Abstract:
A circuit arrangement having a plurality of variable capacitance elements such as varactors is described, the varactors having associated electronic control means which controls the capacitance of the variable capacitance elements over a control range. The control range is such that for any particular variable capacitance element a complete variation from a lowest to a highest capacitance is obtained from only a portion of the control range.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims priority to currently pending United Kingdom Patent Application number 0327284.6, filed Nov. 24, 2003.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     N/A  
       BACKGROUND OF THE INVENTION  
       [0003]     Varactors are commonly used in RF circuits for tuning oscillators, filters and amplifiers.  
         [0004]     One problem with varactors is that their capacitance/voltage characteristics are typically very non-linear as shown in  FIG. 1   a,  which illustrates a typical metal oxide semiconductor varactor (MOSvar) capacitance/voltage characteristic. The non-linear feature of the MOSvar is emphasized by  FIG. 1   b  which shows the first derivative dC/dV of the curve of  FIG. 1   a.    
         [0005]     One device allowing a capacitance/voltage characteristic having an acceptable tuning range and a more linear range to be obtained is a hyper-abrupt varactor. However, the implementation of a hyper-abrupt varactor requires extra processing during manufacture, which is expensive.  
         [0006]     An alternative method of overcoming the non-linearity of a varactor is to use digital techniques to switch in capacitors so as to tune over the required range. However, this solution is complex, can be physically large, and may be too slow.  
         [0007]     It is an object of the present invention to provide a variable capacitance circuit arrangement having a relatively linear characteristic.  
       OBJECTS AND SUMMARY OF THE INVENTION  
       [0008]     Additional objects and advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.  
         [0009]     According to a first aspect of the present invention, there is provided a circuit arrangement having a variable capacitance for a tuning circuit, wherein the circuit arrangement comprises a plurality of variable capacitance elements connected in parallel and, coupled to the capacitance elements, control means for electronically controlling the capacitances of the variable capacitance elements, the control means having a control range over which they cause the capacitance of the circuit arrangement to vary, the control means and the variable capacitance elements being configured such that at least one of the said elements exhibits complete variation of its capacitance in response to the control means over only a portion of the control range.  
         [0010]     The characteristics of the capacitance elements determine the way in which the capacitance of the circuit arrangement varies in response to the control means. For example, the variable capacitance elements may be chosen so as to produce a capacitance versus control voltage response which is more linear over its operating range compared with that of a single varactor. Similarly, the variable capacitance elements may be chosen so as to produce a capacitance response which follows an approximate square law characteristic for a linearized frequency/voltage characteristic when used in conjunction with an inductor to form a resonant network.  
         [0011]     The control means may comprise a common control source and a plurality of different respective offset biases applied to the variable capacitive elements.  
         [0012]     In a preferred embodiment, the variable capacitive elements are varactor (varactors), which are biased by an offset voltage. In this embodiment, the control means is configured to apply a common control voltage and a plurality of different DC offset voltages to the respective varactors. The voltage applied across each varactor is, therefore, the difference between the common control voltage and the respective offset voltage applied to each varactor, (or the sum of these two voltages, depending on the sign of the offset voltage). When tuning, the varactor only exhibits a change in its capacitance if the difference between the control voltage and the respective offset voltage applied to the particular varactor falls within the range of voltages over which the varactor capacitance varies, in terms of the voltage applied across the varactor itself. Alternatively, the control means may be configured to apply a plurality of different control voltages to the respective varactors. A common bias voltage may be applied.  
         [0013]     Each varactor may be arranged to have one of its electrodes coupled to the common control voltage source and its other electrode to a respective DC offset bias voltage source.  
         [0014]     The capacitance characteristic of the circuit arrangement is dependent on the number of variable capacitive elements connected in parallel. The more variable capacitive elements used in the circuit, the closer the capacitance characteristic can be to a desired response i.e., linear, square law, etc.  
         [0015]     Advantageously, the control means are arranged such that the variable parts of the capacitance response characteristics of the variable capacitive elements overlap. By adjustment of the overlaps, it is possible to alter the overall characteristic of the circuit arrangement to be closer to the desired characteristic.  
