Abstract:
Techniques are provided for compensating for variations in capacitance of capacitors used in resonant circuits, particularly varactors used in voltage-controlled oscillators. Indications of actual varactor capacitances are used to determine which of several inductances to use in the resonant circuit with the varactor. The inductances may be composed of a bondwire inductance, and may also be composed of one or more coil inductors. Based on the determined capacitance indication, a bondwire is connected from a common bondpad to a selected bondpad to complete the resonant tank circuit such that an LC product of the tank circuit is within a desired or acceptable LC product range.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to resonant circuits and in particular to voltage-controlled oscillators. 
     2. Background of the Invention 
     Voltage-Controlled Oscillators (VCOs) are used in many systems, such as communications systems and computers, where frequencies are synthesized. Frequency synthesis can be used, e.g., to provide a carrier frequency for a signal in a communications system such as a radio transceiver. 
     VCOs are configured to have an oscillation, or resonant, frequency. For oscillation frequencies higher than about 1 GHz, typically LC oscillators are used because they have low noise and are relatively stable. LC oscillators typically use a tank circuit including an inductance (L) and a capacitance (C) connected in series or in parallel to provide a resonant circuit. The oscillation frequency of the LC tank depends on a product of the inductance and capacitance (the LC product) of the tank. 
     A control voltage called the tuning voltage is used in a VCO to adjust the oscillation frequency. The oscillation frequency can also be adjusted by varying the capacitance of the tank using the tuning voltage. This may be accomplished by implementing the tank capacitor as a varactor, whose capacitance varies with the tuning voltage. The tuning voltage has a range of voltages that can be provided, corresponding to a range of capacitances that can be provided. This range of capacitances corresponds to a range of frequencies producible by the LC tank. The varactor is typically designed to have a desired nominal capacitance, so that the LC tank will oscillate at a desired frequency, when the tuning voltage is at a nominal voltage. The nominal voltage will be approximately in the middle of the tuning voltage range if the change in oscillation frequency is linear relative to the change in tuning voltage. 
     SUMMARY 
     A number of technical advances are achieved in the art, by implementation of an LC VCO for compensating variances in capacitance of an LC tank of the VCO. The LC VCO may be broadly conceptualized as a system in which the inductive portion of an LC tank of the VCO is selectively adjusted, thus helping to ensure that an LC product of the LC tank is within a desired range to help ensure that the VCO can output a desired frequency or frequencies. 
     For example, an LC VCO may utilize a system architecture in which inductance is adjusted to compensate for variances between actual capacitance and designed/desired capacitance to achieve a desirable LC product. An implementation of the system architecture may include selectable inductances for connection to a capacitor of an LC tank of the VCO. Inductances of different values can be connected to the capacitor to effectively alter the capacitance, by actually altering the LC product. The different inductance values can be provided by different lengths of bondwire, or by different-valued, coils, inductors and the like, or by combinations of these techniques. 
     Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures 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 invention, and be protected by the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The invention can be better understood with reference to the following figures. Components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principals of the invention. 
     FIG. 1 is a schematic block diagram of a phase-locked loop circuit including a voltage-controlled oscillator. 
     FIG. 2 is a schematic circuit diagram of the voltage-controlled oscillator, shown in FIG. 1, including an LC tank. 
     FIG. 3 is a schematic circuit diagram of a voltage-controlled oscillator using a variable inductance arrangement for an LC tank. 
     FIG. 4 is a schematic diagram of a wafer including multiple semiconducting chips. 
     FIG. 5 is a schematic diagram of a system for attaching bondwires as part of the LC tank shown in FIG.  3 . 
     FIG. 6 is a block flow diagram of a process of adjusting an inductance of any of the LC tank shown in FIG.  3 . 
     FIG. 7 is a schematic circuit diagram of another voltage-controlled oscillator using a variable inductance arrangement, different than that shown in FIG. 3, for an LC tank. 
     FIG. 8 is a schematic circuit diagram of another voltage-controlled oscillator using a variable inductance arrangement, different than those shown in FIGS. 3 and 7, for an LC tank. 
