Abstract:
A phase synchronous multiple LC tank oscillator includes a plurality of oscillator stages that are configured to oscillate synchronously. The phase of each of the plurality of oscillator stages is substantially the same and the plurality of oscillators are inductively coupled. The synchronous oscillation is substantially caused by magnetic coupling. The oscillator stages may be electrically coupled during a first time period and the electrical coupling and disconnected or reduced during a second time period.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation in part of U.S. patent application Ser. No. 10/004,521 by Beomsup Kim, entitled PHASE SYNCHRONOUS MULTIPLE LC TANK OSCILLATOR filed Nov. 14, 2001, now U.S. Pat. No. 6,900,699, which is incorporated herein by reference for all purposes. 

   FIELD OF THE INVENTION 
   The present invention relates generally to voltage controlled oscillators (VCO&#39;s). An inductively coupled oscillator is disclosed. 
   BACKGROUND OF THE INVENTION 
   VCO&#39;s are commonly used in communication systems, as well as other applications. Multiple oscillators may be cascaded in a ring to improve noise and performance and may be configured with various different phase relationships among the oscillators in the cascade. Kim, which was previously incorporated by reference, discloses a VCO that includes multiple oscillators configured to oscillate synchronously, in phase with each other. The oscillators are electrically coupled to cause the synchronous oscillation. Because the oscillators oscillate in phase, it is possible to arrange the inductors that are included in the oscillators so that the inductors are mutually inductive. 
   One of the ways that noise is introduced into a cascaded oscillator is through the wires and switches that comprise the electrical connections between oscillators that couple the oscillators and cause synchronous oscillation. It would be useful if such noise could be reduced. Furthermore, it would be useful if improved coupling schemes for synchronous oscillator cascades could be developed and if improved inductor layout schemes could be developed for inductors configured to be mutually inductive. 
   SUMMARY OF THE INVENTION 
   A phase synchronous multiple LC tank oscillator is described. In some embodiments, a plurality of oscillator stages are configured to oscillate synchronously. The phase of each of the plurality of oscillator stages is substantially the same and the plurality of oscillators are inductively coupled. In some embodiments, a multiple LC tank oscillator includes a first oscillator stage having a first inductor and a second oscillator stage having a second inductor, wherein the first inductor and the second inductor are mutually inductive. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
       FIG. 1A  is a block diagram illustrating an electrically coupled phase synchronous LC tank oscillator. 
       FIG. 1B  is a block diagram illustrating an alternate design for an electrically coupled phase synchronous oscillator. 
       FIG. 1C  is a diagram illustrating another LC tank oscillator architecture that utilizes four oscillator stages that oscillate synchronously. 
       FIG. 2  is a diagram illustrating an LC tank oscillator. 
       FIG. 3A  is a diagram illustrating a rectangular physical layout for 4 inductors that are mutually inductive. 
       FIG. 3B  is a diagram illustrating an octagonal physical layout for 4 inductors that are mutually inductive. 
       FIG. 3C  is a diagram illustrating a circular layout for 4 inductors that are mutually inductive. 
       FIG. 3D  is a diagram illustrating in detail how one inductor passes below another inductor at a crossing point. 
       FIG. 4A  is a diagram illustrating a 2 oscillator stage synchronous LC tank oscillator where the oscillators are inductively coupled without electrical coupling. 
       FIG. 4B  is a diagram illustrating a layout for 2 inductors that are mutually inductive. 
       FIG. 4C  is a diagram illustrating a 4 oscillator stage synchronous LC tank oscillator where the oscillators are inductively coupled without electrical coupling. 
       FIG. 5  is a diagram illustrating a 4 oscillator stage synchronous LC tank oscillator where the oscillators are inductively coupled with electrical coupling selectively included and controlled by a set of switches. 
   

   DETAILED DESCRIPTION 
   It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer readable medium such as a computer readable storage medium or a computer network wherein program instructions are sent over optical or electronic communication links. 
