Patent Abstract:
This disclosure concerns systems and devices configured to implement impedance matching schemes in a high speed data transmission environment. In one example, an optoelectronic assembly is provided that includes a TO package having a base through which one or more leads pass. The leads are electrically coupled to an optoelectronic device in the TO package, and are electrically isolated from the base. Some or all of the leads include a ground ring that is electrically isolated from the lead and electrically coupled with the base. A circuit interconnect is also included that is electrically coupled to the optoelectronic device and the TO package. The circuit interconnect includes a dielectric substrate having signal traces that are electrically coupled to the signal leads. A ground signal conductor disposed on the dielectric substrate is electrically coupled with the ground rings.

Full Description:
[0001]     The present invention relates generally to optoelectronic devices, and particularly to a circuit interconnect for controlled impedance at high frequencies.  
       BACKGROUND OF THE INVENTION  
       [0002]     An optoelectronic device, such as a laser diode or a photo diode, is generally enclosed in a transistor outline (TO) package, which provides a conductive housing for the optoelectronic device. A laser diode converts an electrical signal into an optical signal for transmission over a fiber optic cable, while a photo diode converts an optical signal into an electrical signal. In order for a laser diode to convert an electrical signal into an optical signal, the electrical signal must be sent through the TO package of the laser diode. Similarly, an electrical signal from a photo diode must be sent through the TO package of the photo diode to external electrical circuitry. For high frequency operation, it is important to control the impedance seen by the electrical signals that flow into and out of the TO package.  
         [0003]     Conventional signal and ground connections to TO packages, which including distinct signal and ground connections, result in uncontrolled impedances that degrade data signal integrity at high frequencies (e.g., at or above 3 GHz).  
       BRIEF SUMMARY OF AN EXEMPLARY EMBODIMENT OF THE INVENTION  
       [0004]     In general, exemplary embodiments of the invention are concerned with systems and devices configured to implement impedance matching schemes in a high speed data transmission environment. In one example, an optoelectronic assembly is provided that includes a TO package having a base through which one or more leads pass. The leads are electrically coupled to an optoelectronic device in the TO package, and are electrically isolated from the base. Some or all of the leads include a ground ring that is electrically isolated from the lead and electrically coupled with the base. A circuit interconnect is also included that is electrically coupled to the optoelectronic device and the TO package. The circuit interconnect includes a dielectric substrate having signal traces that are electrically coupled to the signal leads. A ground signal conductor disposed on the dielectric substrate is electrically coupled with the ground rings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     Additional objects and features of the invention will be more readily apparent from the following detailed description and appended claims when taken in conjunction with the drawings, in which:  
         [0006]      FIG. 1  is a diagram of an optoelectronic assembly in accordance an embodiment of the invention.  
         [0007]      FIG. 2  depicts the ground signal conductor side of a circuit interconnect.  
         [0008]      FIGS. 3A and 3B  depict the back of a TO package in accordance with the first and second embodiments.  
         [0009]      FIG. 4  is a diagram of a transmitter assembly in accordance with an embodiment of the invention.  FIGS. 4A, 4B ,  4 C,  4 D and  4 E are circuit diagrams of the transmitter assembly of  FIG. 4 .  
         [0010]      FIG. 5  is a diagram of a transmitter assembly in accordance with an alternate embodiment of the invention.  
         [0011]      FIG. 6  is a diagram of a receiver assembly in accordance with an embodiment of the invention.  FIG. 6A  is a circuit diagram of the receiver assembly of  FIG. 6 .  
         [0012]      FIG. 7  is a diagram of a transceiver assembly in accordance with an embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]     Referring to  FIG. 1 , there is shown an embodiment of an optoelectronic assembly  100  in accordance with the present invention. The optoelectronic assembly may be a transmitter optoelectronic assembly or a receiver optoelectronic assembly. The optoelectronic assembly includes a transistor outline (TO) package  102  that houses an optoelectronic device. If the optoelectronic assembly is a transmitter optoelectronic assembly, the optoelectronic device is a light source such as a laser diode. If the optoelectronic assembly is a receiver optoelectronic assembly, the optoelectronic device is a detector such as a photo diode.  
