Method and apparatus for tuning filters

A highly accurate tuning circuit for a tunable filter is provided which trims an RC time constant based on variances in both a formed capacitive component as well as variances in formed resistive components. A capacitor and resistor based tuning control circuit includes both a formed capacitor based tuning reference current generator and a formed resistor based tuning voltage reference generator. Each generates a voltage reference which is compared to the other to determine control signals for tuning a tunable resistive component forming the resistive portion of the RC time constant of the relevant filter. The resistive component is tuned by shorting selective resistors in the tunable resistive component. By tuning the RC time constant to the particular variances in capacitor and resistor components formed in the integrated circuit, the RC time constant of the tunable filter can be tuned to a desired absolute value within a tighter tolerance range than was previously available with conventional tuning circuits which provided tuning control based only on the variances of formed resistor elements.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates generally to tuned filters. More particularly, it
 relates to a tuning method and apparatus which provides high absolute
 accuracy in an RC time constant of an integrated circuit filter.
 2. Background of Related Art
 It is known that process variation in the tolerance of the absolute value
 of individual components formed in an integrated circuit (e.g., resistors
 and/or capacitors) can be quite great, but that similar components can be
 matched quite closely in value because it is likely that both will be
 affected equally by process and/or temperature variations. Thus,
 individual integrated circuit resistors can typically be manufactured to
 have a value only within a large range or tolerance.
 One method to overcome this problem is the use of matched components.
 However, in certain applications such as where the absolute value of an RC
 time constant is important, the tolerance in the resistance and/or
 capacitance value providing the RC time constant dictates the accuracy of
 any filter basing its operation thereon. The accuracy of an RC time
 constant and thus a filter based thereon can be improved significantly by
 in-circuit tuning of the resistance forming the RC time constant to
 compensate for fluctuations in process and/or temperature.
 A continuous time filter is a traditional technique, useful for removing
 high frequency out-of-band power of a signal (e.g., above 2 times the
 Nyquist rate). A continuous time filter is particularly useful in a
 circuit including sampling by an analog-to-digital (A/D) converter to
 provide higher accuracy in the samples. The present inventors desired to
 provide a more accurately tuned continuous time filter than those
 presently available using resistor based tuning.
 Some applications, e.g., a central office codec used to digitize telephone
 speech, will use a switched capacitor filter switched at a high speed,
 e.g., 1 megahertz (MHz) to achieve a desired band pass (e.g., 4 kilohertz
 (kHz) band pass) together with a continuous time filter (e.g., a smoothing
 filter) having a wide tolerance due to the large tolerance of certain
 components such as resistors and/or capacitors forming an RC time constant
 therein. Such applications typically use both a band pass filter and a
 continuous time filter having low tolerance requirements, e.g., in the
 neighborhood of 35 kHz.
 FIGS. 6 to 10 show a conventional technique for tuning the resistive
 portion of an RC time constant in a continuous time filter based on the
 variances of formed resistors only.
 In particular, FIG. 6 shows a block diagram of an embodiment of a
 conventionally tuned continuous time filter operating on an input signal
 410 to provide an output signal 420. A filter 600, e.g., a continuous time
 filter, filters an input signal 410 to provide an appropriately filtered
 output signal 420. The response of the filter 600 is controlled by one or
 more RC time constant(s). To provide the desired accuracy, the filter 600
 is tuned by hand with a trimmed current source resistor tuning control
 circuit 400.
 FIG. 7 is a schematic depiction of an embodiment of the conventionally
 tuned filter 600 shown in FIG. 6.
 In particular, the trimmed current source resistor tuning control circuit
 400 includes a trimmed current portion 502, and a comparators and latches
 portion 520. A relevant segment of the filter 600 is also shown in FIG. 7,
 as are the input signal 410 and the output signal 420.