    
    
     A detailed description of the invention as illustrated in the figures will now be provided. While the invention will be described in connection with these figures, there is no intent to limit the invention to the embodiment or embodiments disclosed in these figures. On the contrary, the intent is to cover all alternatives, modifications, and equivalents included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to the embodiment of FIG. 1, a typical phase-locked loop (PLL) system  10  is illustrated that includes a phase comparator  12 , a low-pass filter (LPF)  14 , a voltage-controlled oscillator (VCO)  16 , and an N-divider  18 . As shown, the phase comparator  12  receives a reference signal having a reference frequency f r , and a feedback signal having a frequency f o /N. The comparator  12  compares the reference and feedback signals and outputs a DC signal, indicative of a difference in phase between the reference and feedback signals, to the LPF  14 . The LPF  14  filters the output signal from the comparator  12  and outputs the filtered signal to the VCO  16 . The VCO  16  uses the signal from the LPF  14  to adjust a frequency f o  of an output signal of the VCO  16 . The VCO&#39;s output signal is output on a line  20  and is also fed back to the N-divider  18 . The N-divider  18  divides the frequency f o  of the VCO&#39;s output signal by a factor N, and outputs a signal having a frequency f o /N to the phase comparator  12 . The adjustment of the VCO&#39;s output signal frequency f o  continues so that the output frequency f o  locks to a desired frequency that is related to the reference frequency f r  by the factor N. 
     Referring to the embodiment of FIG. 2, the VCO  16  may be implemented using an active device  22  having a negative resistance −R, and an LC tank  24 . The LC tank  24  includes an inductive element  26 , having an inductance L, and a varactor  28  having a variable capacitance C. The varactor&#39;s capacitance C may be varied by a tuning voltage V tune  received by an input  30  that is coupled to the varactor  28 . The varactor  28  may be implemented as a reverse-biased diode. The varactor  28  is shown coupled in parallel with the inductive element  26 , although other couplings, such as a serial coupling, may be employed. Also, the inductive element  26  is shown as a single inductor, but the element  26  may be a combination of multiple inductive apparatus, e.g., coupled in serial, or parallel, or combinations of serial and parallel connections. 
     Referring to FIG. 3, a VCO  40  includes an inductance arrangement  42  for the inductive element  26  of FIG. 2, the varactor  28 , and the active device  22 . In a preferred embodiment, each of the components  22 ,  28 , and  42  are formed on a semiconducting die or chip. By forming the varactor  28  on a semiconducting chip, manufacturing tolerances, e.g., process variations, affect a nominal capacitance C nom  of the varactor  28 , resulting in a tolerance range of nominal varactor capacitance C nom  due to the manufacturing tolerances. Thus, on a given wafer, varactors  28  on different chips may provide different actual nominal capacitances C nom-act  when subjected to the same bias and tuning voltages. The same is true for varactors  28  on different wafers, and on wafers in different batches of wafers. To help compensate for the variance in the varactor&#39;s actual nominal capacitance C nom-act  relative to a designed/desired nominal varactor capacitance C nom-des , the inductance arrangement  42  provides for different inductances to be selectively coupled to the varactor  28 . 
     In at least this embodiment, the inductance arrangement  42  includes a common bondpad  44 , five selectable bondpads  46   1 - 46   5 , and a bondwire  48 . The bondwire  48  is connected from the common bondpad  44  to one of the selectable bondpads  46 , here bondpad  46   3 . The bondwire  48  has an inductance that is dependent on its length and the selectable bondpads  46  are each disposed at different distances relative to the common bondpad  44  such that the bondwire length, and thus inductance, depends upon to which selectable bondpad  46  the bondwire  48  is connected. The distances from the common bondpad  44  to the selectable bondpads  46  are arranged such that an inductance range of the corresponding bondwires  48  will help to compensate for the tolerance range of the varactor&#39;s nominal capacitance C nom . The inductances providable by the arrangement  42  are much more precise/reliable than the varactor capacitance C. 
     The number of, and distances from the common bondpad to, the selectable bondpads  46  are designed to provide a sufficiently broad range of compensation and sufficiently fine resolution to appropriately compensate for any actual nominal varactor capacitance C nom-act  within the expected tolerance range. The range of compensation is broad enough so that an LC product of the inductance L of the inductive element  26  (FIG. 2) and the nominal varactor capacitance C nom  at either extreme of the tolerance range can be brought within a desired, or at least acceptable, range of LC product values. The resolution is such that an effective nominal capacitance C nom-eff  can be adjusted from any value within the tolerance range to within the acceptable capacitance range such that the LC product is within the acceptable LC product range. Thus, a center frequency of oscillation of the tank will be within an acceptable range of frequencies. 
     The following provides a design example for the bondpads  46 . Suppose the actual nominal varactor capacitance C nom-act  can vary ±10% from the designed nominal varactor capacitance C nom-des , and an acceptable range is ±2.5% of C nom-des . In this case, the inductance arrangement needs to be configured to effectively adjust the nominal varactor capacitance C nom  (i.e., adjust an effective nominal varactor capacitance C nom-eff ) from either −10% or +10% of design to between −2.5% and 2.5% of designed nominal capacitance C nom-des . 