         [0014]     Signal contacts  112 , also called signal leads, extend through apertures in the base  124  of the TO package  102  and contact corresponding signal traces  114  on a circuit interconnect  104 . The signal traces  114  are mechanically and electrically connected to the signal contacts  112  by solder, conductive epoxy or any other appropriate conductive attachment mechanism. Signal contacts  112  and signal traces  114  convey power and data signals between an external circuit  118  and the device or devices in the TO package  102 .  
         [0015]     The circuit interconnect  104  is preferably made of an elongated piece of flexible dielectric  120 . The dielectric  120  serves as an insulator between a ground signal conductor  116  on one side of the dielectric  120  and the data signal traces  114  on the other side of the dielectric. The ground signal conductor  116  conveys ground current between the external circuit  118  and the device or devices in the TO package  102 . While the embodiment shown in  FIG. 1  has two signal contacts  112  and two corresponding signal traces  114 , in other embodiments the number signal contacts and signal traces may be greater or fewer, depending on the number of power and data connections needed by the device or devices inside the TO package  102 .  
         [0016]     The external, back surface of the base  124  is sometimes called the “ground plate,” because the base  124  of the TO package is grounded by a connection between the ground plate and the ground conductor  116  on the circuit interconnect  104 . The ground connection to the base  124  provides a circuit ground voltage source and ground current connection for the electrical and optoelectronic components in the TO package  102 .  
         [0017]     To avoid signal reflections and other signal degradations, the impedance of the signal path from the device in the TO package  102  to the external circuit  118  must be kept as consistent as possible. The impedance of the circuit interconnect  104  (i.e., the characteristic impedance of the transmission line formed by the circuit interconnect) is precisely determined by the thickness of the dielectric and the width of the data signal traces, and the circuit interconnect is configured so that for frequencies in the range of 3 to 10 GHz its impedance approximately matches the impedances of both the circuitry inside the TO package and the external circuit  118 . As used in this document, two impedances are defined to “approximately match” when the two impedances are either exactly the same or one of the impedances is larger than the other, but no more than 50% larger. In other words, the impedance of the circuit interconnect is within a factor of about 1.5 of the impedance of the circuitry inside the TO package and the external circuit  118 . Preferably the impedance of the circuit interconnect will be within 25% (i.e., within a factor of about 1.25) of the impedances of the circuitry inside the TO package and the external circuit  118 . The impedance of the circuit interconnect  104  of the present invention is typically between 20 and 30 ohms.  
         [0018]     In other embodiments, the circuit interconnect  104  may be optimized for impedance matching (to the high frequency signal leads of the TO package  102 , and also to an external circuit) for a different range of operating frequencies than 3 to 10 GHz. Typically, the range of operating frequencies at the circuit interconnect  104  of the present invention approximately matches impedances at both ends of the circuit interconnect  104  will include a range of frequencies above 3 GHz.  
         [0019]     In a preferred embodiment the circuit interconnect  104  has a thickness between 0.003 and 0.012 inches, and the dielectric substrate  120  of the circuit interconnect is preferably polyimide or polyester. Other insulating materials may be used besides polyimide or polyester. Moreover, the insulator  120  does not necessarily need to be flexible; however, the flexibility is useful for fitting the optoelectronic assembly  110  into a housing (not shown), such as the housing of an optoelectronic transmitter, receiver or transceiver. The flexible dielectric substrate  120  is coated on each side with a conductive material such as copper, a copper alloy, or other malleable, highly conductive metal or metal alloy. The data signal traces  114  are fabricated from the conductive material on one side of the circuit interconnect  104 , while the entire second side of the circuit interconnect  104  (excluding circular regions corresponding to the positions of the signal leads traversing the base of the TO package) serves as the ground signal conductor  116 . Other methods of creating the conductive signal traces may be used as is understood by one skilled in the art.  
         [0020]     In an alternate embodiment, only a portion of the second side of the circuit interconnect  104  serves as the ground signal conductor  116 , leaving room for one or more additional signal traces (e.g., for power or low frequency data signals) on the second side of the interconnect  104 . In this alternate embodiment, the ground signal conductor  116  would be positioned relative to the traces on the first side of the circuit interconnect so as to provide connections with well controlled impedance.  