 The trimmed current portion 502 of the trimmed current source resistor
 tuning control circuit 400 includes a current mirror formed by two
 p-channel metal oxide semiconductor field effect transistors (PMOSFETs)
 532, 534. A first side of the current mirror is trimmed with a current
 trimming element 536 (e.g., a variable resistor) to provide a desired
 current through the MOSFET 532. The current value set in this first side
 of the current mirror is then duplicated in the other side of the current
 mirror circuit, i.e., through MOSFET 534. The duplicated current is driven
 through a plurality of series connected resistors, e.g., four resistors
 504, 506, 508 and 510.
 Three comparators 522, 524 and 526 are fed on their respective positive
 inputs by nodes between each of the respective resistors 508 and 506, 506
 and 504, and above resistor 504. The negative input of each of the three
 comparators 522, 524 and 526 is tied to a desired reference voltage VREF.
 The reference voltage VREF may be either internally generated on the
 integrated circuit or externally provided to the trimmed current source
 resistor tuning control circuit 400 from a source external to the
 integrated circuit.
 The outputs of the comparators 522, 524 and 526 are respectively latched by
 latches 542, 544 and 546. The outputs of the comparators 522, 524 and 526
 control the switching in or out of individual resistor in a tunable
 resistor element in the filter 600.
 For example, tunable resistor components 613, 615 in the segment of the
 filter 600 shown in FIG. 7 are adjusted or `tuned` in accordance with the
 state of the outputs of the latches 542, 544 and 546. For instance, if the
 voltage reference VREF is at a level such that comparator 526 is saturated
 (i.e., the voltage level of the node between resistors 508 and 506 is
 greater than that of the voltage reference VREF), then latch 546 would
 have an active output thus turning on respective MOSFET switches 653a and
 653b in the tunable filter 600, and accordingly short resistor 634 in the
 first resistive component 613 and resistor 644 in the balanced resistive
 component 615. Accordingly, the resistive components 613 and 615 (which
 are the balanced resistive portions of an RC time constant in the filter
 600) are tuned within the allowable tolerance to the resistance of formed
 resistors 631-633 and 641-643, respectively, based on the performance of
 resistors 504-510 formed in the trimmed current portion 502.
 Similarly, if the resistors 504-510 in the trimmed current portion 502 are
 such that a voltage level between resistors 506 and 504 exceeds that of
 the voltage reference VREF, then latches 544 and 546 have active outputs
 to cause closure of MOSFET switches 653a, 653b, 652a and 652b, to tune the
 resistive components 613 and 615 to the values of resistors 631-632 and
 641-642, respectively. If the formed resistors 504-510 are such that the
 voltage level above the resistor 504 exceeds that of the voltage
 reference, then all three comparators 522-526 will become saturated and
 all three latches 542-546 will have active outputs when enabled, thus
 shorting all resistors except for resistor 631 in the first resistive
 element 613 and except for resistor 641 in the second resistive element
 615.
 For completeness, more detailed schematics of an embodiment of the
 conventional tunable continuous time filter shown in FIG. 6 are shown in
 FIGS. 8 to 10. In particular, FIG. 8 is a schematic diagram of the trimmed
 current portion 502 of the embodiment of the trimmed current source
 resistor tuning control circuit 400 shown in FIGS. 6 and 7. FIG. 9 is a
 schematic diagram of the comparators and latches portion 520 of the
 trimmed current source resistor tuning control circuit 400 shown in FIGS.
 6 to 8. FIG. 10 is a schematic diagram of the filter 600 shown in FIG. 6.
 Accordingly, FIGS. 6-10 show a conventional technique wherein a tunable
 resistor portion of an RC time constant in a filter 600 can be tuned based
 on formed resistors and a trimmed current. However, such conventional
 resistance-only based designs do not provide the desired precision or
 tolerance in the tuned filter, largely because variances in the other
 portion of the RC time constant, i.e., in the capacitive portions still
 leave a significant amount of error in the absolute value of the RC time
 constant. Moreover, many integrated circuit technologies do not allow for
 the manufacture of high density capacitors necessary for switched
 capacitor techniques.
 Untuned filters in these technologies would likely have +/-50% tolerance.
 Even techniques based only on resistor tuning can reduce the variation to
 only approximately +/-25%.
