Abstract:
Programmable filters are used for the purpose of changing the filter cutoff frequency as may be necessary for the operation of a wireless transmitter or receiver. Frequencies may be changed by selecting a desirable value of a capacitor and/or a resistor. The programmable filter controls the frequency according to the disclosed method. Furthermore, in order to reduce the area consumed by the programmable filter a three-dimensional layout is used. In accordance with the disclosed invention it is possible to program the input of the programmable filter to have a higher or lower input resistance as may be required while maintaining the desired programmed cutoff frequency by switching the respective capacitors in a capacitor bank, thereby combining the elements needed for frequency programmability and input impedance level selection.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to programmable filters, and more specifically relates to programmable cut-off or center frequency filters. 
         [0003]    2. Prior Art 
         [0004]    The use of operational amplifiers for the purpose of creating a variety of low-pass, high-pass and band-pass filters is well known in the art. Referring to  FIG. 1 , there is shown a simple integrator  100 , also known in the art as the Miller integrator. The integrator is comprised of an operational amplifier  110 , a resistor  120  connected to the inverting input of operational amplifier  110 , and a capacitor  130  providing a feedback path from the output to the inverting input of operational amplifier  110 . The characteristics of integrator  100 , i.e., the specific corner frequency of integrator  100 , depend on the values of resistor  120  and capacitor  130 . 
         [0005]    For the purpose of providing a plurality of corner frequencies, it is customary to connect one or more additional capacitors, each in series with a switch, to enable capacitor connections in parallel to capacitor  130 . Such a modified Miller integrator  200  is shown in  FIG. 2 . As a result, by switching switch  235  to a connecting position, capacitor  230 -B is connected in parallel to capacitor  230 -A. It is well known in the art that the total capacitance of capacitors connected in parallel is the sum of the capacitance of each of capacitors  230 -A and  230 -B. In another prior art embodiment, frequencies are adjusted by the use of connecting additional resistors through switches, such as shown in  FIG. 3 . In some embodiments, and in particular MOSFET-C filters, the ohmic resistors are implemented by means of MOS transistors. These provide for both the ohmic resistance and a switch in a single device. In the case where the capacitor bank is implemented using metal-insulator-metal implementation, the overall area of the filters is significantly impacted by the combined areas of the operational amplifiers and the capacitor bank, as shown schematically in  FIG. 8 . Specifically, capacitor bank  810  occupies one area of the layout and the operational amplifiers  820  occupy another area of the layout, while the MOS portion  830 , which may further contain the MOS resistors, occupy a third area of the layout. Prior art solutions are deficient in providing a constant cutoff frequency of the programmable filter when there is a need to change the input resistance, for example for the purpose of controlling the signal-to-noise ratio. Specifically noted are “Adaptive analog IF signal processor for a wide-band CMOS wireless receiver,” by Behbahani et al., IEEE Journal of Solid-State Circuits, vol. 36, pp. 1205-1217, August 2001 (hereinafter “Behbahani”), and “Dynamically power-optimized channel-select filter for zero-IF GSM”, by Ozgun et al., Digest, IEEE International Solid-State Circuits Conference, pp. 504, 505 and 613, San Francisco, February 2005 (hereinafter “Ozgun”. 
         [0006]    It would therefore be advantageous to provide a programmable filter configured to enable frequency programmability, while at the same time achieving a desired input resistance, and it would be further desirable if the elements required for these two purposes could be combined. It would be further advantageous if it would be possible to reduce the area occupied by such a filter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic diagram of a Miller integrator used for a filter application (prior art). 
           [0008]      FIG. 2  is a schematic diagram of a Miller integrator of a programmable filter having a plurality of capacitors connected in parallel by means of switches (prior art). 
           [0009]      FIG. 3  is a schematic diagram of a Miller integrator of a programmable filter having a plurality of capacitors connected in parallel by means of switches and a plurality of resistors connected in parallel by means of switches (prior art). 
           [0010]      FIG. 4A  is a schematic drawing of a first type of a capacitor bank in accordance with the disclosed invention. 
           [0011]      FIG. 4B  is a schematic drawing of a second type of a capacitor bank in accordance with the disclosed invention. 
           [0012]      FIG. 5A  is a schematic drawing of a first type of a resistor bank in accordance with the disclosed invention. 