         [0021]     The side of the circuit interconnect  104  that serves as the ground signal conductor  116  is depicted in  FIG. 2 . The small circular regions  130  represent holes in the dielectric substrate  120  of the interconnect, through which the signal leads of the TO package extend. The annular circular regions  132  surrounding the smaller holes  130  represent non-conductive, unmetalized regions in which the conductive material has been removed from the second side of the circuit interconnect  104  so as to prevent electrical shorts between the signal leads and the ground signal conductor  116 .  
         [0022]     Returning to  FIG. 1 , the data signals are transmitted between the optoelectronic device in the TO package  102  and electrical circuitry  118 . The data signal contacts  112  extend through apertures in the base  124  of the TO package  102  and contact the data signal traces  114 . For each data signal contact  112 , a separate, respective ground ring  106  surrounds the data signal contact  112  and is attached to the base  124  of the TO package  102 . The base  124  is a circular (actually, cylindrical) metal plate, generally held at the circuit ground voltage during operation of the optoelectronic device. The base  124  is the foundation of the TO package  102 . In a preferred embodiment the base  124  is made of a metal known as “Alloy  42 ,” which is an alloy of iron and nickel. In other embodiments the base  124  may be made of other appropriate metals. The primary purpose of the ground rings is to form a low reflection connection between the data signal contacts  112  and the circuit interconnect  104 , so as to minimize signal reflections at the interface between the data signal contacts  112  and the circuit interconnect  104 . In some embodiments, ground rings  106  are used only with high frequency data signal contacts  112  (e.g., carrying data signals at frequencies at or above 2 or 3 GHz), but not with the power signal contact and any lower frequency data signal contacts, because ground rings  106  are not needed to form low reflection connections between the signal contacts  112  and the circuit interconnect  104  for low frequency connections.  
         [0023]      FIG. 3A  shows the ground rings  106  on the back surface of the base  124 . The ground rings  106  are preferably highly conductive, thin metal rings that are bonded to the back, planar surface of the base  124 , such as by solder, conductive epoxy or any other appropriate bonding or conductive attachment mechanism. As a result, the ground rings are mechanically and electrically connected to the back surface of the base  124 . The ground rings  106  rise slightly above the back planar surface of the base  124 , which facilitates the bonding of the ground signal conductor  116  of the circuit interconnect  104  to the ground rings. Alternately, the ground rings  106  may be implemented as raised annular regions of the base  124 , i.e., as integral parts of the base. The circuit ground connection provided by the ground signal conductor  116 , which is electrically and mechanically bonded to the ground rings  106 , and possibly other portions of the base as well, keeps the entire base  124  at the circuit ground voltage during normal operation. While the ground rings  106  are shown in  FIG. 3A  as being circular or annular in shape, in other embodiments other shapes could be used. For instance, the ground rings  106  could be oval shaped structures.  
         [0024]     Although there are two ground rings  106  surrounding the two data signal contacts, only one ground ring is seen in  FIG. 1  because of the angle of the perspective view shown in  FIG. 1 . The ground signal conductor  116  directly contacts the ground rings  106 , and carries ground current from the ground rings  106  to a circuit ground terminal  122  ( FIG. 1 ). In a preferred embodiment, the ground signal conductor  116  also directly contacts the base  124  at the back surface of the TO package  102  so as to provide a high quality ground connection to the entire TO package and the devices therein. These contacts between the ground signal conductor  116  and the ground rings  106  and the back surface of the base  124  are preferably implemented by bonding these components together using solder, conductive epoxy or any other appropriate bonding or conductive attachment mechanism.  
         [0025]     The ground signal and the data signals are maintained in a close relationship to each other, separated by the insulator  120 . This provides for a controlled impedance at high frequencies.  
         [0026]     Referring again to  FIG. 1 , the electrical circuitry  118  is electrically connected to the circuit interconnect  104 . The signal traces  114  contact the electrical circuitry  118  while the ground conductor  116  contacts the electrical circuitry&#39;s circuit ground node  122 . The electrical circuitry is typically mounted on or includes a circuit board (not shown) and the circuit interconnect is electrically connected to that circuit board. The electrical circuitry  118  amplifies and processes the electrical signals transmitted to a laser diode (in one embodiment) or from a photo diode (in another embodiment), or both (in yet another embodiment). Thus, the electrical circuitry  118  may include a laser driver circuit, a received signal recovery circuit, or both. Further, the electrical circuitry  118  may include digital signal processing circuits, such as serializing circuits and deserializing circuits, and circuits that perform data conversions, such as the 8b/10b conversion for converting a data stream into a “balanced” data stream that is balanced with respect to 1 and 0 bits, and that provides sufficient data transitions for accurate clock and data recovery.  
