Abstract:
A programmable phase locked-loop (PLL) active filter circuit is provided which includes networks of cooperating bandwidth tuning components to select bandwidth ranges. The values and arrangement of the network of selectable series input (R 1 ) resistors are chosen to be useful in both low band and high band settings. Likewise, the opamp network of feedback resistors (R 2 ) and capacitors (C 1 ) values are chosen to be useful in both low band and high band applications, automatically pairing with the R 1  selection in response to a bandwidth range selection. These tuning components, internal to an integrated circuit, can be used for a plurality of wideband loops. External components can be used to supplement the internal components for low and high bandwidth applications.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to integrated circuit (IC) active loop filter circuitry and, more particularly, to a filter network that can be programmed to operate over multiple bandwidth ranges in a phase locked-loop system. 
     2. Description of the Related Art 
     Phase locked loop circuits, including active filters, are used in communication systems and also as part of larger digital system applications such as clock de-skewing, clock synchronization, high speed serial transmission, and clock recovery. The challenges of the complicated larger systems configured from integrated circuits include variable supply noise frequency spurs (system dependent), variable bandwidth requirements (system application dependent), and reliable operation of the phase locked-loop systems under the stringent requirements. Thus, the challenge is to design a phase-locked loop (PLL) that is configured to satisfy the application dependent bandwidth requirements, as well as variable power supply noise frequency requirements. 
     FIG. 1 is a schematic block diagram illustrating a generic PLL circuit  10  using an active filter architecture (prior art). An active filter  12  is a key component of the loop. The bandwidth (BW) of the PLL is given by the following equation: 
       PLL BW= ( K   PD /2π)*( K   V   /N )*( R   2 / R   1 )* A   
     
       
         loop damping=0.5*( sqrt{K   PD **( K   V   /N )* A *( R   2   2   *C )/ R   1 } 
       
     
     where K PD  and K V  are phase detector and VCO gain parameters, respectively. A is the external attenuation factor of the PLL. Typically, A=1 for on-chip PLL implementations. R 1 , R 2 , and C are components of the active filter  12 . 
     FIG. 2 is a schematic diagram illustrating the active filter  12  of the PLL of FIG. 1 (prior art). The bandwidth and stability parameters of the PLL are determined by the values of R 1  ( 16 ), R 2  ( 18 ), C ( 20 ), and attenuation factor A, which is defined by the ratio of R 4  ( 24 ) to R 3  ( 22 ). For discrete integrated circuit applications, these tuning components have been conventionally located external to the IC, on the printed circuit board (PCB). However, these components take up PCB real estate and their exposure can lead to the injection of noise at critical circuit nodes, which degrades performance. Prior art on-chip integrated circuit PLL circuits are also known. However, due to the limited range of IC internal tuning components, a corresponding limited range of PLL bandwidths are available. Other schemes use multiplexor (MUX) circuits to provide a great range of tuning components, but the multiplexor circuit connections are necessarily numerous, permitting the injection of noise into the more sensitive nodes of the PLL system. Other designs provide PLL bandwidth ranges by providing an IC with parallel tuning circuits. The limited number of node connections in the chosen circuit reduces the noise injection problem, but space and power consumption on the IC is wasted. 
     Parallel circuits of duplicate parts are required, where each parallel circuit contains its own amplifier, tuning components, and oscillator. 
     It would be advantageous if an IC could be devised for configuring PLL bandwidth ranges with the minimum number of internal IC components. Likewise, it would desirable if the bandwidth ranges could be selected with the minimum number of instructions, such as with a user programmable configuration register. 
     It would be advantageous if the above-mentioned bandwidth range selectable IC could be devised to optionally operate with external components to provide configurable bandwidth ranges. 