   A detailed description of one or more preferred embodiments of the invention are provided below along with accompanying figures that illustrate by way of example the principles of the invention. While the invention is described in conjunction with such embodiments, it should be understood that the invention is not limited to any one embodiment. On the contrary, the scope of the invention is limited only by the appended claims and the invention encompasses numerous alternatives, modifications and equivalents. For the purpose of example, numerous specific details are set forth in the following description in order to provide a thorough understanding of the present invention. The present invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the present invention is not unnecessarily obscured. 
     FIG. 1A  is a block diagram illustrating an electrically coupled phase synchronous LC tank oscillator. Each oscillator stage,  102 ,  104 ,  106 , and  108  includes a pair of inputs and a pair of outputs. A preferred design for each oscillator stage is shown in  FIG. 2 . Each output is connected to an input of the next oscillator in the chain. This configuration results in each of the oscillators oscillating in a synchronous manner. 
     FIG. 1B  is a block diagram illustrating an alternate design for an electrically coupled phase synchronous oscillator. Each oscillator stage,  112 ,  114 ,  116 , and  118  includes a pair of inputs and a pair of outputs. A preferred design for each of the LC tank oscillators is shown in  FIG. 2 . Each output is connected to an input of the next oscillator in the chain, with a different input selected than the input selected in  FIG. 1 . This configuration results in each of the oscillators oscillating in a synchronous manner. 
     FIG. 1C  is a diagram illustrating another LC tank oscillator architecture that utilizes four oscillator stages,  122 ,  124 ,  126 , and  128  that oscillate synchronously. Each oscillator stage includes four inputs and two outputs. The inputs and outputs are again configured to cause the oscillator stages to oscillate in phase. 
     FIG. 2  is a diagram illustrating an LC tank oscillator. LC tank oscillator  200  includes input node  202  and input node  204 . The input nodes are coupled to the LC portion of the circuit  206  via a network of transistors. Because the individual LC tank oscillators oscillate synchronously, the inductors may be configured in a mutually inductive manner. Each inductor is laid out so that the area in which its flux is contained substantially coexists with the flux area of the other inductors that comprise the other oscillators in the ring. 
     FIG. 3A  is a diagram illustrating a rectangular physical layout for 4 inductors that are mutually inductive in the manner described above. The layout includes inductors  302 ,  304 ,  306  and  308 . The layout is symmetric and the flux area of the inductors substantially overlaps. Dashed lines are used to illustrate when one inductor wire drops to a lower layer to pass under another inductor wire. This layout is implemented using a multilayer metal structure as described in  FIG. 3D . Each conductor that forms a loop begins at a first upper layer, then transitions to a second lower layer to pass under another inductor when that is required and then transitions back to the first layer. 
     FIG. 3B  is a diagram illustrating an octagonal physical layout for 4 inductors that are mutually inductive in the manner described above. The layout includes inductors  312 ,  314 ,  316  and  318 . The layout is symmetric and the flux area of the inductors substantially overlaps. Dashed lines are used to illustrate when one line passes under another line. This layout is implemented using a multilayer metal structure as described  110  in  FIG. 3D . The octagonal layout shown has the advantage that the angle of the turns is less acute, which reduces current crowding. 
     FIG. 3C  is a diagram illustrating a circular layout for 4 inductors that are mutually inductive in the manner described above. The layout includes inductors  322 ,  324 ,  326  and  328 . The layout is symmetric and the flux area of the inductors substantially overlaps. Dashed lines are used to illustrate when one line passes under another line. This layout is implemented using a multilayer metal structure as described in  FIG. 3D . The circular layout shown has the advantage eliminating the angular turns, which reduces current crowding. 
     FIG. 3D  is a diagram illustrating in detail how one inductor passes below another inductor at a crossing point. Preferably, the inductors are laid out on a top metal layer. Depending on the manufacturing process, the top metal layer may be thicker (on the order of two or three times thicker in some embodiments) than the second layer that is just below the top layer. When the top layer is thicker than the lower layers, the lower layers tend to have higher resistance. This problem can be ameliorated by connecting the top layer to two or more lower layers in parallel. At a crossing point, one inductor is interrupted in the top layer and is connected to one or more lower layers using one or more vias. Once the crossing point has passed, the inductor resumes on the top layer and one or more vias connect the lower layers to the top layer. 
