Abstract:
An RF choke is understood to be used in the cable television (“CATV”) systems to separate RF signals from the AC power signal, both of which initially co-exist when transmitted from the headend of the CATV system. The RF choke of the present invention, is modified by separating the predetermined number of turns from the winding adjacent to the “hot” (RF/AC) side of the RF choke in a CATV circuit. By evenly spreading these turns from each other with the purpose of reducing the series parasitic capacitance between the “hot” terminal and the rest of the choke. A useful parallel resonance at high frequency range in series with the rest of the choke is created. In addition, such a RF choke being connected (in shunt) to the RF signal path affects its performance to a much lesser degree than prior art chokes. Because of its reduced parasitic parameters, the choke of the present invention can be placed right at (or on) the AC/RF connector (terminal) of the PCB. This eliminates the oversized conductive trace that connected the CATV cable to the RF choke in prior art implementations. Thus, this placement reduces the parasitic parameter interference contributed by the now eliminated PCB conductive trace, which had to be sized to carry the AC signal of up to 25 Amps.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This patent application claims the priority of US. Provisional Patent Application No. 60/214,907 filed Jun. 29, 2000. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a Radio Frequency coil (“RF choke”); and more specifically, to an improved RF choke with improved RF performance and improved method for implementing the RF choke in a cable television (“CATV”) system carrying both an AC signal and RF frequencies. 
     BACKGROUND OF THE INVENTION 
     It is within the general knowledge of those skilled in the art that RF signals and an AC power signal are commonly transmitted from the headend to the subscriber, over coaxial cable, in a typical CATV system. The AC power signal is used to power electronic devices along the network, such as amplifiers. Thus, prior to being received by the subscriber, it is customary to separate the RF signals from the AC power signal (“AC signal”). It is known in the art that RF chokes are used to separate (“bypass”) the AC signal from the RF signals. The AC power signal typically ranges in frequency from 50 to 60 Hz and typically carries from 8 to 25 Amperes (“Amps”). The RF signals transmitted from the headend typically range from 50 MHz to upward of 870 MHz. It is also commonly known in the art that in systems allowing a subscriber to transmit RF signals to the headend, typically from 5 MHz to 42 Mhz and above, such RF signals may be combined with the AC signal prior to reaching the headend. 
     There are several problems with using RF chokes to bypass the AC signal. The first problem is the high currents being bypassed by the RF chokes creates hum distortion, which distorts the information contained within the RF signals. Thus, there is a need in the art to diminish this distortion and allow the RF signals to pass with minimum interference. 
     Handling the currents that rise up to 25 amps, while effectively maintaining the RF signals from 5 MHz to above 870 MHz, increases the requirements of the RF choke. For example, an RF choke which must handle currents rising up to 25 Amps, must be wrapped with a wire conductor of sufficient size (up to 14 AWG). The increased wire size increases the RF choke cost and degrades its performance. Thus, there is also a need to provide RF chokes whose increased wire size minimally effect distortion of the RF signals. 
     A Printed Circuit Board (“PCB”) is used to make the electrical connections at and around the juncture used for separating the AC current from the RF signals in most CATV systems. The CATV cable carrying both the AC signal and the RF signals is connected at a node on the PCB. After separation, the RF signals are connected to a matching network (“tuning network”); whereas, the AC signal is connected to the power network through the RF choke. The connection from the CATV cable to the RF choke has typically been made by using electrical conducting traces on the PCB. Due to the high circuit current received by the RF choke, the trace leading from the CATV cable to the “hot side” of the RF choke had to be of sufficient size to carry up to 25 amps. This large electrical trace distorts the RF signals. Specifically, the trace interferes with precision RF adjustments of the tuning network. Thus, there is a further need in the art to avoid this high current carrying trace which interferes with RF precision tuning. 