     SUMMARY OF THE INVENTION 
     Accordingly, a programmable active filter architecture is provided for IC designs such as phase-locked loop systems. The invention represents a significant improvement over conventional active filter implementations in terms of the flexibility offered in the selection of the desired range (low or high) bandwidth and desired filter mode (external or internal implementation). The bandwidth ranges are available without having to implement a system of different selectable loops. The invention describes the circuit implementation details of the active filter components, loop stability considerations, programmable features for selecting the bandwidth range (low or high) and filter mode (external or internal). 
     More specifically, a PLL active filter integrated circuit with selectable bandwidth ranges is provided. The PLL active filter comprises an amplifier and a filter network. The filter network is coupled to the amplifier and supplies a plurality of PLL bandwidth ranges in response to the bandwidth range commands. 
     For example, when the filter network accepts an external mode, low bandwidth range command, the filter network supplies a large value of R 1  resistance, a small value of R 2  resistance, and a large Cl capacitance in parallel to the R 2  resistance. An external capacitor can also be selectively connected between the filter network and the amplifier output for improved damping. When the filter network accepts an external mode, high bandwidth command, the filter network supplies a low value of R 1  resistance, a large value of R 2  resistance, and a small C 1  capacitance in parallel to the R 2  resistance. Again, the external capacitor can be selectively added to the filter to modify the damping factor. The filter network also has an internal mode, high bandwidth range command. Actually, there are a plurality of internal mode, high bandwidth range commands, where each specific command corresponds to a selected value of R 1  resistance. The internal mode, high bandwidth range commands also select a (fixed) low value of R 2  resistance and a (fixed) large value of C 1  capacitance in parallel to the R 2  resistance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic block diagram illustrating a generic PLL circuit using an active filter architecture (prior art). 
     FIG. 2 is a schematic diagram illustrating the active filter of the PLL of FIG. 1 (prior art). 
     FIG. 3 is a schematic block diagram illustrating the present invention integrated circuit PLL including an active filter with programmable bandwidth ranges. 
     FIG. 4 is a schematic diagram of the selectable R 1  resistor of FIG.  3 . 
     FIG. 5 is a schematic diagram illustrating the selectable parallel R 2  resistance/C 1  capacitance network of FIG.  3 . 
     FIG. 6 is a schematic diagram of the present invention selectable PLL bandwidth range active filter using a differential signal amplifier and a VCO. 
     FIG. 7 is a flowchart illustrating the present invention method for varying the bandwidth range of an integrated circuit (IC) phase locked-loop (PLL) active filter including a filter network and an amplifier. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 is a schematic block diagram illustrating the present invention integrated circuit PLL  100  including an active filter  102  with programmable bandwidth ranges. Generally, the invention describes the circuit implementation details of the active filter components, loop stability considerations, programmable features for selecting the bandwidth range (low or high) and filter mode (external or internal). Specifically, the invention describes a programmable filter network  104  that includes a selectable R 1  resistance  106  that is a network of resistors and switches, and a selectable parallel R 2 /C 1  network  108  that is a network of resistors, capacitors in parallel with the resistors, and switches. Unlike multiplexor networks however, the resistor and capacitor values are chosen for cooperation between networks  106  and  108 . That is, the networks  106 / 108  are commanded to work in one of three pre-configured arrangement of components. As explained in detail below, the filter network  104  can be used in conjunction with external resistors and capacitors to extend the bandwidth range selection. 
     The active filter  102  comprises a single-ended amplifier  110  having an input and an output. The filter network  104  has a first port on lo line  112  to accept an input signal, a second port connected to the amplifier input on line  114 , a third port coupled to the amplifier output on line  116 , and a fourth port to accept bandwidth range commands on line  118 . The filter network  102  supplies a plurality of PLL bandwidth ranges in response to the bandwidth range commands. As noted above, the selectable R 1  resistance  106  and selectable parallel R 2 /C 1  network cooperate by automatically configuring themselves into one of three basic bandwidth ranges in response to the bandwidth range commands on line  118 . The bandwidth range commands are typically provided from a user configurable register. 
