Abstract:
In some embodiments of the present invention, multiple voltage controlled capacitor branches may be employed in a tuning circuit. Each of the multiple voltage controlled capacitors branches may be set to somewhat apart resonance frequency so that a composite resonance derivative curve forms an equi-ripple curve with an improved tuning sensitivity linearization.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Phase Locked Loop (PLL) tuning circuitry is a common solution for stabilizing tuning frequency in Radio Frequency (RF) circuits. In order to maintain the frequency of a PLL circuit within very tight limits a calibration process may be carried out, which is able to compensate for variances in the electrical features of the tuning circuitry components over time and frequency. In particular, when the component used in the PLL circuit to change the PLL frequency is a voltage controlled capacitor (varactor), it may be desirable to compensate for the tuning sensitivity of the varactor to changes in its electrical features over frequency (also defined “tuning linearity”). For improved linearity in a tuning circuit it may be desirable to compensate the tuning linearity also for variance due to other factors, such as temperature.  
           [0002]    Metal Oxide Semiconductor Field Effect Transistor (MOSFET) varactors (also known as MOSCAPs) are regarded as inferior to bi-polar varactors with respect to tuning linearity, yet MOSCAPs are generally capable of handling dual-polarity voltage swing while bi-polarity varactors are not generally capable of doing so.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0003]    The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanied drawings in which:  
         [0004]    [0004]FIG. 1 is a simplified electrical diagram illustration of a compensated tuning circuitry constructed and working according to some embodiments of the present invention;  
         [0005]    [0005]FIG. 2 is a schematic tuning frequency derivative versus control voltage graph of some embodiments of the present invention;  
         [0006]    [0006]FIGS. 3 and 4 are schematic electrical diagrams of tuning circuits according to some embodiments of the present invention; and  
         [0007]    [0007]FIG. 5 is a high-level schematic block diagram illustration of a high frequency section in a radio receiver using the tuning circuitry of some embodiments of the present invention. 
     
    
       [0008]    It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0009]    In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However it will be understood by those of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention.  
         [0010]    It should be understood that the present invention may be used in a variety of applications. Although the present invention is not limited in this respect, the circuit disclosed herein may be used in many apparatuses such as in the receivers of a radio system. Radio systems intended to be included within the scope of the present invention include, by way of example only, cellular radiotelephone communication systems, two-way radio communication systems, one-way pagers, two-way pagers, personal communication systems (PCS), and the like.  
         [0011]    Many high frequency analog circuits, such as filters and PLL tuning circuits are based on an Inductor/Capacitor (L/C) resonance or filtering loop. The filtering features of an L/C filter may be tunable, by changing the electrical properties of one or more of its electrical components, such as the capacitance of the capacitor (C) or the inductance of the inductor (L). Similarly, tuning circuits may have a tunable resonance frequency that may be changed by changing the electrical properties of one or more of its electrical components, such as the capacitance of the capacitor (C) or the inductance of the inductor (L). The linearity of a tuning circuit is determined mainly by the derivative of tuning frequency (also known as “Kvco”) vs. tuning voltage. For better tuning linearity a constant ratio of tuning-frequency-to-tuning-voltage is required over as wide tuning voltage as possible. Thus, in a derivative-of-tuning-frequency vs. tuning voltage graph, where a flat portion of the graph represents a constant tuning-frequency-to-tuning-voltage ratio, the bigger the flat portion is, the wider is its tuning linearity range.  
