BACKGROUND 
       [0001]    Electronic devices often incorporate controllable attenuation devices and/or components for varying the amount of resistance applied to electronic signals such as radio frequency (RF) signals. Such attenuators are used in, among other things, automatic gain control circuits, position locating systems, telephone systems, television systems, and microwave circuit applications. 
         [0002]    Some controllable attenuation devices such as monolithic microwave integrated circuit (MMIC) digital attenuators incorporate high frequency field effect transistors (FETs), such as gallium arsenide (GaAs) metal semiconductor FETs, arranged in a variety of network configurations (which may include other circuit elements, e.g., discrete resistor, among others). These devices operate by turning certain transistors on and off to adjust or select the desired attenuation. 
         [0003]    Digital attenuators vary the strength of input signals in response to digital control signals. In a typical 1-bit digital attenuator, the amount of attenuation offered by the attenuator varies depending on whether the bit of the control signal has a value of “0” (logic low) or “1” (logic high). Typically, if a 2-bit or other multiple-bit digital attenuator is desired, a plurality of 1-bit digital attenuators are cascaded according to known techniques to produce the desired m-bit digital attenuator (where m≧2). For example, if a 3-bit digital attenuator is desired, three of the 1-bit digital attenuators are cascaded to produce the 3-bit digital attenuator. 
         [0004]    Due to the cascading of bits, however, a reference insertion loss in conventional multi-bit digital attenuators tends to be high resulting in higher Voltage Standing Wave Ratio (VSWR), which represents the amount of reflected power. A high VSWR increases noise, which degrades system performance. Further, having multiple bits that are cascaded in the digital attenuator deteriorates the attenuation accuracy when multiple bits are switched on at the same time. 
         [0005]    For certain applications, it is desirable to have a constant phase over attenuation states. Usually, for lower bits and at lower frequencies, phase shift may be less and manageable using techniques such as cascading lower bits. At higher frequencies, however, conventional digital attenuators may experience significant phase difference between the on and off states. 
         [0006]    Accordingly, there exists the need for an improved digital attenuator to provide constant phase at higher frequencies. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  illustrates one embodiment of a constant phase digital attenuator circuit. 
           [0008]      FIG. 2  illustrates one embodiment of a multi-stage constant phase digital attenuator. 
           [0009]      FIG. 3  illustrates an on-chip layout of one embodiment of a multi-stage constant phase digital attenuator. 
       
    
    
     SUMMARY 
       [0010]    Various embodiments are directed to providing constant phase digital attenuation. In one embodiment, a digital attenuator circuit comprises an input node to receive an input signal to be attenuated, an output node to output an attenuated signal, a reference loss path between the input node and the output node, and an attenuation path between the input node and the output node. The reference loss path comprises switching elements and matching circuitry to improve Voltage Standing Wave Ratio (VSWR), and the attenuation path comprises switching elements and attenuating circuitry to attenuate the input signal when the digital attenuator circuit is switched from a reference loss state to an attenuation state. An effective phase length of the reference loss path and an effective phase length of the attenuation path may be equalized to provide a constant phase when the digital attenuator circuit is switched between states. 
         [0011]    In one embodiment, a multi-stage digital attenuator includes a plurality of digital attenuator circuits with each digital attenuator circuit comprising a stage of the multi-stage digital attenuator circuit. The multi-stage digital attenuator includes a plurality of interstage inductance elements implemented as high impedance transmission lines to match the output of a previous stage and the input of a next stage. The plurality of digital attenuator circuits forming the multi-stage digital attenuator may be implemented on a single chip. 
         [0012]    In one embodiment, a method includes providing a digital attenuator circuit on a chip. The digital attenuator circuit includes a reference loss path comprising switching elements and matching circuitry to improve VSWR and an attenuation path comprising switching elements and attenuating circuitry to attenuate an input signal when the digital attenuator circuit is switched from a reference loss state to an attenuation state. An effective phase length of the reference loss path and an effective phase length of the attenuation path may be equalized to provide a constant phase when the digital attenuator circuit is switched between states. A plurality of digital attenuator circuits may be provided on the chip to implement a multi-stage digital attenuator, and interstage inductance elements comprising high impedance transmission lines may be provided to couple digital attenuator circuits. 