     More specifically, when the filter network  102  accepts an external mode, low bandwidth range command, a large value of R 1  resistance is supplied between the first and second ports on lines  112  and  114 , respectively. Simultaneously, a small value of R 2  resistance is supplied between the second port on line  114  and the third port coupled to line  116 , and a large C 1  capacitance is supplied in parallel to the R 2  resistance. Details of the selectable R 1  resistor  106  and selectable R 2 /C 1  network  108  are provided below. 
     When the filter network  102  accepts an external mode, high bandwidth range command, a low value of R 1  resistance is supplied between the first and second ports on lines  112  and  114 , respectively. Simultaneously, a large value of R 2  resistance is supplied between the second port on line  114  and the third port coupled to line  116 , and a small C 1  capacitance is supplied in parallel to the R 2  resistance. 
     The filter network  102  accepts a plurality of internal mode, high bandwidth range commands. In response to the plurality of internal, high bandwidth range commands a corresponding plurality of R 1  resistances are supplied between the first and second ports on lines  112  and  114 , respectively. Simultaneously, a low (fixed) value of R 2  resistance is supplied between the second port on line  114  and the third port coupled to line  116 , and a large (fixed) value of C 1  capacitance is supplied in parallel to the R 2  resistance. Since no external components are selected in this mode of operation, optimal noise performance is obtained. 
     FIG. 4 is a schematic diagram of the selectable R 1  resistor  106  of FIG.  3 . The selectable R 1  resistor  106  includes a first resistor  120 , shown surrounded by dotted lines, with a plurality of selectable series resistors. The first resistor  120  has an input connected to the first port of the filter network on line  112 , and the first resistor  120  has an output. A second resistor  122  has an input connected to the output of the first resistor  120  and an output connected to the second port of the filter network on line  114 . A first switch  124  has an input connected to the first port of the filter network on line  112  and an output connected to the output of the first resistor  120 . In one aspect of the invention, the first switch  124 , and the switches introduced below, are implemented with P-channel and N-channel transistors with differential select signals. 
     The first resistor  120  includes a third resistor  126  having an input connected to the input of first switch  124  and an output. A fourth resistor  128  has an input connected to the output of the third resistor  126  and the fourth resistor  128  has an output. A fifth resistor  130  has an input connected to the output of the fourth resistor  128  and the fifth resistor  130  has an output. A sixth resistor  132  has an input connected to the output of the fifth resistor  130  and the sixth resistor  132  has an output. A seventh resistor  134  has an input connected to the output of the sixth resistor  132  and the seventh resistor  134  has an output. An eighth resistor  136  has an input connected to the output of the seventh resistor  134  and the eighth resistor  136  has an output. A ninth resistor  138  has an input connected to the output of the eighth resistor  136  and the ninth resistor  138  has an output connected to the input of the second resistor  122 . 
     A second switch  140  has an input connected to the input of the fourth resistor  128  and an output connected to the output of the fourth resistor  128 . A third switch  142  has an input connected to input of the sixth resistor  132  and an output connected to the output of the seventh resistor  134 . A fourth switch  144  has an input connected to input of the seventh resistor  134  and an output connected-to the output of the eighth resistor  136 . 
     A dotted line is shown connecting sixth resistor  132  and seventh resistor  134 . The dotted line is intended to represent that additional resistors could be added between the sixth resistor  132  and the seventh resistor  134  in other aspects of the invention. Likewise, additional switches could be added to connect these additional resistors (not shown) to create further R 1  resistance combinations. 
     The network has the three fundamental select modes. When the filter network receives an external mode, low bandwidth range command, it is desirable to have a large value of R 1  resistance. Therefore, the first switch  124  is opened. Likewise, the second, third, and fourth switches  140 / 142 / 144  are opened. When the external mode, high bandwidth range command is received on line  118  (see FIG.  3 ), a lower value of R 1  is required. Therefore, the first switch  124  is closed, and the R 1  value is approximately equal to the resistance of the second resistor  122  and the resistance across the first switch  124 . 