           [0013]      FIG. 5B  is a schematic drawing of a second type of a resistor bank in accordance with the disclosed invention. 
           [0014]      FIG. 5C  is a schematic drawing of a resistor bank in accordance with the disclosed invention using MOS devices for combined switches and resistors. 
           [0015]      FIG. 6  is a schematic diagram of a Miller integrator of a programmable filter designed in accordance with the disclosed invention. 
           [0016]      FIG. 7A  shows an exemplary capacitor bank switch table operable in accordance with the disclosed invention for a nominal input resistance. 
           [0017]      FIG. 7B  shows an exemplary capacitor bank switch table operable in accordance with the disclosed invention for an increased input resistance. 
           [0018]      FIG. 8  is a top-level view of the layout of a filter having a plurality of operational amplifiers and capacitor banks (prior art). 
           [0019]      FIG. 9  is a top level-view of the layout of a filter having a plurality of operational amplifiers and a capacitor bank laid out in accordance with the disclosed invention. 
           [0020]      FIG. 10  is a layout of a chip having a programmable filter laid out in accordance with the disclosed invention. 
           [0021]      FIG. 11  is a cross section illustrating the lower level interconnect metal layers and the two-upper metal layers forming the capacitor bank. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    For the purpose of overcoming the deficiencies of the prior art, a plurality of capacitors are used in a capacitor bank in a negative feedback loop of an amplification means, for example an operational amplifier, each capacitor of the capacitor bank being capable of being either connected or disconnected to the feedback loop by means of a respective switch. In addition, a resistor bank is connected to the inverting input of the amplification means of the filter, each resistor being capable of being connected or disconnected from the circuit input by means of a respective switch. In one embodiment of the disclosed invention all but one of the capacitors and all but one of the resistors are connected to a switch while one capacitor and one resistor are permanently connected. All switches are connected to the virtual ground of the op amp, thus minimizing variations of the gate-source and/or gate-drain voltages, which would otherwise cause a variation in switch resistance and would result in signal distortion. The switches are operative under control means, the control means being capable of programming the cutoff frequency while providing the desired input resistance of the filter. The setting of the input resistance may be necessary, for example, for the purpose of controlling signal-to-noise ratio and impact thereof. Therefore the disclosed invention enables frequency programmability and impedance level selection. In another embodiment of the disclosed invention all capacitors and all resistors are connected in series with a switch. A control unit is used to ensure that at least one capacitor and at least one resistor are always connected. 
         [0023]    In accordance with the disclosed invention, a capacitor bank, such as one of the capacitor banks  400 A and  400 B shown in  FIGS. 4A and 4B  respectively, is used in the negative feedback loop. Capacitor bank  400 A comprises a plurality of capacitors capable of parallel connection by means of respective switches. In the exemplary and non-limiting  FIG. 4A , there are shown four capacitors  420 ,  421 ,  422 , and  423 , each having a respective switch  410 ,  411 ,  412 , and  413 . Each of the plurality of switches may be turned ‘on’, i.e., in a conducting position, or ‘off’, i.e., in a non-conducting position, independently of any other of the plurality of switches of capacitor bank  400 A. In another embodiment of the disclosed invention, and as shown in more detail below, the capacitor bank  400 B shown with respect to  FIG. 4B  is used in the feedback loop. In comparison to the exemplary embodiment  400 A, a switch, for example switch  410 , may be permanently in the “on” position, or replaced by a shunt, as in  400 B. Furthermore, in accordance with the disclosed invention, a resistor bank, such as one of the resistor banks  500 A or  500 B shown in  FIGS. 5A and 5B  respectively, and used to connect the input of the filter to the amplification means. A person skilled-in-the-art would note that for a MOSFET-C filter implementation, the ohmic resistors, for example resistors  520  and  521 , are replaced by MOS transistors, for example MOS transistors  570  and  571  shown in  FIG. 5C , which further provide the respective switching means equivalent to switches  510  and  511 , through the control of the gate voltage, for example at gates  560  and  561 , of each of the MOS transistors. In one embodiment of the disclosed invention a MOS resistor, for example MOS resistor  570 , may be permanently “on”, by providing the desired voltage at gate  560 . It is to be understood that, although the circuits are shown here unbalanced, a fully-balanced configuration would be required in order to cancel MOSFET nonlinearities in a MOSFET-C configuration. Such fully-balanced circuits maybe found in U.S. Pat. No. 7,049,875 entitled “One-pin automatic tuning of MOSFET resistors”, assigned to common assignee and which is hereby incorporated by reference for all the useful information it may contain. 