       DETAILED DESCRIPTION 
       [0013]      FIG. 1  illustrates one embodiment of a constant phase digital attenuator circuit  100 . Digital attenuator circuit  100  comprises RF input node  102  and RF output node  104 . While it can be appreciated that digital attenuator circuit  100  comprises a symmetrical structure, it will be assumed for purposes of illustration that the left node comprises RF input node  102  and right node comprises RF output node  104 . The embodiments, however, are not limited in this regard. 
         [0014]    Each of RF input node  102  and RF output node  104  is connected to reference loss path  106  and attenuation path  108 . Digital attenuator circuit  100  may control the switching between reference loss path  106  and attenuation path  108  to achieve a desired amount of attenuation. In various embodiments, the desired amount of attenuation or loss corresponds to a difference in attenuation between reference loss path  106  and attenuation path  108 . Although it is desirable for reference loss path  106  to have very low insertion loss, zero insertion loss generally is not attainable. Because reference loss path  106  will always have some insertion loss, the actual amount of attenuation or loss (e.g., 1 dB, 2 dB, 4 dB, 8 dB, 16 dB) provided by digital attenuator circuit  100  is achieved by virtue of the difference in attenuation between reference loss path  106  and attenuation path  108 . 
         [0015]    As shown in  FIG. 1 , digital attenuator circuit  100  may implement a double pole double throw (DPDT) switching structure. Namely, input (e.g., left) side  110  and output (e.g., right) side  112  of digital attenuator circuit  100  together may form a DPDT switch. The top and bottom portions of input side  110  form a first single pole double throw (SPDT) switch. For example, at the input node  102 , a signal may be switched to either reference path  106  or to attenuation path  108 . Likewise, the top and bottom portions of output side  112  form a second SPDT switch. Accordingly, a DPDT switch is implemented by input side  110  and output side  112  of digital attenuator circuit  100 . 
         [0016]    Digital attenuator circuit  100  is designed such that reference path  106  and attenuation path  108  are switched by the DPDT switching structure. It can be appreciated that there are trade-offs in designing such a switching structure. In particular, digital attenuator circuit  100  must be designed to provide attenuation accuracy, phase balance, low insertion loss, and good VSWR. In addition, low VSWR must be maintained both for individual bits and for combinations of bits. In an exemplary embodiment, each switched element pair (reference loss path  106  and attenuation path  108 ) is designed to have 50 ohm (Ω) impedance to minimize impedance differences as bits are switched in and out. 
         [0017]    In various embodiments, digital attenuator circuit  100  is designed such that the phase length between reference loss path  106  and attenuation path  108  is equal. It is noted that having different effective path lengths for reference loss path  106  and attenuation path  108  may result in significant phase difference between the reference loss and attenuation states at higher frequencies. Because the effective phase lengths of reference loss path  106  and attenuation path  108  are equal, the phase length through the digital attenuator circuit  100  does not change when switching between states. For example, if the insertion phase of the attenuator is 17 degrees in the reference loss state, the phase length will be 17 degrees in the attenuation state for 8 dB attenuation. It is noted that the absolute value of the phase does not matter so long as it is equivalent through reference loss path  106  and attenuation path  108 . 
         [0018]    In various implementations, digital attenuator circuit  100  comprises a single stage to be cascaded with one or more other stages. In such implementations, excess shunt capacitance may exist at the common junctions of the DPDT switches, which will affect the phase accuracy when cascaded. Each stage having poor VSWR may exhibit reflections. While small reflections may have minimal affect on attenuation accuracy, even small reflections must be minimized to avoid creating phase errors. By eliminating reflections, good VSWR is achieved and phase accuracy is maintained for higher frequencies. This is especially important for higher frequencies such as 4 GHz. 