     The internal mode, high bandwidth range command is actually a plurality of commands which selectively control the operation of the first switch  124 , second switch  140 , third switch  142 , and fourth switch  144 . The first switch  124  is open in all cases. Combining the opening and closing of the second, third, and fourth switches  140 / 142 / 144  creates a plurality of R 1  resistances corresponding to the number of internal mode, high bandwidth range commands. 
     FIG. 5 is a schematic diagram illustrating the selectable parallel R 2  resistance/C 1  capacitance network  108  of FIG.  3 . The selectable parallel R 2  resistance/C 1  capacitance network  108  includes a tenth resistor  200  having an input connected to the second port of the selectable filter network on line  114  and an output. A multi-pole network  202  has an input connected to output of the tenth resistor  200  and an output. An eleventh resistor  204  has an input connected to the output of the multi-pole network  202  and an output connected to the third port of the filter network. 
     A second capacitor  206  has an input connected to the second port of the filter network on line  114  and an output connected to the third port of the filter network. A fifth switch  208  has an input connected to the output of the tenth resistor  200  and an output connected to the output of the multi-pole network  202 . 
     A sixth switch  210  has an input connected to the second port of the filter network on line  114  and an output. A third capacitor  212  has an input connected to the output of the sixth switch  210  and an output. A seventh switch  214  has an input connected to the output of the third capacitor  212  and an output connected to the third port of the filter network. 
     The multi-pole network  202  includes a twelfth resistor  216  having an input connected to the output of the tenth resistor  200  and an output. A thirteenth resistor  218  has an input connected to the output of the twelfth resistor  216  and an output. A fourteenth resistor  220  has an input connected to the output of the thirteenth resistor  218  and an output. A fifteenth resistor  222  has an input connected to the output of the fourteenth resistor  220  and an output connected to the input of the eleventh resistor  204 . A fourth capacitor  224  has an input connected to the input of the thirteenth resistor  218  and an output connected to the third port of the filter network. A fifth capacitor  226  has an input connected to the input of the fifteenth resistor  222  and an output connected to the third port of the filter network. 
     When the filter network accepts an external mode, low bandwidth range command at the fourth port, the fifth switch  208  is closed to minimize the R 2  resistance. Note, the resistance of the fifth switch  208  is small relative to the resistance of the tenth resistor  200  and the eleventh resistor  204 . The sixth switch  210  and the seventh switch  214  are closed to maximize the C 1  capacitance. The addition of a large C 1  capacitance insures closed loop stability of the amplifier  110 . The total resistance is the result of the tenth resistor  200 , the eleventh resistor  204 , and the resistance of the fifth switch  208 . The total capacitance is the result of the second capacitor  206  and the third capacitor  212 , which has significantly more capacitance than the second capacitor  206 . This allows independent control of the opamp  110  closed-loop compensation in the external mode, low bandwidth range mode without interfering with the high bandwidth range mode phase-locked loop system stability. The value of the third capacitor  212  is optimized with the worst-case closed loop opamp  110  stability considerations. 
     When the filter network accepts an external mode, high bandwidth range command at the fourth port, the fifth switch  208  is opened to maximize the R 2  resistance. The sixth switch  210  and the seventh switch  214  are opened for the reduction of the C 1  capacitance needed to insure amplifier  110  closed loop stability. For PLL closed loop stability, a series of relatively high frequency poles are created, optimally placed as explained below. One high frequency pole is created with the cooperation of second capacitor  206  with the combined resistance value of the tenth resistor  200 , the twelfth resistor  216 , the thirteenth resistor  218 , the fourteenth resistor  220 , the fifteenth resistor  222 , and the eleventh resistor  204 . A second high frequency pole is created by the cooperation of fourth capacitor  224  and the combined resistance of the thirteenth resistor  218 , the fourteenth resistor  220 , the fifteenth resistor  222 , and the eleventh resistor  204 . A third high frequency pole is created by the cooperation of the fifth capacitor  226  and the combined resistance of the fifteenth resistor  222  and the eleventh resistor  204 . The optimum placement for the capacitors is dictated by the highest bandwidth range target and the worst-case PLL phase margin considerations. The high frequency poles for the PLL are generally intended to improve the stability of the amplifier  110  in the closed loop, and do not interfere with the PLL stability. The dotted lines shown connecting the thirteenth resistor  218  and the fourteenth resistor  220  are intended to represent the potential for the addition of resistors and capacitors, to place additional high frequency poles. 