 Thus, there is a need for a tuning circuit for a continuous time filter
 which allows more accurate tuning of the RC time constant of a filter such
 that variances in both the resistor and the capacitor in the RC time
 constant are compensated for.
 SUMMARY OF THE INVENTION
 In accordance with the principles of the present invention, a control
 circuit for tuning a filter comprises a capacitor based tuning reference
 current generator, a resistor based tuning reference voltage generator,
 and a combining circuit to combine reference signals output from the
 capacitor based tuning reference generator and the resistor based tuning
 reference generator to provide at least one control signal to tune the
 filter.
 A method of tuning a filter in accordance with another aspect of the
 present invention comprises forming a capacitor based tuning reference
 generator including a first capacitance in an integrated circuit. A first
 resistance is formed in the integrated circuit. A second capacitance and a
 second resistance are formed in combination to form an RC combination. One
 of the second capacitance and the second resistance is tuned based on a
 signal from the first capacitance and a signal from the first resistance.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
 The present invention provides a highly accurate tuning circuit for a
 tunable filter which sets an RC time constant in the tunable filter based
 on the absolute value of a formed capacitive component as well as on the
 absolute values of formed resistive components. A capacitor and resistor
 based tuning control circuit includes both a formed capacitor based tuning
 voltage reference generator and a formed resistor based tuning voltage
 reference generator. The two resultant voltage references are compared to
 one another to determine control signals for switching in or out any of a
 plurality of individual resistors in a tunable resistive component forming
 the resistive portion of the RC time constant of the relevant tunable
 filter. By tuning the RC time constant based on variances in similarly
 formed capacitor and resistor components, the RC time constant of the
 tunable filter can be more accurately tuned to a desired absolute value
 within a tighter tolerance range than was previously available with
 conventional tuning circuits which provided a tuning signal based only on
 formed resistor elements or only on formed capacitive elements.
 Thus, the present invention provides a method and apparatus for
 compensating for process and temperature variations in capacitor formation
 in an integrated circuit as well as in resistor formation, as opposed to
 the conventional tuning techniques which compensated for variations in
 resistor formation or capacitor formation only. The present invention,
 while applicable in a wide variety of tunable circuits, has particular
 application in circuits wherein the absolute values of a resistance and
 capacitance are important, e.g., in a filter having an RC time constant.
 FIG. 1 shows a block diagram of an embodiment of a tunable filter tuned in
 accordance with the principles of the present invention.
 In particular, in FIG. 1, an input signal 410 is filtered in the base band
 by a tunable filter 120 using a capacitor and resistor based RC tuning
 control circuit 100. The tunable filter 120 is preferably a continuous
 time filter such as, e.g., an anti-aliasing filter useful in combination
 with sampling circuits.
 FIG. 2 shows a more detailed block diagram of the capacitor and resistor
 based RC tuning control circuit 100 shown in FIG. 1.
 In FIG. 2, the capacitor and resistor based RC tuning control circuit 100
 includes both a formed capacitor based tuning reference voltage generator
 217 and a formed resistor based tuning reference voltage generator 219.
 Respective reference voltages from both the formed capacitor based tuning
 reference voltage generator 217 and from the formed resistor based tuning
 reference voltage generator 219 combine to provide suitable tuning control
 signals for use by the tunable portion of an RC time constant in a
 relevant tunable filter. The respective reference voltages may be combined
 in any suitable fashion, e.g., in parallel using a comparison device, or
 in series by cumulatively combining the respective reference voltages such
 that one reference voltage generator 217 or 219 feeds an output to the
 other reference voltage generator 219 or 217.
 FIG. 3 is a schematical depiction of the formed capacitor based tuning
 reference voltage generator 217 shown in FIG. 2.
 In FIG. 3, a formed capacitor 106 is alternately charged to a reference
 voltage level VREF, and then discharged to ground (e.g., an analog
 ground), based on the cycles of a reference clock signal. One phase .PHI.1
 of a clock signal output from a non-overlapping reference clock generator
 102 controls a charging switch 102 (e.g., a MOSFET), while an opposite
 phase .PHI.2 (e.g., 180 degrees out of phase with the first phase .PHI.1)
 of the clock signal controls a discharge switch 104 (e.g., another
 MOSFET).