         [0019]    As shown in  FIG. 1 , digital attenuator circuit  100  comprises interstage inductance element  114 . Interstage inductance element  114  may be implemented at the common junction (e.g., RF output node  104 ) of cascaded DPDT switches. In various embodiments, interstage inductance element  114  may be used to couple stages or bits as well as to couple a stage or bit to the output or to the input. 
         [0020]    Interstage inductance element  114  may be designed to match out the parasitic reactance (e.g. excess shunt capacitance) between stages or bits at higher frequencies and minimize reflections. For example, the inductance provided by interstage inductance element  114  may be used to match the output of a previous stage and the input of the next stage. In an exemplary embodiment, interstage inductance element  114  may provide an inductance of 0.8 nH between stages. 
         [0021]    In various embodiments, interstage inductance element  114  may comprise a high impedance transmission line. In such embodiments, the high impedance transmission line may have a higher Q factor and lower insertion loss as compared to a standard spiral inductor. Accordingly, use of interstage inductance  114  may achieve wideband constant phase digital attenuation with low insertion loss. 
         [0022]    Interstage inductance element  114  (e.g., high impedance transmission line) also may be designed to maximize separation between stages to minimize coupling between stages. By avoiding coupling between stages or bits, phase interaction with the actual attenuator bits is minimized. 
         [0023]    In the embodiment of  FIG. 1 , digital attenuator circuit  100  is designed such that the effective phase lengths between reference loss path  106  and attenuation path  108  are equal. As shown, reference loss path  106  and attenuation path  108  of digital attenuator circuit  100  may be implemented by a network configuration of transistors and resistors. Each transistor may comprise, for example, a field effect transistor (FET) such as a junction FET (JFET), a metal-oxide semiconductor FET (MOSFET), a metal semiconductor FET (MESFET), a pseudomorphic high electron mobility transistor (PHEMT), or other type of suitable transistor in accordance with the described embodiments. The transistors may comprise various n-type or p-type semiconductor materials such as silicon, GaAs, and so forth. Each resistor may comprise, for example, a thin-film resistor or other suitable resistor in accordance with the described embodiments. 
         [0024]    Reference path  106  between input node  102  and output node  104  comprises first series FET  116 . The gate of first series FET  116  may be connected to biasing node  118  through resistor  120  and resistor  122 . The source and drain of first series FET  116  may be connected to resistor  124 . In one exemplary embodiment, resistor  120  may comprise a 2 kΩ resistor, resistor  122  may comprise a 10 kΩ resistor, and resistor  124  may comprise a 10 kΩ resistor. 
         [0025]    First series FET  116  is connected to cascaded first shunt FET  126  and second shunt FET  128 , which is connected to ground. The gate of first shunt FET  126  may be connected to biasing node  130  through resistor  132  and resistor  134 . The gate of second shunt FET  128  may be connected to biasing node  130  through resistor  132  and resistor  136 . In one exemplary embodiment, resistor  132  may comprise a 2 kΩ resistor, resistor  134  may comprise a 10 kΩ resistor, and resistor  136  may comprise a 10 kΩ resistor. In another embodiment, cascaded first shunt FET  126  and second shunt FET  128  may be replaced by a single dual gate shunt FET. 
         [0026]    First series FET  116  is also connected to shunt resistor  138  which is connected to ground. In various embodiments, shunt resistor  138  is designed to keep the VSWR on the reference path  106  as good (e.g., low) as possible and to minimize reflections. In one exemplary embodiment, shunt resistor  138  may comprise a 2011Ω resistor. 
         [0027]    Reference path  106  comprises second series FET  140  connected to shunt resistor  138 . The gate of second series FET  140  may be connected to biasing node  118  through resistor  120  and resistor  142 . The source and drain of second series FET  140  may be connected to resistor  144 . In one exemplary embodiment, resistor  120  may comprise a 2 kΩ resistor, resistor  142  may comprise a 10 kΩ resistor, and resistor  144  may comprise a 10 kΩ resistor. 