     When the filter network also accepts internal mode, high bandwidth range commands at the fourth port, the fifth switch  208 , the sixth switch  210 , and seventh switch  214  are closed to minimize the R 2  resistance but maximize the C 1  capacitance. The compensation scheme utilized for the external mode, low bandwidth range works for this mode with the same of value of R 2  and the programmable R 1  configuration described above. 
     Returning to FIG. 3, a sixth capacitor  300  is included having an input connected to the third port of the filter network and an output connected to the amplifier output on line  116 . In addition, an eighth switch  302  has an input connected to the third port of the filter network and an output connected to an external IC port. A seventh capacitor  304 , external to the integrated circuit  100 , has an input connected to the output of the eighth switch  302  and an output. A ninth switch  306  has an input connected to an external IC port to interface with the output of the sixth capacitor  304 . The ninth switch  306  has output connected to the output of the amplifier on line  116 . The eighth switch  302  and the ninth switch  306  are closed in response to the external mode, low bandwidth range command to control the damping factor. Likewise, the eighth switch  302  and the ninth switch  306  are closed in response to the external mode, high bandwidth range command to improve and control the damping factor. 
     A sixteenth resistor  308  is included in some aspects of the invention, having an input connected to the amplifier output on line  116  and an output. A tenth switch  310  has an input connected to the output of the amplifier on line  116  and an output connected to the output of the sixteenth resistor  308 . The tenth switch is opened, to include the sixteenth resistor  308 , in response to external mode, low bandwidth range and external mode, high bandwidth range commands. Together, the sixteenth resistor  308  and an optionally connected seventeenth resistor  316 , external to the IC  100 , form an attenuation network. Because seventeenth resistor  316  is an external component, bandwidths can be targeted in the range from tenths of kHz to hundreds of kHz in the external mode, low bandwidth range. Likewise, when the external mode, high bandwidth range command is given, the seventeenth resistor  316  is used. The tenth switch  310  is opened, and sixteenth resistor  308  and seventeenth resistor  316  form an attenuation network. Bandwidths in the range from MHz onwards to tens of MHz are possible in this configuration. 
     In some aspects of the invention when an internal mode, high bandwidth command is used, the tenth switch  310  is closed to bypass the sixteenth resistor  308 . In these circumstances the seventeenth resistor is typically not used. The attenuation network formed by the sixteenth resistor  308  and the seventeenth resistor  316 , connected between the amplifier  110  output and an input to a voltage controlled oscillator  318 , is bypassed. That is, the external attenuation factor A is equal to one. 
     FIG. 6 is a schematic diagram of the present invention selectable PLL bandwidth range active filter using a differential signal amplifier  110  and the VCO  318 . The amplifier  110  has a positive input connected to the second port of a first filter network  102  on line  114 . The amplifier  110  further includes a negative input connected to line  400 . A second filter network  402  is included having a first port on line  404  to accept an input signal, a second port connected to the amplifier negative input on line  400 , a third port, and a fourth port on line  406  to accept bandwidth range commands. As with the first filter network  102  described above, the second filter network  402  supplies a plurality of active filter bandwidth ranges in response to the bandwidth range commands. The first and second filter networks  102 / 402  provide equivalent resistance and capacitance values simultaneously. Capacitor  420  corresponds to sixth capacitor  300 . Capacitor  422  corresponds to seventh capacitor  304 . Resistor  424  corresponds to sixteenth resistor  308 , resistor  426  corresponds to seventeenth resistor  316 , and capacitor  428  provides an AC ground. 
