Abstract:
An integrated digitally controlled linear-in-decibels attenuator circuit in which one or more sets of selection switches establish a desired attenuation by selectively connecting the input signal electrode to one or more corresponding resistive ladder networks connected in series, thereby providing a substantially more constant signal attenuation value over a wider frequency bandwidth. With a single resistive ladder network, attenuation control is achieved using a thermometer switching code. With multiple resistive ladder networks, coarse and fine attenuation control can be achieved using thermometer and bubble switching codes, respectively.

Description:
RELATED APPLICATIONS 
       [0001]    This is a continuation-in-part of application Ser. No. 11/500,024, filed Aug. 7, 2006, which is a continuation-in-part of application Ser. No. 11/160,210, filed Jun. 14, 2005. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to signal attenuation circuits, and in particular, to digitally controlled signal attenuation circuits. 
         [0004]    2. Description of the Related Art 
         [0005]    Digitally controlled attenuator circuits are well-known in the art. Such attenuator circuits are generally used in controlled impedance environments, and allow the attenuation to be controlled in units or fractions of decibels (dB). One particular type of such attenuator is referred to as a linear-in-dB attenuator, in which a thermometer code type of switching, or control, signal causes the attenuation to vary in single dB steps. 
         [0006]    Referring to  FIG. 1 , a conventional digitally controlled linear-in-dB attenuator includes a resistive ladder circuit with series resistances Rs 2 -Rs 7  and shunt resistances Rp 1 -Rp 7 , interconnected substantially as shown, to which the input voltage signal Vin is applied. The voltages at nodes N 1 -N 7  are applied to the throw electrodes of the single-pole, single-throw switch circuits S 1 -S 7 . The pole electrodes of these switches S 1 -S 7  are mutually connected to provide the output signal Vout. The switches S 1 -S 7  are controlled with a thermometer code control signal to selectively close the individual switches, depending upon the desired attenuation. (As one example embodiment, the series resistances Rs 2 -Rs 7  would have nominal resistance values of 109 ohms, while the shunt resistances Rp 1 -Rp 7  would have nominal resistances of 8170 ohms.) 
         [0007]    Referring to  FIG. 1A , a problem with such conventional attenuator circuits is the limited bandwidth caused by the circuit topology. As seen in  FIG. 1A , at or near a certain frequency Fc, the attenuation is no longer constant and begins to increase. This is due to the switch circuits S 1 -S 7 , which are typically implemented using metal oxide semiconductor field effect transistor (MOSFET) switches with low turn-on resistances. As is well-known in the art, such devices typically have relatively high parasitic capacitances at their drain and source electrodes. It is this parasitic capacitance that causes the bandwidth to be limited, thereby causing the attenuation characteristics to no longer be constant above a certain frequency Fc. Further, also as shown in  FIG. 1A , the bandwidth decreases as the attenuation increases. This is caused by the increased capacitance due to more of the switches S 1 -S 7  being in their off states. 
       SUMMARY OF THE INVENTION 
       [0008]    An integrated digitally controlled linear-in-decibels attenuator circuit in which one or more sets of selection switches establish a desired attenuation by selectively connecting the input signal electrode to one or more corresponding resistive ladder networks connected in series, thereby providing a substantially more constant signal attenuation value over a wider frequency bandwidth. With a single resistive ladder network, attenuation control is achieved using a thermometer switching code. With multiple resistive ladder networks, coarse and fine attenuation control can be achieved using thermometer and bubble switching codes, respectively. 
         [0009]    In accordance with one embodiment of the presently claimed invention, an integrated digitally controlled linear-in-decibels attenuator circuit includes: 
         [0010]    a plurality of attenuation control electrodes to convey a plurality of digital control signals corresponding to a signal attenuation value in accordance with a thermometer code; 
         [0011]    an input signal electrode to convey an input signal having a magnitude; an output signal electrode to convey an output signal corresponding to the input signal and having a magnitude which is less than the input signal magnitude in relation to the signal attenuation value; and 
         [0012]    a resistive network coupled between the input and output signal electrodes and responsive to the plurality of digital control signals by attenuating the input signal to provide the output signal. 