         [0028]    Second series FET  140  is connected to cascaded third shunt FET  146  and fourth shunt FET  148 , which is connected to ground. The gate of third shunt FET  146  may be connected to biasing node  130  through resistor  132  and resistor  150 . The gate of fourth shunt FET  148  may be connected to biasing node  130  through resistor  132  and resistor  152 . In one exemplary embodiment, resistor  132  may comprise a 2 kΩ resistor, resistor  150  may comprise a 10 kΩ resistor, and resistor  152  may comprise a 10 kΩ resistor. In another embodiment, cascaded third shunt FET  146  and fourth shunt FET  148  may be replaced by a single dual gate shunt FET. 
         [0029]    Attenuation path  108  between input node  102  and output node  104  comprises third series FET  154 . The gate of third series FET  154  may be connected to biasing node  156  through resistor  158  and resistor  160 . The source and drain of third series FET  154  may be connected to resistor  162 . In one exemplary embodiment, resistor  158  may comprise a 2 kΩ resistor, resistor  160  may comprise a 10 kΩ resistor, and resistor  162  may comprise a 10 kΩ resistor. 
         [0030]    Third series FET  154  is connected to cascaded fifth shunt FET  164  and sixth shunt FET  166 , which is connected to ground. The gate of fifth shunt FET  164  may be connected to biasing node  168  through resistor  170  and resistor  172 . The gate of sixth shunt FET  166  may be connected to biasing node  168  through resistor  170  and resistor  174 . In one exemplary embodiment, resistor  170  may comprise a 2 kΩ resistor, resistor  172  may comprise a 10 kΩ resistor, and resistor  174  may comprise a 10 kΩ resistor. In another embodiment, cascaded fifth shunt FET  164  and sixth shunt FET  166  may be replaced by a single dual gate shunt FET. 
         [0031]    Third series FET  154  is also connected to PI network  176  comprising shunt resistor  176 - 1  connected to ground, shunt resistor  176 - 2  connected to ground, and series resistor  176 - 3  connected to shunt resistor  176 - 1  and shunt resistor  176 - 2 . In various embodiments, shunt resistor  176 - 1 , shunt resistor  176 - 2 , and series resistor  176 - 3  are structured and arranged in accordance with the desired amount of attenuation or loss provided by digital attenuator circuit  100 . 
         [0032]    Attenuation path  108  comprises fourth series FET  178  connected to PI network  176 . The gate of fourth series FET  178  may be connected to biasing node  156  through resistor  158  and resistor  180 . The source and drain of fourth series FET may be connected to resistor  182 . In one exemplary embodiment, resistor  158  may comprise a 2 kΩ resistor, resistor  180  may comprise a 10 kΩ resistor, and resistor  182  may comprise a 10 kΩ resistor. 
         [0033]    Fourth series FET  178  is connected to cascaded seventh shunt FET  184  and eighth shunt FET  186 , which is connected to ground. The gate of seventh shunt FET  184  may be connected to biasing node  168  through resistor  170  and resistor  188 . The gate of eighth shunt FET  186  may be connected to biasing node  168  through resistor  170  and resistor  190 . In one exemplary embodiment, resistor  170  may comprise a 2 kΩ resistor, resistor  188  may comprise a 10 kΩ resistor, and resistor  190  may comprise a 10 kΩ resistor. In another embodiment, cascaded seventh shunt FET  184  and eighth shunt FET  186  may be replaced by a single dual gate shunt FET. 
         [0034]    In various embodiments, digital attenuator circuit  100  may vary the strength of input signals in response to voltages applied to the biasing nodes which turn on attenuation path  108  and turn off reference loss path  106  and vice versa. In operation, an RF signal applied at RF input node  102  of digital attenuator circuit  100  may be attenuated appropriately to generate an attenuated signal at RF output node  104  by applying voltages to the biasing nodes to turn certain FETs on and off for achieving a desired amount of attenuation or loss. For example, applying an approximately zero bias voltage to the gate of a FET places the FET in a low impedance or on state. Applying a negative bias voltage such as −5 V to the gate of a FET places the FET in a high impedance or off state. It is noted that the negative bias voltage may vary such as from −3 V to −8 V, for example. 