     Generally RF chokes, utilize the combination of clockwise windings connected to a series of counterclockwise windings around a core, and a dampening resistor interposed between a predetermined winding amongst the counterclockwise windings and the last of the counterclockwise winding. The clockwise windings are connected to the “hot” lead of the RF choke, which is connected to the CATV cable carrying both the RF signals and the AC signal. The general design, while suitable for its intended purpose, creates distortion with the RF signals. Thus, there is a need in the art for a revised RF choke design to help diminish the distortion and allow the RF signals to pass with minimum interference. 
     Prior art RF chokes, which utilized increased wire size to handle the high AC signal currents, introduced more parasitic capacitance. Thus, there is also a need in the art to provide RF chokes whose increased wire size minimally effect distortion of the RF signals. 
     SUMMARY OF THE INVENTION 
     The invention is a modified RF choke that is implemented in an innovative manner in an RF CATV circuit. In the RF CATV circuit, the RF choke is used to separate the AC signal from the RF signal in an RF circuit. The RF choke has a core that is normally made of a ferrite material. The core is usually cylindrical in shape, and has two ends. On the first end, a conductor is wrapped in a clockwise direction around the core and is considered the first winding. The first lead of the RF choke is electrically connected at the first end of the first winding. The number of times the first winding is wrapped around the core is the number of first winding turns. Hereinafter, the number of first winding turns is referred to as “N1”. In the newly designed RF choke, the first winding implements a spacing (“offset”) between a predetermined number of first winding turns. The predetermined number of first winding turns is hereinafter referred to as “X”. 
     The second end of the first winding is connected to the second winding, which circumvents the core in the counter clockwise direction. The number of times the second winding wraps around the core is the number of second winding turns . Hereinafter the number of second winding turns is referred to as “N2”. The second lead is electrically connected to the second end of the second winding. In addition, there is a dampening resistor that is electrically connected between the second lead and a predetermined place amongst the second winding. 
     The present invention, the improved RF choke, may be implemented in an CATV system which carries the RF signal and the AC signal. The RF choke will be used to isolate the AC signal from the RF signal. The electrical connections are implemented on a printed circuit board (“PCB”). A CATV cable is connected to a node on this PCB. The first lead of the RF choke and a RF jumper conductor that carries the isolated RF signal to a tuning network, is also connected at this node. The AC signal goes through the RF choke to a power network. The RF signal goes through the RF jumper conductor to a connected RF tuning network. The RF jumper conductor may be used to improve RF tuning and impedance matching in the RF tuning network. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The construction designed to carry out the invention will hereinafter be described, together with other features thereof. 
     FIG. 1 is a pictorial diagram  100  of the RF choke as previously known in the art. 
     FIG.  2 . is a pictorial diagram of the RF choke  200  of an exemplary embodiment of the present invention. 
     FIG. 3 is a pictorial diagram  300  illustrating the physical placement of the RF choke in the CATV system on a PCB in an exemplary embodiment of the present invention. 
     FIG. 4 is a schematic diagram  400  illustrating the electrical connections to the RF choke in the CATV system as used in an exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now in more detail to the drawings, the invention will now be described in more detail. 
     FIG. 1 is a pictorial diagram of the RF choke  100  as previously known in the art. As previously known, the central body of the RF choke  100  is a core  105 , often made up of a ferrite material. The core  105  normally extends the length of the RF choke  100 . It is often cylindrical in shape and serves as the body around which the windings of the RF choke  100  are wound. 
     The windings of the RF choke  100  extends from the primary (“Hot”) lead  110 . The primary (“Hot”) lead  110  is utilized to electrically connect the RF choke  100  to other parts of the CATV system. The primary (“Hot”) lead  110  is normally electrically connected to the CATV system cable which carries both the RF signals and the AC signal. Thus, the primary (“Hot”) lead  110  electrically connects the CATV system cable to the windings of the RF choke  100 . 
