Patent Publication Number: US-2007109065-A1

Title: Methods and apparatus for the reduction of local oscillator pulling in zero intermediate frequency transmitters

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims the benefit of U.S. Provisional Patent Application No. 60/679,239 filed May 10, 2005. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to the design of wireless transmitters, and more particularly to the design and layout of wireless transmitters targeted at reducing the phenomenon of local oscillator pulling.  
      2. Prior Art  
      The “pulling” of a local oscillator (LO) in zero intermediate frequency (IF) transmitters with “on” frequency voltage controlled oscillator (VCO), is well known in the art to drastically degrade system performance of wireless transmitters. Specifically, pulling is the modulation of a LO signal, due to unwanted coupling, from an interference, appearing at the same frequency. This phenomenon is graphically explained with reference to  FIG. 1 . The VCO generates a LO frequency f LO    110 . However there is also present an interference  120  around the same frequency of f LO    110 . Due to coupling effects there is a pulling of the f LO    110  frequency resulting in a frequency scheme  140 , undesirable for the operations of transmitters in general.  
      A person skilled in the art would easily identify the dominant sources for coupling mechanisms and would generally group them into three categories: a) magnetic coupling of inductive components; b) electrical coupling through a common substrate; and, c) electromagnetic coupling through power supplies (see  FIG. 4  for details). The magnetic coupling between a transmitter  210  and a VCO  220  are shown with respect to  FIG. 2 . The magnetic coupling occurs when inductors are in a close enough proximity to have meaningful coupling between the inductive components, such as inductor  215  in the transmitter circuit and inductor  225  in the VCO circuit. Electrical coupling is shown with reference to  FIG. 3  where an electrical coupling from a transmitter circuit to a VCO circuit through a substrate  310  is shown. It is well known in the art that coupling, such as demonstrated by circuits  312  and  314 , are common in circuits placed on the same substrate, causing coupling effects and thereby the undesirable phenomenon of LO frequency pulling. The effects of electromagnetic coupling are shown with reference to  FIG. 4  where the connection of a transmitter  410  and a VCO  420  to the power supply are shown. Specifically, transmitter  410  is connected to V cc  via a bonding wire represented by inductor  430 , a pad  440  placed on the chip to which the bonding wire connects, and further through parasitic interconnect depicted by element  450 -A. Transmitter  410  is connected to V ss  via a bonding wire represented by inductor  470 , a pad  460  placed on the chip to which the bonding wire connects, and further through parasitic interconnect depicted by element  450 -B. Similarly, VCO  420  is connected to V cc  via a bonding wire represented by inductor  430 , a pad  440  placed on the chip to which the bonding wire connects, and further through parasitic interconnect depicted by element  450 -C. VCO  420  is connected to V ss  via a bonding wire represented by inductor  470 , a pad  460  placed on the chip to which the bonding wire connects, and further through parasitic interconnect depicted by element  450 -D. Coupling between parasitic interconnects  450 -A to  450 -C and coupling between parasitic interconnects  450 -B to  450 -D results in this third mode of interference.  
      In order to avoid the phenomenon of LO pulling, different approaches have been used in prior art solutions. For example, in one prior art solution the VCO oscillates at double the LO frequency, then a frequency divider is used to produce the final LO frequency. In a more complicated prior art solution the frequency scheme uses a VCO running at ⅔ of the LO frequency followed by a mixer that mixes the ⅔ LO with a ⅓ LO thereby producing the final LO frequency. Although the above mentioned techniques overcome the LO “pulling”, they lead to a more complicated system, increasing in this way the IC area as well as the power consumption.  
      It would therefore be advantageous to provide for a solution to the LO frequency pulling that does not require complex circuitry. It would be further advantageous if such a solution would be based on specific design practices that can be made compatible with standard chip design practices and techniques.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic frequency diagram showing the phenomenon of frequency pulling (prior art).  
       FIG. 2  is a schematic diagram describing the interference caused by coupling between a transmitter and a VCO circuit (prior art).  
       FIG. 3  is a schematic diagram describing the interference caused by electrical coupling through a common substrate (prior art).  
       FIG. 4  is a schematic diagram describing the interference caused by electromagnetic coupling through the power supply interconnect (prior art).  
       FIG. 5  is a circuit showing the placement of inductive elements having opposite currents.  
       FIG. 6  is a layout of a transmitter chip with a layout in accordance with the teachings of the disclosed invention.  
       FIG. 7  is a layout of a transmitter and VCO portions each having a grounded guard ring.  
       FIG. 8A  is a schematic of RC decoupling network to at least reduce the power supply interference.  
       FIG. 8B  is an exemplary bonding diagram for 90° power supply bonding.  
       FIG. 9  is a design flowchart in accordance with the teachings of the disclosed invention.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      By contrast to the methods and techniques taught in prior-art solutions, a different approach is proposed in the teachings made by the inventor. Specifically, the approach taught for the purpose of minimizing voltage controlled oscillator (VCO) “pulling” is to minimize the effects that generate the phenomenon. Therefore specific techniques must be used and adhered to in order to ensure the minimization or elimination of the VCO pulling effect.  