         [0035]    When digital attenuator circuit  100  is in the attenuation state, reference loss path  106  is off and attenuation path  108  is on. In references loss path  106 , a negative bias voltage (e.g., −5 V) may be applied to biasing node  118  so that first series FET  116  and second series FET  140  are off, and an approximately zero bias voltage may be applied to biasing node  130  so that first shunt FET  126 , second shunt FET  128 , third shunt FET  146 , and fourth shunt FET  148  are on. In attenuation path  108 , an approximately zero bias voltage may be applied to biasing node  156  so that third series FET  154  and fourth series FET  178  are on, and a negative bias voltage (e.g., −5 V) may be applied to biasing node  168  so that fifth shunt FET  164 , sixth shunt FET  166 , seventh shunt FET  184 , and eighth shunt FET  186  are off. 
         [0036]    To switch digital attenuator circuit  100  to the reference loss state, opposite biasing voltages may be applied to biasing nodes  118 ,  130 ,  156 , and  168 . When in the reference loss (low attenuation) state, reference loss path  106  is on and attenuation path  108  is off. In reference loss path  106 , first series FET  116  and second series FET  140  are on, first shunt FET  126 , second shunt FET  128 , third shunt FET  146 , and fourth shunt FET  148  are off, and shunt resistor  138  provides good VSWR. In attenuation path  108 , third series FET  154  and fourth series FET  178  are off, and fifth shunt FET  164 , sixth shunt FET  166 , seventh shunt FET  184 , and eighth shunt FET  186  are on. 
         [0037]    In one embodiment, digital attenuator circuit  100  may be designed to achieve 1 dB of attenuation. In an exemplary 1 dB circuit, shunt resistor  176 - 1  may comprise a 637.9Ω resistor, shunt resistor  176 - 2  may comprise a 637.9Ω resistor, and series resistor  176 - 3  may comprise a 5.0Ω resistor. 
         [0038]    In another embodiment, digital attenuator circuit  100  may be designed to achieve 2 dB of attenuation. In an exemplary 2 dB circuit, shunt resistor  176 - 1  may comprise a 301.4Ω resistor, shunt resistor  176 - 2  may comprise a 301.4Ω resistor, and series resistor  176 - 3  may comprise a 9.3Ω resistor. 
         [0039]    In another embodiment, digital attenuator circuit  100  may be designed to achieve 4 dB of attenuation. In an exemplary 4 dB bit circuit, shunt resistor  176 - 1  may comprise a 171.7Ω resistor, shunt resistor  176 - 2  may comprise a 171.7Ω resistor, and series resistor  176 - 3  may comprise a 20.8Ω resistor. 
         [0040]    In another embodiment, digital attenuator circuit  100  may be designed to achieve 8 dB of desired attenuation or loss. In an exemplary 8 dB bit circuit, shunt resistor  176 - 1  may comprise a 98.2Ω resistor, shunt resistor  176 - 2  may comprise a 98.2Ω resistor, and series resistor  176 - 3  may comprise a 46.4Ω resistor. 
         [0041]    In another embodiment, digital attenuator circuit  100  may be designed to achieve 16 dB of desired attenuation or loss. In an exemplary 16 dB bit circuit, shunt resistor  176 - 1  may comprise a 56.2Ω resistor, shunt resistor  176 - 2  may comprise a 56.2Ω resistor, and series resistor  176 - 3  may comprise a 129.2Ω resistor. 