     FIG. 7 is a flowchart illustrating the present invention method for varying the bandwidth range of an integrated circuit (IC) PLL active filter including a filter network and an amplifier. Although the method is depicted as a sequence of numbered steps for clarity, no order should be inferred from the numbering unless explicitly stated. The method begins at Step  500 . Step  502  accepts an input signal. Step  504  accepts a bandwidth range command. Step  506 , in response to the bandwidth range command, selects the value of the R 1  resistance in series to the amplifier input. Step  508  selects the value of R 2  resistance from the amplifier output to the amplifier input in cooperation with the R 1  resistance selected in Step  506 . Step  510  selects the value C 1  capacitance in parallel with the R 2  resistance, in cooperation with the R 1  resistance selected in Step  506  and the R 2  resistance selected in Step  508 . 
     In some aspects of the invention, accepting a bandwidth range command in Step  504  includes accepting an external mode, low bandwidth range command. Then, selecting an R 1  resistance in Step  506  includes selecting a large value of the R 1  resistance in series to the amplifier input. Selecting an R 2  resistance in Step  508  includes selecting a small value of R 2  resistance from the amplifier output to the amplifier input. Selecting a C 1  capacitance in Step  510  includes selecting a large value C 1  capacitance in parallel with the R 2  resistance. 
     In some aspects of the invention an internal sixth capacitor connects the R 2  resistor and the amplifier output, and an external seventh capacitor (see FIG. 3) is included. A further step, Step  512  connects the external capacitor between the R 2  resistance and the amplifier output in response to the external mode, low bandwidth range command in Step  504 . 
     In some aspects of the invention, a voltage controlled oscillator (VCO) is included, as well as an external seventeenth resistor having an input connected to the input of the VCO and an output connected to ground. Then, Step  514  connects a sixteenth resistor between the amplifier output and the input of the seventeenth resistor. The sixteenth and seventeenth resistors form an attenuation network between the amplifier output and the VCO input. 
     In some aspects of.the invention, accepting a bandwidth range command in Step  504  includes accepting an external mode, high bandwidth range command. Then, selecting an R 1  resistance in Step  506  includes selecting a small value of the R 1  resistance in series to the amplifier input. Selecting an R 2  resistance in Step  508  includes selecting a large value of R 2  resistance from the amplifier output to the amplifier input. Selecting a C 1  capacitance in Step  510  includes selecting a small value C 1  capacitance in parallel with the R 2  resistance. 
     Step  512  connects the external capacitor between the R 2  resistance and the amplifier output in response to the external mode, high bandwidth range commands in Step  504 , and Step  514  connects the sixteenth resistor value from the amplifier output to the seventeenth resistor and the VCO inputs. 
     In some aspects of the invention, accepting a bandwidth range command in Step  504  includes accepting a plurality of internal mode, high bandwidth commands. Then, selecting an R 1  resistance in Step  506  includes selecting a plurality of R 1  resistances in series to the amplifier input in response to the corresponding plurality of internal mode, high bandwidth commands. Selecting an R 2  resistance in Step  508  includes selecting a small value of R 2  resistance from the amplifier output to the amplifier input. Selecting a C 1  capacitance in Step  510  includes selecting a large value C 1  capacitance in parallel with the R 2  resistance. 
     A system and method have been described for a cooperating network of resistor and capacitor components with values and switches which permit the same components to be automatically configured into a plurality of bandwidth ranges. External components are added to further extend the bandwidth range and improve the damping factors. A specific example has been provided of an active filter in the context of a PLL circuit. However, the selectable bandwidth range concept of the present invention is applicable to a wider range of uses. Other variations and embodiments will occur to those skilled in the art.