         [0027]      FIG. 3A  shows the base  124  at the back of the TO package  102  in one embodiment of the present invention. The signal contacts (leads)  112  carrying data signals and/or a power supply voltage extend through apertures in the base  124  of the TO package  102 . The data signal contacts  112  contact the data signal traces  114  ( FIG. 1 ) of the circuit interconnect. The signal contacts  112  do not contact the base  124  of the TO package  102 ; rather, they extend through a dielectric  140 , preferably a ring of glass, embedded in the base  124 . Each dielectric ring  140  is concentric with one of the signal contacts  112 . When the circuit interconnect  104  is bonded to the base of the TO package  102 , the unmetalized insulator region  132  ( FIG. 2 ) on the second side of the circuit interconnect overlaps the dielectric ring  140  in the base  124 . For each data signal contact  112  (or at least each high frequency data signal contact), there is a conductive ground ring  106  that surrounds the dielectric  140 , concentric with the contact  112  and the dielectric ring  140 .  
         [0028]     In some embodiments, the ground rings  106  are the only parts of the TO package that directly contact the ground signal conductor  116  of the circuit interconnect. In one embodiment, however, the ground signal conductor  116  is mechanically and electrically bonded to a large portion of the external, back surface of the base  124 , in addition to the ground rings  106 . Alternatively, additional ground contacts may be provided by signal leads connected to the TO package.  
         [0029]      FIG. 3B  depicts an alternate embodiment, in which a ground lug  150  is used instead of the ground rings  106  to provide a high quality ground connection to the base  124  and to prevent signal reflections in the high frequency data signal paths. The ground lug  150  is a preferably a highly conductive, thin metal lug bonded to the back, planar surface of the base  124 , such as by solder, conductive epoxy or any other appropriate bonding or conductive attachment mechanism. The ground lug  150  rises above the back planar surface of the base  124 , which facilitates the bonding of the ground signal conductor  116  of the circuit interconnect  104  to the ground lug. Alternately, the ground lug  150  may be implemented as a raised regions of the base  124 , i.e., as an integral part of the base. The ground lug has two round (i.e., cylindrical) holes in it, aligned with the dielectric rings  140  surrounding the data signal contacts  112 .  
         [0030]     The use of a ground lug, instead of ground rings, typically does not require any change in the design of the circuit interconnect  104 . As shown in  FIG. 3B , the ground lug  150  is preferably positioned so as to surround the data signal contacts  112 . If the TO package includes more than two high frequency data signal contacts  112 , either the ground lug may be made larger or one or more additional ground lugs  150  may be positioned around those additional signal contacts  112  so as to provide a ground current path that is precisely positioned with respect to the data signal current flowing each of the data signal contacts  112 .  
         [0031]     The low impedance connection or bond between the ground signal conductor and the ground lug  150  is preferably formed by placing solder on the top surface of the ground lug or on the back surface of the ground signal conductor  116  and then soldering the ground signal conductor  116  to the ground lug  150 . Alternately, the ground signal conductor  116  may be mechanically and electrically connected to the ground lug  150  using a conductive epoxy or any other appropriate conductive attachment mechanism.  
         [0032]     In yet another alternate embodiment, the base  124  of a TO package  102  may include both ground rings and ground lugs for forming ground current connections to the ground signal conductor  116  of the circuit interconnect  104 .  
         [0033]     Referring to  FIG. 4 , there is shown a transmitter optoelectronic assembly  400  in accordance with an embodiment of the present invention. The transmitter optoelectronic assembly  400  includes: 
        a laser diode  402 , such as an edge emitter or other type of laser diode;     a laser submount  404 , on which the laser diode is mounted; the laser submount  404  may be made of aluminum nitride or alumina ceramic; the laser submount  404  preferably incorporates one or more integrated or attached passive components, such as resistors, capacitors, and inductors, to provide improved impedance matching and signal conditioning;     a laser pedestal  406  to which the submount  404  is attached; the laser pedestal  406  is a grounded, conductive structure having a partially concentric shape with respect to data signal contacts  412 ,  414  that extend through the base  124 ;     a monitor photo diode  408  for detecting the light emitted from a back facet of the laser diode  402  in order to monitor the intensity of the light emitted by the laser diode  402 ;     a monitor photo diode sub-mount  410  on which the monitor photo diode  408  is mounted; and     a Transistor Outline (TO) package  420  incorporating controlled impedance glass-metal feedthroughs.        