         [0042]      FIG. 2  illustrates one embodiment of a multi-stage digital attenuator  200  comprising RF input node  202 , RF output node  204 , and attenuation stages  206 - 1  through  206 - 5 . As shown, multi-stage digital attenuator  200  may implement a 5 bits constant digital attenuator. It can be appreciated that multi-stage digital attenuator  200  may comprise or implement a greater or fewer number of stages in accordance with the described embodiments. 
         [0043]    In this embodiment, attenuation stage  206 - 1  is connected to attenuation stage  206 - 2  via interstage inductance element  208 - 1  arranged to match the output of attenuation stage  206 - 1  and the input of the attenuation stage  206 - 2 . Attenuation stage  206 - 2  is connected to attenuation stage  206 - 3  via interstage inductance element  208 - 2  arranged to match the output of attenuation stage  206 - 2  and the input of the attenuation stage  206 - 3 . Attenuation stage  206 - 3  is connected to attenuation stage  206 - 4  via interstage inductance element  208 - 3  arranged to match the output of attenuation stage  206 - 3  and the input of the attenuation stage  206 - 4 . Attenuation stage  206 - 4  is connected to attenuation stage  206 - 5  via interstage inductance element  208 - 4  arranged to match the output of attenuation stage  206 - 4  and the input of the attenuation stage  206 - 5 . 
         [0044]    Each of attenuation stages  206 - 1  through  206 - 5  may comprise or be implemented by digital attenuator circuit  100  of  FIG. 1 , and each of interstage inductance elements  208 - 1  through  208 - 4  may comprise or be implemented as a high impedance transmission line having a high Q factor and low insertion loss. Interstage inductance elements  208 - 1  through  208 - 4  may be designed to match out the parasitic reactance (e.g. excess shunt capacitance) between stages or bits at higher frequencies and minimize reflections. Interstage inductance elements  208 - 1  through  208 - 4  also may be designed to maximize separation between stages to minimize coupling between stages. By avoiding coupling between stages or bits, phase interaction with the actual attenuator bits is minimized. 
         [0045]      FIG. 3  illustrates an on-chip layout of one embodiment of multi-stage digital attenuator  200 . In this embodiment, multi-stage digital attenuator  200  comprises RF input node  202 , RF output node  204 , attenuation stages  206 - 1  through  206 - 5  and interstage inductance elements  208 - 1  through  208 - 4  arranged to match the output and input of attenuation stages. As shown, multi-stage digital attenuator  200  may be implemented on a single chip  210 . Accordingly, multi-stage digital attenuator  200  may achieve wideband constant phase digital attenuation with low insertion loss and on-chip matching circuitry. 
         [0046]    Numerous specific details have been set forth herein to provide a thorough understanding of the embodiments. It will be understood by those skilled in the art, however, that the embodiments may be practiced without these specific details. In other instances, well-known operations, components and circuits have not been described in detail so as not to obscure the embodiments. It can be appreciated that the specific structural and functional details disclosed herein may be representative and do not necessarily limit the scope of the embodiments. 
         [0047]    It is also worthy to note that any reference to “various embodiments,” “some embodiments,” “one embodiment,” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, appearances of the phrases “in various embodiments,” “in some embodiments,” “in one embodiment,” or “in an embodiment” in places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
         [0048]    While certain features of the embodiments have been illustrated as described above, many modifications, substitutions, changes and equivalents will now occur to those skilled in the art. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the embodiments.
Various embodiments are directed to providing constant phase digital attenuation. In one embodiment, a digital attenuator circuit comprises an input node to receive an input signal to be attenuated, an output node to output an attenuated signal, a reference loss path between the input node and the output node, and an attenuation path between the input node and the output node. The reference loss path comprises switching elements and matching circuitry to improve Voltage Standing Wave Ratio (VSWR), and the attenuation path comprises switching elements and attenuating circuitry to attenuate the input signal when the digital attenuator circuit is switched from a reference loss state to an attenuation state. An effective phase length of the reference loss path and an effective phase length of the attenuation path may be equalized to provide a constant phase when the digital attenuator circuit is switched between states. Other embodiments are described and claimed.