         [0024]    Reference is now made to  FIG. 6  where an exemplary and non-limiting schematic diagram  600  of a Miller integrator of a programmable filter is shown. Amplification means is implemented by using an operational amplifier  610  having the non-inverting input grounded. The output of operational amplifier  610  is connected to the inverting input of operational amplifier  610  by means of capacitor bank  400 , where one capacitor is permanently connected in the feedback loop. The input of the programmable filter is connected to the inverting input of operational amplifier  610  by means of resistor bank  500 , where one resistor is permanently connected in the input path. The switches of both capacitor bank  400 , regardless whether for example  400 A or  400 B, and resistor bank  500 , regardless whether for example  500 A,  500 B or  500 C, are controlled, for example, by a control unit  620 . The specific operation of control unit  620  is explained in more detail below. It should be further noted that if capacitor bank of the type  400 A and/or a resistor bank of type  500 A are used, it is essential to ensure that a conducting path exists for at least a capacitor and/or at least a resistor, that is provided by means of control unit  620 . Specifically, in a preferred embodiment control unit  620  is enabled to ensure that at least a capacitor of capacitor bank  400 A is connected in the feedback loop, and/or at least a resistor of resistor bank  500 A is connected in the input path. 
         [0025]    Merely for the purpose of illustration, a particular application of the disclosed invention is presented. However, this example should not be construed as limiting the scope of the disclosed invention. A person skilled-in-the-art would readily note that the characteristic frequencies (cutoff frequencies for low-pass or high-pass filters, or center frequencies for bandpass filters) of active RC filters are inversely proportional to resistance-capacitance products. Assume three distinct cutoff frequencies are required from programmable filter  600 , for example 2.5, 5 and 10 MHz. Capacitor bank  400 B is comprised of a group of capacitors having respective exemplary values of C/2, C/2, C, and 2C, for capacitors  420 ,  421 ,  422 , and  423  respectively, as shown for the capacitor bank  400 B in  FIG. 4B  as may be used in the circuit of  FIG. 6 . Such binary weighting is discussed in “A segmented u-255 law PCM voice encoder utilizing NMOS technology”, by Tsividis et al., IEEE Journal of Solid State Circuits, vol. SC-11, no. 6, pp. 740-747, December 1976. For a known input resistance, discussed in more detail below, to achieve a 2.5 MHz filter the switches are closed to create a total capacitance of 4C; for a 5 MHz filter the 2C capacitance is used; and, for a 10 MHz capacitance the C capacitance is used. Now, for the purpose of achieving the goals of the disclosed invention, described hereinabove, resistor bank  500  is comprised of a plurality of resistors connected in parallel, for example resistors  520  and  521  of  FIG. 5A  as used in  FIG. 6 , each having a resistive value of R, and where resistor  520  is permanently connected in the input path. As a result, in this case, two distinct impedance levels are available: R and R/2. The value of R/2 is used for the nominal impedance level, while the resistance R may be used to support a higher input resistance. Hence, in the example above, the given resistance value is for the nominal impedance. If the higher input resistance is desired, then switching of the switches of both resistor bank  500  and capacitor bank  400  are required, performed, for example, under the control of control unit  600 . For merely illustrative purposes, the following example is now provided for a programmable filter having a cutoff frequency of 2.5 MHz. For nominal operation switch  511  is closed to achieve an effective input resistance of R/2. The capacitance required is  4 C and therefore switches  411 ,  412 , and  413  are all closed, causing the four capacitors  420  through  423  to be connected in parallel. To switch to the higher input resistance mode of operation, the switch  511  must be opened, thereby increasing the input resistance to a value of R. It is further necessary to reduce the capacitance in the feedback loop in order to maintain the desired cutoff frequency, i.e.., changing the effective capacitance of capacitor bank  400  from 4C to 2C, thereby maintaining the same resistance-capacitance product. To achieve this, switches  411  and  412  are kept in the closed position while switch  413  is caused to be in the open position, leaving an effective capacitance of 2C comprising of C/2, C/2 and C capacitors connected in parallel in the feedback loop. 