     Specifically, the primary (“Hot”) lead  110  connects to the clockwise winding. This connection allows the AC signal from the CATV system cable to be separated from the RF signals, through the RF choke  100 . As indicated by the name, the clockwise winding  115  is wound around the core  105  in a clockwise direction, when observing the core  105  longitudinally from the end that the clockwise winding  115  begins. The windings are normally made of a conductive wire material. The number of times in which the winding is wound around the core  105  are commonly referred to as the number of turns. The number of turns are one parameter of the RF choke  100  that directly effects its inductive properties. A single turn of the clockwise winding  115  is shown and referred to as a single turn cut-away clockwise winding  120  for illustrative purposes. Normally, the clockwise winding  115  does not span the entire length of the core  105 . 
     The clockwise winding  115  is electrically connected to a counterclockwise winding  125 . The counterclockwise winding  115  completes the total number of turns (when added to the clockwise winding) that spans the length of the core  105 . A single turn of the counterclockwise winding  125  is shown and referred to as a single turn cut-away counterclockwise winding  130  for illustrative purposes. The counterclockwise winding  125  connects to the secondary lead  135 . The secondary lead  135  is normally connected to an RF tuning network when implemented in the CATV system. 
     Finally, the prior art RF choke  100  also includes a dampening resistor  140 . The dampening resistor  140  is interposed between a predetermined turn in the counterclockwise winding  125  and the secondary lead  135 . The dampening resistor  140  dampens resonance (another inherent characteristic of an RF choke  100 ). Resonance stems from the interaction of parasitic capacitance created when current flows through the windings of the RF choke  100 , and the inductive characteristics of the inductor. 
     FIG.  2 . is a pictorial diagram of the RF choke  200  of an exemplary embodiment of the present invention. Similar to the previously known RF choke  100 , the RF choke  200  includes a central body commonly known as a core  105 . The core is generally manufactured from a ferrite material for improved inductive qualities. 
     The primary (“Hot”) lead  210  extends from the windings of the RF choke  200 . The primary (“Hot”) lead  210  is utilized to electrically connect the RF choke  200  to other parts of the CATV system. The primary (“Hot”) lead  210  is normally electrically connected to the CATV cable which carries both the RF signals and the AC signal. Thus, the primary (“Hot”) lead  210  electrically connects the CATV cable to the windings of the RF choke  200 . 
     Specifically, the primary (“Hot”) lead  210  connects to the clockwise winding  215 . The clockwise windings have a specific number of turns, N1  217 . A single turn of the clockwise winding  215  is shown and referred to as a single turn cut-away clockwise winding  220  for illustrative purposes. Unlike prior art RF choke  100 , the clockwise winding  215  of RF choke  200  has a predetermined number of its turns, X  219 , which are separated by a predetermined offset. Though this embodiment of the invention utilizes a uniform offset between each of the separated windings, other embodiments may have predetermined offsets that are not all uniform. One purpose for adding the offset is reducing the series parasitic capacitance between the primary (“Hot”) lead  210  and the ground. It also reduces the series parasitic capacitance between the primary (“Hot”) lead  210  and the remainder of the RF choke  200 . In addition, this offset in the clockwise winding  215  creates a useful parallel resonance at high frequency ranges in series with the rest of the RF choke  200 . 
     As shown with the prior art RF choke  100 , the clockwise winding  215  is connected to a counterclockwise winding  225 . The counterclockwise windings have a specific number of turns, N2  227 . The counterclockwise winding  225  completes the total number of turns (when added to the clockwise winding  215 ) that spans the length of the core  205 . A single turn of the counterclockwise winding  225  is shown and referred to as a single turn cut-away counterclockwise winding  230  for illustrative purpose. The counterclockwise winding  225  connects to the secondary lead  235 . The secondary lead  235  is normally connected to a power network, which further distributes the AC power. 
     The RF choke  200  also includes a dampening resistor  240 . The dampening resistor  240  is interposed between a predetermined turn in the counterclockwise winding  225  and the secondary lead  235 . The dampening resistor  240  dampens resonance (another inherent characteristic of an RF choke  100 ). Resonance stems from the interaction of parasitic capacitance created by the windings of the RF choke  200 , and the inductive characteristics of the inductor. 