     The location of the selectable bondpad  46   3  (assuming five bondpads  46  will be used) may be chosen first. The location of the selectable bondpad  46   3  is chosen such that an inductance L 3  of the bondwire  48  from bondpad  44  to bondpad  46   3  is a desired inductance L des , where the LC product L des C nom-des  yields a desired (and possibly ideal) LC product (an LC product within a desired range and possibly equal to an ideal value). 
     Locations of extreme bondpads are designed in accordance with the nominal varactor capacitance tolerance range and acceptable range. Bondpad  46   1  is located such that a bondwire inductance L 1  will adjust the effective capacitance C nom-eff  from the lower end of the tolerance range C tol-min =0.9C nom-des , to the lower limit of the acceptable range C acc-min =0.975C nom-des . In this case, the inductance L 1 =(C acc-min /C tol-min )L 3 =(0.975/0.9)L 3 , or approximately 1.083L 3 . Similarly, bondpad  46   5  is located such that a bondwire inductance L 5  will adjust the effective capacitance C nom-eff  from the upper end of the tolerance range C tol-max =1.1C nom-des , to the upper limit of the acceptable range C acc-max =1.025C nom-des . In this case, the inductance L 5 =(C acc-max /C tol-max )L 3 =(1.025/1.1)L 3 , or approximately 0.932L 3 . 
     Intermediate bondpad locations, if needed or desired, are chosen to compensate for actual nominal capacitances C nom-act  between the extremes of the tolerance range, especially capacitances that cannot be compensated for by the boundary inductances L 1  and L 5 . For example, the effective capacitance C nom-eff  will not he within the acceptable range of ±2.5% of C nom-des  if L 1  is used and 
     C nom-act ≧(C acc-max *C tol-min /C acc-min )=(1.025*0.9/0.975)C nom-des ≈0.946C nom-des . Thus, using a maximum of 0.94C nom-des  for safety, the bondpad  46   2  can be located such that L 2 =(C acc-min/ 0.94C nom-des )L 3 =(0.975/0.94)L 3 ≈1.037L 3 . The inductance L 2  can compensate actual capacitances C nom-act  of 0.94C nom-des  or greater into effective capacitances C nom-eff  that are within the acceptable capacitance range until 
     C nom-des ≧(C acc-max /X)=(1.025/1.037)C nom-des ≈0.988C nom-des , where X=L 1 /L 3 . As the maximum capacitance for which L 2  can be used is within the acceptable range, no more bondpads with inductances between L 1  and L 3  are needed. Similarly, the effective capacitance C nom-eff  will not be within the acceptable range of ±2.5% of C nom-des  if L 5  is used and 
     C nom-acc ≧(C acc-min *C tol-max /C acc-max )=(0.975*1.1/1.025)C nom-des ≈1.046C nom-des . Thus, using a maximum of 1.05C nom-des  for safety, the bondpad  46   4  can be located such that L 4 =(C acc-max /1.05C non-des )L 3 =(1.025/1.05)L 3 ≈0.976L 3 . The inductance L 4  can compensate actual capacitances C nom-act  of 1.05C nom-des  or less into effective capacitances C nom-eff  that are within the acceptable capacitance range until 
     C nom-act ≧(C acc-min /Y)=(0.975/0.976)C nom-des ≈0.999C nom-des , where Y=L 1 /L 3 . As the minimum capacitance for which L 4  can be used is within the acceptable range, no more bondpads with inductances between L 5  and L 3  are needed. Thus, the assumption of five selectable bondpads  46  was correct. Other numbers of bondpads  46  could be used to provide finer resolution if desired. 
     Predetermined categories of deviations of the actual capacitance C nom-act  relative to the desired/designed nominal varactor capacitance C nom-des  are associated with the selectable bondpads  46 . The associations are based on knowledge of the inductances designed to be provided by bondwire connections to the various selectable bondpads  46 . For example, continuing the above example, selectable bondpads  46   1 ,  46   2 ,  46   3 ,  46   4 , and  46   5  can be associated with actual capacitances C nom-act  in the ranges 0.9-0.94C nom-des , 0.94-0.975C nom-des , 0.975-1.025C nom-des , 1.025-1.05C nom-des , and 1.05-1.1C nom-des , respectively. 