 The voltage reference VREF preferably has a +/-10% or greater if internally
 generated, or a +/-2% to +/-5% tolerance if generated external to the
 integrated circuit.
 The formed capacitor 106 provides a current I based on the product of the
 voltage reference VREF, the capacitance C of the formed capacitor 106, and
 the frequency of the clock signals .PHI.1 and .PHI.2. This current I is
 proportional to the absolute value of the formed capacitor 106.
 Preferably, the capacitor 106 is formed similarly to the capacitor forming
 the RC time constant of the tunable filter 120 such that any process
 and/or temperature variations are likely to affect all capacitors
 substantially equally.
 A smoothing filter 223 allows for a smooth current signal to be output from
 the formed capacitor based tuning reference voltage generator 217.
 FIG. 4 is a schematical depiction of the formed capacitor based tuning
 reference voltage generator 217 shown in FIG. 3 combined together with a
 formed resistor based tuning reference voltage generator 219 to provide
 control signals COUT1 to COUT4. The control signals COUT1 to COUT4 tune an
 RC time constant by selectably switching individual resistor elements in a
 tunable resistance component forming an RC time constant in a relevant
 filter.
 In particular, the formed capacitor based tuning reference voltage
 generator 217 provides a first compensation or reference voltage 398 based
 on the absolute value of the formed capacitor 106. This first reference
 voltage 398 is input to the positive inputs of a series of comparators
 486-489. The negative inputs for each of the respective comparators
 486-489 are connected to sequential nodes in a series combination of a
 plurality of formed resistor elements 304-312. One end of the series
 combination of the formed resistor elements 304-312 is tied to ground
 (e.g., to an analog ground) while the other end of the series combination
 of the formed resistor elements is provided with the reference voltage
 VREF.
 Preferably, the resistor elements 304-312 are formed substantially
 similarly to the resistor element relevant to the RC time constant in the
 tunable filter 120 (FIG. 1) such that any process and/or temperature
 variations will affect all resistors substantially equally.
 The output of the series of comparators 486-489 generates the control
 signals COUT1 to COUT4, which selectably short individual resistor
 elements in a tunable resistor relevant to an RC time constant in the
 tunable filter 120 (FIG. 1).
 FIG. 5 shows an embodiment of a tunable resistive element 413 forming the
 resistive component of an RC time constant in the tunable filter 120.
 In particular, the tuning control signals COUT1 to COUT4 provide switching
 control to shorting switches (e.g., MOSFETs) 566 to 569. As each switch
 566 to 569 is closed, the affected individual resistor 404a-404e is
 effectively removed from the total resistance of the tunable resistive
 element 413.
 The accuracy of the tuning of the RC time constant relates to several
 factors, e.g., the number of individual resistors in the resistive
 components forming the RC constant of the relevant continuous time filter,
 and to the accuracy of the clock. Thus, while the disclosed embodiments
 presume resistive components including five individual resistors each, the
 present invention relates equally to applications utilizing only four
 resistors 631-634 (and 641-644 in a balanced circuit) as shown with
 respect to the conventional techniques shown in FIGS. 6 to 10, to
 applications utilizing fewer than four individual resistors, and to
 applications utilizing greater than five individual resistors. Moreover,
 the clock signals .PHI.1 and .PHI.2 preferably have an appropriate
 accuracy of, e.g., less than 1000 parts per million (ppm). However, the
 particular accuracy desired to provide switching control for the capacitor
 106 will be based on the particular application.
 The present invention is capable of providing an RC time constant having an
 a tolerance in the absolute value of +/-15% or less, depending upon the
 number of individual resistors and corresponding shorting switches used in
 the tuning portion of the relevant filter.
 While the invention has been described with reference to the exemplary
 embodiments thereof, those skilled in the art will be able to make various
 modifications to the described embodiments of the invention without
 departing from the true spirit and scope of the invention.