     Those skilled in the art will understand that specific numerical values for N1  217 , X  219 , and N2  227 , will be dictated by numerous factors such as operation conditions, tolerances, circuit frequency range, cost, needed response, etc. Thus, any specific numerical values discussed herein are not limitations to the possible values of other embodiments of this invention. In an exemplary embodiment of the invention, the induction value of a given RF choke (“L”) was chosen to provide the required isolation to the RF signal for a minimum 5 MHz signal and a maximum of 1000 MHz. A value of at least L=4.7 microhenries was chosen, but is not a limitation. Though other combinations may be implemented, one exemplary embodiment would use N1  217  equals 7, N2  227  equals 14, and X  219  equals 4. These values have shown to be one combination of values that yield the intended results. Though N1  217  and N2  227  could have been equal, the respective values of 7 and 14 were chosen to present a lower amount of turns to the primary (“Hot”) lead  210  to trade-off for better RF performance (lower parasitic capacitance adjacent to the RF signals). 
     Though all clockwise winding could have an offset, it should be noted that the more turns spread, the more the choke inductance is decreased. Thus, the choice of X  219  equals 4, was a compromise between improving RF qualities by decreasing the parasitic capacitance (spreading the turns of the clockwise winding  215 ) and maintaining the design required minimum L (value of the RF choke discussed above). 
     Other embodiments will encompass X  219  having a value which may range from X  219  equals 1 to X  219  equals 30 percent of the total number of turns in the RF choke  200 . The total turns of the RF choke  200  equals N1  217  added to N2  225 . The larger the amount of turns with an offset, the more turns there would have to be in the first winding to maintain the design minimum L. The actual value will be chosen using many factors, including improving RF qualities and maintaining the design required minimum L. 
     Design factors, as explained previously, also dictate the amount of offset between the turns in the clockwise winding  215 . Thus, as a compromise of these factors, exemplary embodiments of the present invention may implement from {fraction (1/10)} the size of the diameter of the in wire used in the winding, to 3 times the diameter of the wire used in the windings. 
     Other exemplary embodiments have demonstrated the desired characteristics, discussed above, by maintaining a ratio of N1  217 , N2  227 , and X  219  having the following respective values: N1  217  equals one-third the total number of turns, N2  227  equals two-thirds the total number of turns, and X  219  ranging from at least 1 turn to 30 percent of the total number of turns. Wherein, the total number of turns is N1  217  added to N2  227 . 
     Turning now to the placement of the inventive RF choke in the CATV system, FIG. 3 is a pictorial diagram  300  illustrating the physical placement of the RF choke in the CATV system. The RF choke  305 , detailed in the description of FIG. 2, is shown with its primary (“Hot”) lead  310  placed into the RF/AC connector  315 . The RF/AC connector  315  is constructed such that it has openings (receptacles) at opposite ends which are electrically connected. Thus if two conductive wires/leads are inserted at the receptacles, they will be electrically connected. The RF/AC connector is secured to the PCB  320  with a fastening device. Inserted at the receptacle opposite of primary (“Hot”) lead  310 , forming an electrical node on the PCB  320 , is the CATV cable  325 . In addition, an RF jumper conductor  330  is electrically connected at the node formed by the primary (“Hot”) lead  310  and the CATV cable  325 . Note that the primary (“Hot”) lead  310  is directly connected to the CATV cable  325 . Unlike prior art, there is no conductive trace on the PCB  320  making the electrical connection. This reduces parasitic parameter interference contributed by the now eliminated PCB conductive trace, which had to be sized to carry the AC signal of up to 25 Amps. 
     The RF jumper conductor  330 , also electrically connected at the node formed by the RF/AC connector  315 , is shown extending from the RF/AC connector to the PCB  320 . The RF jumper conductor  330  is the path of least resistance for the RF signals, thus they are isolated from the AC signal and are connected to the RF tuning network (not shown). 