     Referring to FIG. 4, a wafer  50  includes many semiconducting dies or chips  52 , including operational chips  54  and test chips  56 . The operational chips  54  may include the PLL circuit  10  (FIG. 1) that includes the VCO  40  (FIG.  3 ). The test chips  56  each include test circuitry for use in determining indicia of the actual nominal varactor capacitance C nom-act  of the operational chips  54  relative to the designed capacitance C nom-des . The test circuitry includes at least one varactor, preferably with similar design to the VCO varactors of the operational chips  54  neighboring each test chip  56 . Test pads are coupled to the test chip varactors that can be probed with DC probes attached to test equipment for determining the actual capacitances C nom-act  of the varactors on the test chips  56 . These capacitances serve as indications of the manufacturing variance and thus of the actual varactor capacitances C nom-act  for operational chips  54  in the area of respective test chips  56 . As the actual capacitance C nom-act  can be different for similarly-designed varactors in different areas of the wafer  50 , the test chips  56  are disposed strategically about the wafer  50  to help accurately determine the variance in the actual nominal varactor capacitance C nom-act  for each of the operational chips  54 . 
     Referring also to FIG. 5, a bondwire machine  60  for determining which selectable bondpad  46  to bond to and for connecting the common bondpad  44  to a selectable bondpad  46  includes a controller  62 , a wiring device  64 , and a prober  66 . The controller  62  is a computer such as a personal computer and includes a central processing unit (CPU)  68  and memory  70 . The memory  70  is coupled to the CPU  68  and may include, e.g., random-access memory (RAM), read-only memory (ROM) hard and/or floppy disc drives. Stored in the memory  70  are software instructions that when executed by the CPU  68  may cause the CPU  68  to instruct or otherwise cause the wiring device  64  and the prober  66  to perform various operations. For example, the wiring device is configured to, under control of the controller  62 , connect bondpads  44  and  46  with a bondwire. The prober  66  is configured to help determine indicia of actual capacitances of varactors of the test chips  56  of the wafer  50 . 
     In at least one embodiment, to help determine indicia of actual nominal capacitances of varactors on the wafer  50 , the prober  66  includes probes  68  and a DC power supply  72 . The probes are disposed and configured to contact test pads coupled to the varactors of the test chips  56  when the prober  66  (or a probe card connected to the prober  66 ) is moved, under control of the controller  62 , into contact with the wafer  50 . The power supply  72  can supply DC power to the varactors, and the CPU  68  (or a processor in the prober  66 ) can use response signals received through the probes  68  to determine the actual capacitances of the varactors of the test chips  56 . 
     The CPU  68  may use a difference between the actual nominal capacitances C nom-act  and the designed nominal capacitances C nom-des  of the test chip varactors to select a appropriate selectable bondpads  46  and to control the bondwire device  64  to connect the common bondpad  44  to the selected bondpads  46 . The CPU  68  may access a table or database stored in the memory  70  relating the designed/actual capacitance differences with selectable bondpads  46 . The relation can be of indicia other than this capacitance difference. For example, the relations could be of just the actual nominal capacitance C nom-act . The CPU  68  uses the associated bondpad  46  for all chips  54  in a predetermined area of the test chip  56 , which may be different for different test chips  56  and therefore for different operational chips  54  in different areas of the wafer  50 . Instructions may be issued by the CPU  68  to cause the bondwire device  64  to wire bond the common bondpad  44  to the appropriate selectable bondpad  46  for each operational chip  54 . 
     Referring to  6 , with further reference to FIGS. 1-5, a process  80  of manufacturing the PLL  10  includes stages  82 ,  84 ,  86 , and  88  as shown. The stages and order of the stages shown are exemplary only and not meant to be limiting. Stages may be added, removed, or rearranged without departing from the scope of the invention. 
     At stage  82 , the wafer  50  is manufactured with the operational chips  54  and the test chips  56 . The operational chips  54  lack at least connections between the common bondpad  44  and the selectable bondpads  46 . 
     At stage  84 , differences between the actual nominal capacitances C nom-act  and the designed nominal capacitances C nom-des  of the varactors of the test chips  56  are determined. As shown, the difference may be a ratio, although other ways of determining differences/relationships are possible, such as by subtracting. The CPU  68  executes instructions and causes the prober  66  to cause the probes  68  to contact test pads connected to the test chip varactors. Under control of the CPU  68 , the prober DC reverse biases the varactors to the nominal value. Responses to the nominal biases are used by the CPU  68  to determine indicia of the actual nominal capacitances C nom-act . The actual nominal capacitances C nom-act  are compared to the designed nominal capacitance C nom-des  and the relationship between the actual capacitances C nom-act  and the desired/designed capacitances C nom-des  are determined for the test chip varactors. 