         [0016]     The variable capacitance elements are preferably selected such that the sum of their individual capacitance values is equal to the required total maximum capacitance of the circuit arrangement. Additionally, the varactors may be selected such that the combination of the ranges over which their individual capacitances vary is substantially equal to the total operational range of the circuit arrangement,  
         [0017]     In one embodiment, where a generally linear capacitance/voltage characteristic is required, the individual variable capacitance elements may be chosen such that the maximum capacitance of each element is approximately equal to the maximum required capacitance of the circuit arrangement divided by the number of parallel variable capacitive elements. In addition, the individual variable capacitive elements may be chosen such that the range over which each of their individual capacitances vary is equal to the total operating range of the circuit arrangement divided by the number of capacitive elements connected in parallel. For example, if there are three variable capacitive elements in the circuit, the characteristics of the capacitive elements are such that their individual maximum capacitances are each equal to a third of the total capacitance of the circuit arrangement and their effective operating ranges are each approximately a third of the total operating range of the arrangement.  
         [0018]     In an alternative embodiment the characteristics of the capacitance elements may be selected such that the resultant capacitance/voltage characteristic has a square law characteristic. In this embodiment the characteristic of each capacitance element may be different from the characteristics of the other capacitance elements, unlike the characteristic of capacitance elements that may be selected if a generally linear CN characteristic is desired.  
         [0019]     According to a second aspect of the present invention, there is provided a tunable radio frequency (RF) circuit comprising a circuit arrangement having a variable tuning capacitance, wherein the circuit arrangement comprises a plurality of tuning varactors connected in parallel, and coupled to the tuning varactors, control means for electronically controlling the capacitances of the varactors, and one or more inductors, the control means having a control range over which they cause the capacitance of the circuit arrangement to vary, the control means and the varactors being configured such that at least one of the varactors exhibits complete variation of its capacitance in response to the control means over only a portion of the control range.  
         [0020]     In one embodiment, the circuit includes a modulator which comprises a modulation varactor arranged in parallel with the tuning varactors but isolated therefrom by a DC blocking capacitor, the modulation varactor being coupled to a modulation input. The modulation varactor may have an offset voltage applied to one of its electrodes.  
         [0021]     According to another aspect of the present invention, there is provided a voltage controllable oscillator comprising a circuit arrangement having a variable capacitance for a tuning circuit, wherein the circuit arrangement comprises a plurality of varactors connected in parallel, and coupled to the varactors, control means for electronically controlling the capacitances of the varactors, the control means having a control range over which they cause the capacitance of the circuit arrangement to vary, the control means and the varactors being configured such that at least one of the varactors exhibits complete variation of its capacitance in response to the control means over only a portion of the control range.  
         [0022]     The varactors in the circuit arrangement may be selected such that the capacitance/voltage characteristic of the circuit arrangement has a generally square law characteristic so as to achieve a linearized frequency/voltage response characteristic for the voltage controllable oscillator.  
         [0023]     In one embodiment, the voltage controllable oscillator includes a modulator as described hereinabove. This arrangement provides the oscillator with a tuning circuit wherein the frequency of the oscillator can be modulated by a modulation signal independently of the main frequency control signal.  
         [0024]     According to yet another aspect of the present invention, there is provided a tunable radio frequency (RE) circuit comprising: the resonant combination of a tuning varactor, a modulation varactor and an associated inductance, the tuning varactor and the modulation varactor being respectively coupled to a tuning control input and modulation input which are DC isolated from each other.  
         [0025]     The invention also includes a tunable RF circuit including a plurality of varactors connected in parallel with each other and, coupled to the varactors, capacitance control means operable to apply different variable voltages simultaneously across respective varactors.  