 
         [0040]     The partially concentric shape of the pedestal  406 , which is held at the circuit ground potential, facilitates control of the impedance characteristics of the circuit that runs from the data signal contacts  412 ,  414 , through bond wires  405  to the laser diode  402  and through the laser submount  404  and laser pedestal  406  of the TO package.  
         [0041]     The laser diode  402  is activated when a positive voltage is applied across the p-n junction of the laser diode  402 . In the preferred embodiment, data signal contacts  412 ,  414  form a differential data signal connection. The two contacts  412 ,  414  are electrically connected to the laser submount  404  via bond wires  405  or any another appropriate connection mechanism. One terminal of the laser diode  402  is in direct contact with the laser submount  404  and is therefore electrically connected with one of the differential data signal contacts  414  via a corresponding one of the bond wires  405 . The other data signal contact  412  is electrically connected to the laser diode  402  via a bond wire  405  to the submount  404  and another bond wire connecting the second terminal of the laser diode  402  to the submount  404 . The differential signal provided by data signal contacts  412 ,  414  supplies both a bias voltage and a time varying signal voltage across the p-n junction of the laser diode  402 .  
         [0042]     Improved impedance matching between the circuit interconnect and the electrical circuitry in a TO package is achieved by incorporating resistors, capacitors and/or inductors into the submount  404  for the laser diode to provide a network (e.g., an RL network, or LC network, or RLC network) that compensates for the impedance presented by the bond wires  405  between the data signal contacts  412 ,  414  extending through the TO package and the submount connection points. Typically, the bond wires are made of gold and have inductances of 1 to 5 nanoHenries.  FIG. 4A  is a circuit diagram of the circuit in which data signal contacts  412  and  414  are connected to the laser diode  402  through the laser submount  404 . The resistance, capacitance and/or inductance of the submount  404  are adjusted so that the impedance of the electrical circuitry inside the TO package approximately matches the impedance of the circuit interconnect.  FIGS. 4B, 4C ,  4 D and  4 E are circuit diagrams for alternative impedance compensation networks that may be constructed. In  FIG. 4B  the submount is represented as a single resistor  404 -R. In  FIG. 4C  the submount is represented as two resistors,  404 -R 1  and  404 -R 2 , on either side of the laser diode  402 . In  FIG. 4D  the submount is represented as a capacitor  404 -C connected to ground. Finally in  FIG. 4E  the submount is represented as a resistor  404 -R and a capacitor  404 -C. Typically component values are 10 to 30 ohms (preferably about 20 ohms) for resistor  404 -R and 0.6 to 1.0 picofarads (preferably about 0.75 picofarads) for capacitor  404 -C. The impedance matching provided by the network incorporated into the submount is preferably optimized for a predefined range of operating frequencies, such as 3 GHz to 10 GHz. The predefined range of operating frequencies is preferably the same as the range of operating frequencies at which the optoelectronic device is expected to be used. In the preferred embodiments, the predefined range of operating frequencies includes a range of frequencies above 3 GHz.  
         [0043]     Referring again to  FIG. 4 , as is understood by one skilled in the art, when the laser diode  402  is an edge emitter the laser diode  402  emits light in both the forward direction and the backward direction, from forward and back facets. The forward direction refers to the direction in which light is transmitted through a window of the TO package, while the backward direction refers to the opposite direction. The laser intensity in the backward direction is proportional to the laser intensity in the forward direction. Thus, it is useful to measure the intensity of the laser in the backward direction in order to track the laser intensity in the forward direction. Accordingly, a monitor photo diode  408  is positioned facing the back facet of the laser diode  402 . A power supply voltage contact  416  is connected to the monitor photo diode submount  410  by a bond wire. The monitor photo diode  408  is in contact with the monitor photo diode submount  410  and is connected to the monitor photo diode data signal contact  418  by a bond wire. Thus, the monitor photo diode  408  is reverse biased between the power supply and the data signal contact  418 . The transmitter assembly of  FIG. 4  is operated in conjunction with a circuit interconnect having four data signal traces. The circuit interconnect, not shown, is preferably similar to the one shown in  FIG. 2 , but having four data signal traces  114 . Each data signal trace contacts a respective one of the data signal contacts  412 ,  414 ,  416 , and  418 .  