     Though the RF choke  305  presents a high impedance to the RF signals, it is the path of least resistance for the AC signal (50-60 Hz). Thus, the AC signal is isolated (shunted) by the RF choke  305 . Similar to FIG. 2., there is a damping resistance  335  attached to the coils of the RF choke  305 . This resistance is used for further damping of resonance, which is inherent in the RF choke  305 . 
     Other exemplary embodiments will utilize the RF jumper conductor  330  such that the RF jumper conductor  330  may be sized to assists in impedance matching and tuning of the RF tuning network (not shown). In addition, by raising the RF jumper conductor  330  above the PCB  320 , the parasitic capacitance from the jumper to the ground plane (if connected to ground) would be reduced. However, in this embodiment, the height of the RF jumper conductor  330  from the PCB  320  may also be adjusted for assisting the RF tuning. 
     FIG. 4 is a schematic diagram  400  illustrating the electrical connections to the RF choke  405  in the CATV system as used in an exemplary embodiment of the present invention. The schematic diagram  400  shows RF choke  405 , which was detailed in FIG. 2, and its damping resistor  410 . The damping resistor is connected between one end of the RF choke  405  and a predetermined turn of the windings. A first end (primary (“Hot”) lead  210 ) of the RF choke  405 , is electrically (and physically) connected to the RF/AC connector  415 . The second end (secondary lead  235 ) of the RF choke  405  is the AC connection to the CATV power equipment (not shown). The first end of the RF jumper conductor  420  is also electrically connected at the RF/AC connector  415 . Finally, as illustrated in FIG. 3, the CATV cable  325  is electrically and physically connected to the RF/AC connector  415 . Therefore, the CATV cable  325 , a first end of the RF jumper conductor  420 , and the primary (“Hot”) lead  210  of the RF choke  405  are electrically connected together at RF/AC connector  415 . 
     The second end of the RF jumper conductor  420  is electrically connected to one end of capacitor  425 . The Capacitor  425 , the Capacitor  430 , and the Capacitor  435  (variable capacitance), form the tuning network  440  for RF tuning. The RF jumper conductor  420  may be chosen to assist in impedance matching with the tuning network  440  and assist in RF tuning. 
     FIG. 4 shows the electrical connection of one embodiment of the present invention. Other embodiments may include different tuning networks (various capacitive and inductive combinations). Still other embodiments may have multiple damping resistors  410  (connected in series or parallel). 
     Thus it may be seen that a more advantageous design of an RF choke and implementation may be had according to the present invention. With the present invention and its inventive implementation; the newly designed RF choke being directly connected to the CATV cable (carrying both RF signals and the AC signal), the new RF choke distorts the RF tuning to a much lesser degree than previously designed RF chokes without the offset, or those using enlarged traces for handling the high AC current to the RF choke. The newly designed RF choke that implements the offset between a predetermined number of turns in the winding (whether clockwise or counterclockwise), opposes the increased parasitic capacitance that is introduced when the wire size was increased (as done with prior circuits) to handle the large AC currents. In addition, the oversized conductive trace that connected the CATV cable to the RF choke in prior art implementations has now been eliminated. This reduces parasitic parameter interference contributed by the now eliminated PCB conductive trace, which had to be sized to carry the AC signal of up to 25 Amps. Therefore, the present invention also addresses the need in the art to avoid the high current carrying trace, which interfered with RF precision tuning. 
     The combination of the invention&#39;s innovative features and placement in the circuit optimizes current passing capabilities, maintains sufficient inductance (translating into high impedance) at the low frequency range of the RF signal, adds only a minimum hum modulation and exemplifies tuning and impedance matching characteristics. The newly designed RF choke, having the wire size usable for up to 25 Amps of AC current, exhibits improved performance at the high frequency range. The inventive placement choice relieves the PCB area from the 25 Amp current, normally carried by the now removed trace. The combination of the newly designed RF choke and the new manner of implementation in the CATV system has addressed the highlighted needs of the prior art.