         [0013]    In accordance with another embodiment of the presently claimed invention, an integrated digitally controlled linear-in-decibels attenuator circuit includes: 
         [0014]    a first plurality of attenuation control electrodes to convey a first plurality of digital control signals corresponding to a first signal attenuation value in accordance with a thermometer code; 
         [0015]    a second plurality of attenuation control electrodes to convey a second plurality of digital control signals corresponding to a second signal attenuation value in accordance with a bubble code; 
         [0016]    an input signal electrode to convey an input signal having a magnitude; 
         [0017]    an intermediate signal electrode to convey an intermediate signal corresponding to the input signal and having a magnitude which is less than the input signal magnitude in relation to the first signal attenuation value; 
         [0018]    an output signal electrode to convey an output signal corresponding to the intermediate signal and having a magnitude which is less than the intermediate signal magnitude in relation to the second signal attenuation value; 
         [0019]    a first resistive ladder network coupled between the input and intermediate signal electrodes and responsive to the first plurality of digital control signals by attenuating the input signal to provide the intermediate signal; and 
         [0020]    a second resistive ladder network coupled between the intermediate and output signal electrodes and responsive to the second plurality of digital control signals by attenuating the intermediate signal to provide the output signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic diagram of a conventional digitally controlled linear-in-dB attenuator circuit. 
           [0022]      FIG. 1A  is a graph of attenuation versus frequency for the circuit of  FIG. 1 . 
           [0023]      FIG. 2  is a schematic diagram of a digitally controlled linear-in-dB attenuator circuit in accordance with one embodiment of the presently claimed invention. 
           [0024]      FIG. 2A  is a graph of attenuation versus frequency for the circuit of  FIG. 2 . 
           [0025]      FIG. 3  is a schematic diagram of one example of an implementation of a switch circuit for the attenuator circuit of  FIG. 2 . 
           [0026]      FIG. 4  is a block diagram of a digitally controlled linear-in-dB attenuator circuit in accordance with another embodiment of the presently claimed invention. 
           [0027]      FIG. 5  is a table of thermometer and bubble codes for attenuator control signals in accordance with one embodiment of the presently claimed invention. 
           [0028]      FIG. 6  is a graph of attenuation levels versus time for the attenuator circuit of  FIG. 4  with the attenuator control signals of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    The following detailed description is of example embodiments of the presently claimed invention with references to the accompanying drawings. Such description is intended to be illustrative and not limiting with respect to the scope of the present invention. Such embodiments are described in sufficient detail to enable one of ordinary skill in the art to practice the subject invention, and it will be understood that other embodiments may be practiced with some variations without departing from the spirit or scope of the subject invention. 
         [0030]    Throughout the present disclosure, absent a clear indication to the contrary from the context, it will be understood that individual circuit elements as described may be singular or plural in number. For example, the terms “circuit” and “circuitry” may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together (e.g., as one or more integrated circuit chips) to provide the described function. Additionally, the term “signal” may refer to one or more currents, one or more voltages, or a data signal. Within the drawings, like or related elements will have like or related alpha, numeric or alphanumeric designators. Further, while the present invention has been discussed in the context of implementations using discrete electronic circuitry (preferably in the form of one or more integrated circuit chips), the functions of any part of such circuitry may alternatively be implemented using one or more appropriately programmed processors, depending upon the signal frequencies or data rates to be processed. 
         [0031]    Referring to  FIG. 2 , an integrated digitally controlled linear-in-dB attenuator circuit in accordance with one embodiment of the presently claimed invention includes a resistive ladder circuit, with series resistances Rs 2 -Rs 7  and shunt resistances Rp 1 -Rp 7 , and single-pole, double- throw switch circuits S 1 -S 6 , all interconnected substantially as shown. (It will be understood by one of ordinary skill in the art that fewer or more series and shunt resistances and switch circuits can be used in accordance with the number of dB steps of attenuation desired.) The input signal Vin is applied to the series resistances Rs 2 -Rs 7  via resistance Rp 7 , and to resistances Rp 1 -Rp 6  via the switch circuits S 1 -S 6 . Accordingly, the output signal Vout is provided at the output of the resistive ladder circuit (e.g., as opposed to the mutually connected pole electrodes of the switch circuits S 1 -S 6 ). In conformance with Thevenin&#39;s Theorem, this circuit topology advantageously maintains a sufficient output impedance at the output node No since the pole electrodes of the switch circuits S 1 -S 6  are isolated from the output node No by the shunt Rp 1 -Rp 6  and series Rs 2 -Rs 6  resistances, and the throw electrodes are connected either to the low impedance input node Ni or to low impedance circuit ground GND, depending upon the desired signal attenuation. 
         [0032]    Referring to  FIG. 2A , as a result of this circuit topology, the signal attenuation remains more constant over a wider frequency bandwidth due to the isolation of the parasitic capacitances of the switch circuits S 1 -S 6  from the output node No. 