         [0044]     Other transmitter embodiments may include a Vertical Cavity Surface-Emitting Laser (VCSEL) transmitter assembly  500  as shown in  FIG. 5 . The VCSEL  502  is mounted to a submount  504 , which is preferably a capacitor. The capacitor is mounted to the TO package  510 . The VCSEL is electrically connected to the submount  504  via direct contact. Contact  506  is connected to the submount by a bond wire  505  and contact  508  is connected to the VCSEL by another bond wire  505 . A differential signal is provided through contacts  506  and  508 , which results in a positive voltage across the VCSEL&#39;s p-n junction thereby activating the VCSEL  502 . The transmitter assembly of  FIG. 5  is operated in conjunction with a circuit interconnect having two data signal traces, as well as a power connection trace, similar to the interconnects shown in  FIGS. 1 and 2 . Each data signal trace contacts a respective data signal contact  506 ,  508 .  
         [0045]     Referring to  FIG. 6 , there is shown an embodiment of a receiver optoelectronic assembly  600  in accordance with the present invention. The receiver optoelectronic assembly includes: 
        a photo diode  602 ;     a photo diode submount  604 ;     an integrated circuit preamplifier  606  attached to the photo diode  602  and the submount  604 ;     a capacitor  608  for filtering background noise; and     a Transistor Outline (TO) package  616  incorporating controlled impedance glass-metal feedthroughs.        
 
         [0051]     The photo diode submount  604  is preferably a capacitor that serves to filter noise from the power supply (Vcc)  614 . The photo diode  602  is electrically connected to the submount  604  preferably through direct contact. The photo diode  602  is reverse biased between the charged capacitor  608  and a bond wire  605  to the integrated circuit preamplifier  606 . The integrated circuit preamplifier  606  produces a pair of differential data signals through bond wires  605  to contacts  610  and  612 . Finally, as is understood by those skilled in the art, the capacitor  608  is used by the integrated circuit preamplifier  606  to filter unwanted noise from the data signals. The receiver optoelectronic assembly of  FIG. 6  is operated in conjunction with a circuit interconnect having three data signal traces (not shown, but similar to the circuit interconnects shown in  FIGS. 1 and 2 ). Each data signal trace contacts a respective one of the data signal contacts  610 ,  612 , and  614 . The data signals from the photo diode are typically transmitted through the circuit interconnect to a received signal amplifier that is mounted on the circuit board connected to the circuit interconnect.  
         [0052]      FIG. 6A  is a circuit diagram of the receiver assembly shown in  FIG. 6 . The photo diode  602  is reverse biased so that V2 is less than V1. The output from the photo diode is amplified by the integrated circuit preamplifier  606  and then output through the data signal contacts  610  and  612 . The photo diode submount is represented as a capacitor  604  that filters noise from the power supply (Vcc)  614 . The capacitor  608  filters noise from the data signals.  
         [0053]      FIG. 7  shows an embodiment of an optoelectronic transceiver  700  in accordance with the present invention. The optoelectronic transceiver  700  includes a transmitter TO package  702  and receiver TO package  704 . The transmitter TO package  702  houses a light source such as a laser diode, and the receiver TO package  704  houses a detector such as a photo diode. Data signals are transmitted from external electrical circuitry  710  to the transmitter TO package  702  via the transmitter circuit interconnect  706 . The data signals from the detector are transmitted through the receiver TO package  704  to the external electrical circuitry  710  via the receiver circuit interconnect  708 . Both the transmitter circuit interconnect  706  and the receiver circuit interconnect  708  ground their respective TO package through direct contact with the ground rings  712  (two of which are shown in  FIG. 7 ) surrounding the data signal contacts  714 .  
         [0054]     While the present invention has been described with reference to a few specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.

Technology Classification (CPC): 7