      Reference is now made to  FIG. 5  where an exemplary and non-limiting implementation of the placement of inductive elements is shown. In order to minimize the magnetic coupling of inductive components  510  and  520  a differential topology is used in conjunction with appropriate layout techniques. It is well known that the electromagnetic field of two spiral inductors, for example inductors  510  and  520 , exited by opposite currents (opposite winding sense), is nullified on the horizontal symmetry axes  530 . It would therefore be advantageous to ensure, at least through appropriate layout that such inductors are placed in a manner conducive to such nullification of the coupling effect. It is of particular importance with respect to inductors from different circuits, for example, inductors belonging to a transmitter and a VCO having a coupling due to their proximity to each other. Another important characteristic is that the electromagnetic field is degrading rapidly with the distance. Therefore, as further shown with reference to  FIG. 6 , it would be advantageous to place circuits such as a transmitter  610  and a VCO  620  at a maximal distance possible. As a result of such placement there is ensured the minimization of the inductive coupling between such circuits, for example, transmitter  610  and VCO  620 .  
      Referring now to  FIG. 7 , there is shown an exemplary and non-limiting implementation of circuits each having its own guard ring to minimize electrical coupling. Specifically, in order to minimize the electrical coupling through common substrate  710 , grounded “guard rings”  720  and  730  are to be used around critical blocks, for example, around a transmitter and a VCO. The use of such guard rings is to act as a signal sink, as the coupling circuits  725  and  735  are each grounded. As a result an increase of the isolation between critical blocks is achieved, for example, between a transmitter and a VCO. The isolation is further increased as the distance between the blocks is increased.  
      Reference is now made to  FIG. 8A  where an exemplary and non-limiting schematic of separate power supplies provided to critical blocks, for example a transmitter  810  and a VCO  820 , with an RC network is shown. Specifically, in order to minimize the electromagnetic coupling common to the usage of a single power supply, different power supply connections are used for transmitter  810  and VCO  820 . As a result the coupling due to the common parasitic interconnect depicted by circuits  850 -A and  850 -B of transmitter  810  supply path and parasitic interconnect circuits  850 -C and  850 -D of VCO  820  is significantly reduced. Moreover, external RC decoupling networks  880  and  885  to transmitter  810  path and VCO  820  path respectively, are used to further reduce the power supply coupling. RC networks  880  and  885  may be placed as part of package  895  of the chip. With reference to  FIG. 8B  there is shown an exemplary and non-limiting bonding of pads  840  and  845 . In accordance with the disclosed invention, bonding wires  842  and  847 , that connect the internal power supplies to the external power lines, i.e., connecting chip  890  pads to the pads of package  895 , have a 90° placement of each other in order to minimize electromagnetic coupling between them.  
      Reference is now made to  FIG. 9  where an exemplary and non-limiting design flowchart  900  for a device including certain critical blocks, such as a transmitter and a VCO is shown. In step S 910  the inductive coupling is minimized by placing inductors that have to reside in proximity of each other such that the current flow through the inductors is such that the interference is reduced, i.e. the current flows in the same direction for corresponding wires of the two inductors (or other conductors) that are effectively parallel to each other (opposite winding sense by reversal of the winding direction as in  FIG. 5 , or by providing currents of opposite directions in two windings having the same physical winding sense). A proximity threshold for determination of the effect of a coupling between proximate inductors may vary between applications, some being more sensitive to the coupling thereby establishing a lower value for inductive coupling for the purpose of placing the inductors in the manner suggested in this step S 910 . In step S 920  inductors throughout the layout are placed at a maximum distance from each other. This step takes place as the inductive coupling is reduced as the distance grows larger. In one embodiment of the disclosed invention inductors that have been placed in accordance with step S 910  are not moved with respect to each other, thereby reducing the effective number of inductors that require handling. In another embodiment of the disclosed invention only those inductors that belong to separate critical blocks are placed in a maximal distance from each other. In step S 930  critical blocks that may cause interference with the operation of other blocks or that are impacted from interference of other blocks, are placed within the confines of a guard ring. The guard ring is further grounded to the system ground causing interferences from one block to be grounded in respect of another block. In step S 940  the power supplies of blocks that either suffer from interference or cause interference to other blocks are connected to separate power supply pads from the power supply pads of another block. In step S 950  the bonding of power supplies that are close to each other, for example, neighboring Vcc pads, are bonded in 90° degrees of each other. In step S 960  decoupling resistor-capacitor (RC) networks are placed on the power supply lines to further reduce interference, the RC value being calculated to ensure that the interference frequencies are minimized. A person skilled in the art would realize that some or all of these steps may be used for a specific implementation. A person skilled in the art would further notice that the order of these steps are only exemplary and other sequences may take place.  
      In accordance with the disclosed invention there have been shown special layout techniques for the purpose of minimizing VCO pulling due to the modulated signal of a zero IF transmitter. Measurement results of a circuit, such as the exemplary device shown in  FIG. 6 , prove that the VCO pulling is drastically reduced, resulting in a system that does not present any performance degradation due to pulling effects. A person skilled-in-the-art would readily realize that a computer software product comprising a plurality of instructions designed to be executed on a computer system, and utilizing the methods disclosed herein, may be used to identify the areas where design modifications in accordance with the disclosed inventions are needed. Such a computer software product may further provide directions so as to the necessary course of action to correct an initially problematic design to overcome the frequency pulling in accordance with the principles taught herein.  
      Thus 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.