         [0026]    In another illustrative example the target frequency cutoff is 10 MHz, and hence for nominal input resistance the switches of resistor bank  500  are configured to provide an input resistance of R/2 (i.e., one switch in the closed position). The cutoff frequency of a 10 MHz filter requires a capacitance of C, which can be achieved in accordance with the disclosed invention by having switch  411  in the closed position such that capacitors  420  and  421  are connected in parallel, and as each has a value of C/2, the total feedback capacity is C. When it is desired to move to the higher input resistance mode the input resistance is increased by switching switch  511  of the resistor bank  500  to the open position, thereby causing the input resistance to increase to R. In order to maintain the desired cutoff frequency of 10 MHz, the capacitance must be reduced by half, which may be achieved by opening the closed switch  421  of capacitor bank  400 . As a result there is now only a single capacitor in the feedback loop, namely capacitor  420 , with a value of C/2. 
         [0027]    The above examples show only the portion of the Miller integrator, implemented in accordance with the disclosed invention, rather than a full filter, for clarity purposes, and should not be viewed as limiting the disclosed invention. Control unit  620  may be further configured to be operative in response to a signal-to-noise measurement that may require the increase or decrease of the input resistance of the filter without changing the cutoff frequency, as discussed in Behbahani and in Ozgun. 
         [0028]    A person skilled-in-the-art would readily realize that the disclosed invention may be generalized for more cutoff frequencies and input resistances to the Miller integrator of a programmable filter. Hence, capacitor bank  400  should be viewed as being comprised of a plurality of capacitors and respective switches, and resistor bank  500  should be viewed as being comprised of a plurality of resistors and respective switches. It should be further noted that these are not limited to binary-weighted (power of two) values and in fact other, more complex combinations may be materialized, and are specifically included herein as part of the disclosed invention. A person-skilled-in-the art would further note that in the circuit shown herein the plurality of resistors, for example resistors  520  and  521 , of resistor bank  500  may be used to further determine the specific cutoff frequency of the programmable filter. Furthermore, the circuit may be used in other analog circuits that benefit from the ability to maintain a cutoff frequency while changing the desired input resistance. The amplifier shown is for illustration purposes only and a plurality of amplifiers may also be used without departing from the spirit of the disclosed invention. 
         [0029]    In  FIG. 8 , a traditional layout of the filter is shown. A common problem of programmable filters is the need to have a plurality of metal-insulator-metal (MIM) based capacitors that occupy significant layout area. In order to overcome the deficiency thereof a method of layout is used to overcome this problem, as shown in  FIG. 9 . Firstly, the wiring (interconnect) internal to the operational amplifier circuits, for example operational amplifier  610  shown in  FIG. 6 , employ only bottom-level metals, ensuring that at least two higher levels of metals are still available above the inter routing of the operational amplifiers. Secondly, the capacitor bank  815 , respective of, for example, capacitor bank  400 , is placed over the area of operational amplifiers  825 , respective of, for example, operational amplifier  610 , and as further shown in  FIG. 9 , showing the programmable filter  900  implemented in accordance with the disclosed invention. As a result, significant chip area is saved. Comparison of a traditional channel select filter layout resulted in an area of 2.05 mm 2  in comparison to an area of 1.14 mm 2  using the “3-D” layout approach disclosed herein. Measurement results on the 3-D structure did not reveal any deviation from the traditional approach employed for the same filter. Specifically,  FIG. 10  shows a layout of a chip with a programmable filter  900  laid out in accordance with the disclosed invention.  FIG. 11  illustrates a cross section  1100  showing the lower level interconnect metal layers, for example metal layer  1130 , and the two patterned upper metal layers  1110  and  1120  separated by a deposited dielectric layer  1140 , together forming the capacitor bank. Below lower level interconnect metal layers there resides the active area  1150  in which the MOS transistors, for example those forming the operational amplifiers  825 , are shown. For simplicity of the illustration the details of the MOS transistors that form the operational amplifiers are not shown. 
         [0030]    While certain preferred embodiments of the present invention have been disclosed and described herein for purposes of illustration and not for purposes of limitation, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.