     At stage  86 , the CPU  68  associates the differences between actual and designed nominal capacitances to selectable bondpads  46 . The CPU  68  accesses the memory  70  for stored relationships between capacitance differences and selectable bondpads  46 . For each test chip  56 , the CPU  68  determines which selectable bondpad  46  to use for operational clips  54  associated with each test chip  56 . 
     At stage  88 , the CPU  68  controls the bondwire device  64  to wire bond the common bondpad  44  to the selected bondpad  46  for each operational chip  54 . The CPU  68  causes the device  64  to produce the bondwire  48  connecting the common bondpad  44  to the selectable bondpad  46  that is appropriate for that operational chip  54 . The appropriate bondpad  46  is potentially different from chip  54  to chip  54  based on the area of the wafer  50 . 
     Other embodiments are within the scope and spirit of the appended claims. For example, bondwire connections can be made to more than one selectable bondpad  46  (FIG.  2 ). Bondwire connections can be made from the common bondpad  44  to multiple selectable bondpads  46 . This can be used, e.g., to provide finer resolution than connecting to a single selectable bondpad  46  only. Referring to FIGS. 3-5, the bondwire machine  60  can be programmed directly as to which selectable bondpad  46  to connect to the common bondpad  44 . This could be done by a person that measures the actual capacitances and programs the controller  62  based upon know relationships between actual capacitance and desired selectable bondpads  46 . This technique could be used instead of providing the actual capacitance of a varactor on a test chip  54  through the prober  66  and having the bondwire machine  60  determine which selectable bondpad  46  to use. Also, the length of the bondwire between two bondpads could be varied, while ensuring that the bondwire fits inside the chip package, to adjust the inductance. Still other embodiments are possible, such as those shown in FIGS. 7 and 8. 
     Referring to FIG. 7, a VCO  90  includes the active device  22 , the varactor  28 , and an inductance arrangement  92 . The arrangement  92  includes a common bondpad  94 , selectable bondpads  96   1 - 96   5 , a primary inductor  98 , and secondary inductors  100   1 - 100   4  coupled to respective selectable bondpads  96   1 - 96   4 . The primary and secondary inductors  98  and  100  are coil inductors and may be formed at different layers in the wafer  50 . An inductance L p  of the primary inductor  98  is designed to, when combined with an inductance of a bondwire  102  connected to the bondpad  96   5  be used with the varactor  28  if the actual nominal varactor capacitance C nom-act  is at a lower extreme of the tolerance range relative to the designed capacitance C nom-des  (e.g., 90% of C nom-des ). The secondary inductors  100   1 ,  100   2 ,  100   3  and  100   4  are each designed to, when combined with an inductance of a bondwire  102  connected to the bondpads  96   1 ,  96   2 ,  96   3 , and  96   4 , respectively, be used with the varactor  28  if the actual nominal varactor capacitance C nom-act  differs from the designed capacitance C nom-des  by corresponding ranges. The range corresponding to pad  96   1  is at the upper end of the tolerance range. The inductance of the secondary inductor  100   3  coupled to bondpad  96   3  is designed to be used, in combination with the primary inductor  98  and the bondwire inductance, with the varactor  28  if the varactor&#39;s actual capacitance C nom-act  is within the acceptable range of actual nominal capacitances. Depending on the capacitances and inductances used, the primary inductor  98  may be eliminated. 
     Referring to FIG. 8, a system  108  includes an IC chip package  110 , a die or chip  112  that includes a varactor and active device (not shown) for a VCO, and an inductance arrangement  114 . The package  110  may be a printed circuit board (PCB). The inductance arrangement  114  is similar to the inductance arrangement  92 , except that the arrangement  114  does not include a primary inductor (although a primary inductor could be used), and the inductances of inductors  116   1 - 116   4  are designed accordingly. As shown, the arrangement is disposed off-chip (i.e., off of the chip  112 ). Coil inductors may be used for the inductors  116   1 - 116   4 . 
     Still further embodiments are within the scope and spirit of the invention and the appended claims. For embodiments with coil inductors (e.g., as shown in FIGS.  7  and  8 ), the inductors are trimmed or otherwise modified to be precise. Also, for such embodiments the selectable bondpads can be disposed equidistant from the common bondpad. Also, for arrangements with inductors in addition to the bondwire inductance, the inductances of the primary inductor (if any) and the secondary inductors are factored into the determination of which selectable bondpad to use. 
     While various embodiments of the application have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention. Accordingly, the invention is not to be restricted except in light of the appended claims and their equivalents.