective monitoring and/or testing to, for example, troubleshoot a problem component or PCB circuitry. Further, this capability facilitates connecting several single-end probe units  500  to various probe points so that efficient in-operation probing can be conducted using a single probe amplifier that is connected to each of the single-end probe units  500  as needed. 
     Electrical characteristics of a wideband active probing system, such as the damping resistance of the probe tips, can be efficiently and inexpensively modified by utilizing various embodiments of the single-end probe tip unit  500  that have different electrical characteristics. The electrical characteristics of the single-end probe tip unit  500  may be varied, for example, by modifying the internal probe tip unit circuitry and/or the probe tips. In that regard, various configurations of probe tip units  506 ,  507  may be implemented with the single-end probe tip unit  500 . For example, FIG. 6 shows several configurations of probe tip units, among others, that may be implemented. There are solder-on probe tip units  601  and plug-on probe tip units  602 . There are also SMT (surface-mount technology) grabber probe tip units  603  and wedge probe tip units  604 . Many other configurations of probe tip units can be implemented on the single-end probe tip unit  500  within the scope of the invention. 
     Since the probe tip units  506 ,  507  are connected to the single-end probe tip unit  500  at the connection points  504 ,  505 , the probe tip units  506 ,  507  can be replaced efficiently and inexpensively if, for example, there is a need to modify the electrical characteristics of the probing system or an existing probe tip unit is damaged. In that regard, the probe tip units  506 ,  507  can be replaced with other probe tip units, such those depicted in FIG. 6, by, for example, de-soldering the existing probe tip units  506 ,  507  from the connection points  504 ,  505  and soldering replacement probe tip units to the connection points  504 ,  505 . Alternatively, several single-end probe units  500  can be configured with various probe tip units  506 ,  507  to be implemented as needed for probing. 
     It should be understood that throughout the foregoing discussion of the present invention, references made to the probing of IC devices are merely exemplary, and the probing of other electrical circuits and/or electronic devices applies to such references within the scope of the invention. 
     It should be further understood that the above-described embodiments of the present invention are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiments of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.