         [0012]    Reference is made now to FIG. 1, which is a simplified electrical diagram illustration of a tuning system  6  according to some embodiments of the present invention, and to FIG. 2, which is schematic derivative-of-tuning-frequency vs. control voltage graph  28  of some embodiments of the present invention. Tuning system  6  may comprise, for example, but not limited to, an RF antenna  7 , an RF amplification unit  8  and a tuning circuit  10 . RF antenna  7  may be of any desired kind such as, but not limited to, dipole, Yagi, multi-pole. Tuning circuit  10  may comprise two or more branches, each branch comprising at least one voltage controlled capacitor, such as a MOSCAP. Tuning circuit  10  may comprise also at least one branch comprising at least one inductor. In the particular example shown in FIG. 1, tuning circuit  10  comprises three capacitance branches denoted  12 ,  14  and  16  respectively. The voltage controlled capacitors in branches  12 ,  14  and  16  are of type MOSCAP, but may be of other types as well. Tuning circuit  10  comprises also one inductance branch denoted  17 , with inductors L 1  and L 2 . The working point of the gates of the MOSCAPs of branches  12 ,  14  and  16  may be biased to Va, Vb and Vc volts, respectively, where Va differs from Vb by V1 volts and Vb differs from Vc by V2 volts. A single control voltage terminal Vsteer may be used to control the tuning frequency of all three branches.  
         [0013]    Each of the branches  12 ,  14  and  16  has its corresponding graph of the derivative of tuning frequency vs. tuning voltage denoted  22 ,  24  and  26  respectively. In graphs  22 ,  24  and  26  the pick points correspond to the working point voltages Va, Vb and Vc respectively. The combination of the three independent curves forms a composite curve  28 . Composite curve  28  may have an enlarged portion that is substantially flat ranging from Vmin to Vmax. For a tuning voltage ranging between Vmin and Vmax, tuning circuit  10  may have an improved linearized tuning sensitivity. The curve demonstrates a substantially constant derivative-of-tuning-frequency graph, but in general, a designer might wish to achieve any tuning sensitivity profile that fits the system needs with this method.  
         [0014]    Similarly, in a L/C analog filter (not shown) the filter properties, such as central frequency, shape of response curve, etc. may be controlled by applying biased voltages to different branches of the filter.  
         [0015]    Reference is made now to FIGS. 3 and 4, which are schematic electrical diagrams of tuning circuits  30 ,  40  according to some embodiments of the present invention. Tuning circuit  30  is constructed and working according to some embodiments of the present invention. In order to set working point bias voltages for the various branches of tuning circuit  30 , resistors R 1 , R 2  and R 3  may be used, arranged in a voltage splitter arrangement. Additionally, stabilizing shunt capacitors (not shown) may be placed over resistors R 1 , R 2  and R 3 , in order to reduce variances in the voltage drop over R 1 , R 2  and R 3 . In tuning circuit  40  a current source  42  may be added, in order to control the voltage drop over resistors R 11 , R 12 . It will be appreciated by persons of ordinary skill in the art that different arrangements and topologies may be used in order to set the voltage differences between the working points of the voltage-controlled capacitors in the various branches. It will also be appreciated that the number of branches comprising voltage-controlled capacitors may be two or more, and that the number of voltage controlled capacitors in a branch may be one or more. It will also be appreciated that the voltage controlled capacitors used in the linearized tuning circuit according to some embodiments may be other than MOSCAP, such as but not limited to, bi-polar voltage-controlled capacitors.  
         [0016]    Reference is made now to FIG. 5, which is a high-level schematic block diagram illustration of a high frequency section in a radio receiver using tuning circuitry of some embodiments of the present invention. Radio frequency (RF) section  500  may comprise an RF antenna  502 , a RF amplification unit  504 , a tuner  506 , an Intermediate Frequency (IF) amplification unit  508  and a detector  510 . Tuner  506  may comprise an oscillator, which may comprise a PLL circuit  512  according to some embodiments of the present invention. It would be apparent to a person of ordinary skill in the art that the embodiments of the invention may be used in any other systems, where a voltage controlled capacitor is involved in controlling working frequency, or where it is desirable to shape the derivative-of-tuning-frequency vs. tuning voltage graph. These systems may comprise, but not limited to, RF receivers/transmitters, with or without a connection to antenna, RF amplifiers and RF filtering systems.  
         [0017]    It will be appreciated by persons of ordinary skill in the art that according to some embodiments of the present invention other L/C analog circuits, such as, but not limited to, filters may comprise two or more branches each comprising at least one voltage controlled capacitor, where the working point of one voltage controlled capacitor is shifted from that of any other voltage controlled capacitor, to control the response features of that circuit.  
         [0018]    While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill 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 invention.