         [0033]    Referring to  FIG. 3 , an example embodiment of a switch circuit, e.g., the first switch circuit S 1 , includes pairs of N-type and P-type MOSFETs interconnected as transmission gates. For example, complementary pairs N 1 , P 1  and N 2 , P 2  of MOS transistors are interconnected with mutually coupled drain and source electrodes as shown. The incoming control signal drives the gate electrodes of transistors N 1  and P 2 , while the inverted control signal (inverted by an inverter circuit INV) drives the gate electrodes of transistors P 1  and N 2 . Hence, when the control signal is asserted high, the N 1 -P 1  transistor pair is turned on while the N 2 -P 2  transistor pair is turned off. Conversely, when the control signal is de-asserted low, transistor pair N 2 -P 2  is turned on while transistor pair N 1 -P 1  is turned off. Alternatively, instead of transmission gates, single transistors can be used as pass transistors. For example, transistors N 1  and P 2  can be used with transistors P 1  and N 2  omitted. 
         [0034]    Referring to  FIG. 4 , a digitally controlled linear-in-dB attenuator circuit  400  in accordance with another embodiment of the presently claimed invention includes at least two stages  200 ,  100  connected in series, with the first stage  200  being a circuit in conformance with  FIG. 2 , and the second stage  100  being a circuit in conformance with  FIG. 1  (with the output node No of  FIG. 2  connected to the input node N 7  of  FIG. 1 ). Accordingly, with the two stages  200 ,  100  implemented as the example circuits of  FIGS. 2 and 1 , the first stage  200  will have M=6 stages of resistive attenuators (as well as M=6 switches) for M steps of coarse adjustment, and the second stage  100  will have N=7 stages of resistive attenuators (as well as N=7 switches) for N steps of fine adjustment. This results in having M*N=42 possible adjustments while needing only M+N=13 stages of resistive attenuators (with M+N=13 switches), which is significantly less than M*N=42 stages of resistive attenuators (as well as M*N=42 switches) as required in a conventional linear-in-dB attenuator circuit. 
         [0035]    Based upon a reference resistance value Rref, preferred relative values of the resistances in the first stage  200  (Rs 2 -Rs 7  and Rp 1 -Rp 7 ) and second stage  100  (Rs 2 -Rs 7  and Rp 1 -Rp 7 ) are as follows (where a&lt;1 and k&lt;1): 
         [0036]    First Stage  200   
         [0000]      Rs2=Rs3=Rs4=Rs5=Rs6=Rs7=Rref/a-Rref 
         [0000]      Rp1=Rp2=Rp3=Rp4=Rp5=Rp6=Rp7=Rref/(1−a) 
         [0037]    Second Stage  100   
         [0000]      Rs2=Rs3=Rs4=Rs5=Rs6=Rs7=Rref 
         [0000]      Rp1=Rp2=Rp3=Rp4=Rp5=Rp6=Rp7=(Rref*k./(1−k))*((Rref*k/(1−k))+Rref)/Rref 
         [0038]    For example, with a reference resistance value of Rref=500, and a=0.9441 and k=0.7079, coarse and fine steps of 3 dB and 0.5 dB, respectively, can be realized. 
         [0039]    While the minimum attenuation of such an attenuator circuit  400  is equal to one coarse attenuation step and not zero (0 dB), due to resistors Rp 7  and Rs 2 -Rs 7  in the first stage  200  ( FIG. 2 ), it will be readily appreciated by one of ordinary skill in the art that such minimum signal loss can be compensated by the gain of an output buffer amplifier (not shown) following the second attenuator stage  100 . 
         [0040]    Referring to  FIG. 5 , in accordance with one embodiment of the presently claimed invention, the attenuator control signals, i.e., the switch control signals CONTROL ( FIGS. 1 and 2 ), for a digitally controlled linear-in-dB attenuator circuit  400  in accordance with  FIG. 4  would be as shown for a first stage  200  having M=5 stages of resistive attenuators and M=5 switches for M steps of coarse adjustment (i.e., switch S 6  and resistances Rp 6 , Rp 7  and Rs 7  are not used, and the input signal Vin is applied to resistance Rs 6 ), and a second stage  100  having N=7 stages of resistive attenuators and N=7 switches for N steps of fine adjustment. Also in accordance with the presently claimed invention, the first stage  200  provides coarse attenuation control in accordance with thermometer code, while the second stage  100  provides fine attenuation control in accordance with bubble code. 
         [0041]    In the case of the first stage  200  providing coarse attenuation control, an advantage to using thermometer code for such a R-2R resistive ladder network is the ability to provide linear-in-dB attenuation. This is in contrast to the use of binary code which would provide linear-in-voltage  control. 
         [0042]    Referring to  FIG. 6 , attenuation levels versus time are shown for the attenuator circuit of  FIG. 4  using the thermometer and bubble codes of  FIG. 5  for the attenuator control signals. 
         [0043]    Various other modifications and alternations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and the spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. It is intended that the following claims define the scope of the present invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.