   In the example shown, inductor  350  is disposed in the top layer until a crossing point with inductor  360  is reached. Inductor  350  is interrupted in the top layer and is connected to the second layer and the third layer by via  351 . Second layer portion  352  and third layer portion  354  of inductor  350  run below inductor  360 . Once the crossing point is passed, via  355  connects second layer portion  352  and third layer portion  354  back to the top layer and inductor  350  resumes on the top layer. In different embodiments, different numbers of layers may be used. In general, only 2 layers are required, but additional layers may be used to lower the resistance of the portion of the inductor that drops down to a lower layer. 
   In addition to the disclosed architectures using 4 oscillator stages, other 4 oscillator stage architectures may be used as well as architectures having different numbers of oscillators. 
     FIG. 4A  is a diagram illustrating a 2 oscillator stage synchronous LC tank oscillator where the oscillators are inductively coupled without electrical coupling. Oscillator  402  and oscillator  404  are inductively coupled through their inductors  403  and  405 . Each of the inductors includes a center tap that is connected to a bias voltage Vdd. The inductive coupling causes the oscillator stages to oscillate synchronously without electrical coupling. By replacing electrical coupling with inductive coupling, the noise associated with the electrical coupling circuit is eliminated. 
     FIG. 4B  is a diagram illustrating a layout for 2 inductors that are mutually inductive in the manner described above. The inductors are laid out on a top layer with inductor  414  dropping down to a lower layer in order to cross under inductor  412  at the appropriate point. Each inductor includes a center tap. The area in which the magnetic flux is contained for each inductor substantially coexists with the flux area of the other inductor. In other embodiments, different shaped inductors are used, such as the hexagonal shape and circular shape shown above. 
     FIG. 4C  is a diagram illustrating a 4 oscillator stage synchronous LC tank oscillator where the oscillators are inductively coupled without electrical coupling. Oscillators  432 ,  434 ,  436 , and  438  are inductively coupled through their inductors  442 ,  444 ,  446 , and  448 . The inductors are disposed in a manner as shown in  FIGS. 3A–3C  so that the areas in which flux is contained substantially overlap. The inductive coupling causes the oscillator stages to oscillate synchronously without electrical coupling. By replacing electrical coupling with inductive coupling, the noise associated with the electrical coupling circuit is eliminated. 
     FIG. 5  is a diagram illustrating a 4 oscillator stage synchronous LC tank oscillator where the oscillators are inductively coupled with electrical coupling selectively included and controlled by a set of switches. Electrical coupling is selectively connected or disconnected using the switches. Oscillators  452 ,  454 ,  436 , and  438  are inductively coupled through their inductors  462 ,  464 ,  466 , and  468 . Switches  471 ,  472 ,  473 ,  475 ,  476 ,  477 , and  478  selectively connect or disconnect electrical coupling among the oscillators. The inductors are disposed in a manner as shown in  FIGS. 3A–3C  so that the areas in which flux is contained substantially overlap. When the electrical coupling is deactivated using the switches, the inductive coupling causes the oscillator stages to oscillate synchronously without electrical coupling. By replacing electrical coupling with inductive coupling, the noise associated with the electrical coupling circuit is eliminated. A number of embodiments of LC tank oscillators have been described that include coupled inductors. In some embodiments, two or more oscillators are synchronized using inductive coupling without electrical coupling or with reduced electrical coupling. The synchronous coupling improves the phase noise of the oscillators. Also, the inductive coupling increases the Q, which improves the phase noise. Lower phase noise for the same power or lower power for the same phase noise is achieved compared to non-synchronous oscillators. The coupled inductors occupy less area since they overlap. Several embodiments of exemplary overlapping inductor layouts have been illustrated. 
   Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. It should be noted that there are many alternative ways of implementing both the process and apparatus of the present invention. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.