         [0026]     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate at least one presently preferred embodiment of the invention as well as some alternative embodiments. These drawings, together with the description, serve to explain the principles of the invention but by no means are intended to be exhaustive of all of the possible manifestations of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1   a  is a graph illustrating a typical capacitance/voltage (CN) characteristic of a varactor;  
         [0028]      FIG. 1   b  is a graph illustrating the first derivative (dC/dV) of the characteristic of  FIG. 1   a;    
         [0029]      FIG. 2  is a schematic diagram of a circuit arrangement in accordance with the present invention;  
         [0030]      FIG. 3  is a graph illustrating a CN characteristic of the circuit arrangement shown in  FIG. 2 ;  
         [0031]      FIG. 4  is a graph illustrating the CN characteristics shown in  FIGS. 1   a  and  3  in a single representation;  
         [0032]      FIG. 5  is a graph illustrating the first derivative (dC/dV) of the capacitance response of the circuit arrangement of  FIG. 2 ;  
         [0033]      FIG. 6  is a schematic diagram of a circuit arrangement in accordance with the invention, including a modulator; and  
         [0034]      FIG. 7  is a schematic diagram of a voltage controlled oscillator including a circuit arrangement in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     Reference now will be made in detail to the presently preferred embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, which is not restricted to the specifics of the examples. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment, can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims and their equivalents. The same numerals are assigned to the same components throughout the drawings and description.  
         [0036]     Referring to  FIG. 2 , a variable capacitance circuit arrangement  100  has a variable capacitance formed by three varactors  110 ,  112 ,  114  which are connected in parallel. The varactors  110 ,  112 ,  114  are each connected at one of their electrodes to a respective DC offset voltage source V 1 , V 2 , V 3  and, at the other electrode, to a common control input  116  for supplying a control voltage V control  via a series impedance  118 . In this embodiment each of the offset voltage sources V 1 , V 2 , V 3  is connected in series between the anode of the respective varactor and one of the supply rails of the arrangement, in this case to ground, the varactors cathodes being coupled to the control input. The DC offset voltage sources V 1 , V 2 , V 3  each have a different offset voltage level such that each of the varactors  110 ,  112 ,  114  contributes to the variation in overall capacitance only when the difference between the control and the respective offset bias falls within the voltage range (in terms of the voltage across the varactor) over which the varactor exhibits a variation in capacitance.  
         [0037]     In this embodiment, the different offset voltages V 1 , V 2 , and V 3  are such that V 1  is smaller than V 2  which is, in turn, smaller than V 3 . Therefore, assuming that the individual characteristics of the varactors are similar, if the control voltage is raised progressively from the lower limit of its range to its upper limit, the operation of the circuit  100  will sequentially bring the first varactor  110  into the variable part of its characteristic, followed by the second varactor  112 , and finally the third varactor  114 .  
         [0038]     The different offset voltage levels may be achieved by a number of different arrangements, as would be known by a person of ordinary skill in the art. These arrangements can include the use of a voltage divider circuit, zener diodes, individual DC power sources and the like.  
         [0039]     The characteristics of the varactors  110 ,  112 ,  114  are selected such that the sum of their maximum respective capacitances is equal to the required total maximum capacitance of the circuit arrangement  100 . In addition, the varactor characteristics are selected such that the sum of the maximum ranges of variation in capacitance of the varactors is equal to the required total variation in capacitance of the circuit arrangement  100 . If a substantially linear variation of the overall, capacitance with control voltage is required, the varactors are selected so as to have the same or generally similar characteristics, at least insofar as they have at least approximately equal capacitance ranges and capacitance-versus-voltage slopes. If the overall capacitance is to follow an approximate square law characteristic with respect to voltage, as may be required to achieve a linearized tuning frequency-versus-voltage characteristic in a resonant inductance/capacitance circuit such as in an RF voltage-controlled oscillator (VCO), the varactors  110 ,  112 ,  114  may be selected to have different capacitance ranges. For instance, the varactor associated with the highest offset voltage may be chosen to have a greater range of variation of capacitance and a steeper capacitance-versus-voltage slope.  
         [0040]     The offset bias voltages applied to the varactors are preferably set such that there is an overlap, with respect to control voltage, between the high capacitance part of the variable capacitance range of one varactor and the low capacitance part of the range of capacitance of another of the varactors. Overlapping of the variable portions of respective characteristics in this way, as depicted in  FIG. 3 , contributes to the linearity of the capacitance/voltage characteristics of the composite arrangement  100 .  
         [0041]     The circuit arrangement  100  is operable such that as the control voltage is increased from a minimum to a maximum voltage, each of the varactors is sequentially operated. That is to say, as the control voltage is increased the varactors are activated such that there is an overlap between the high capacitance range of one varactor and the low capacitance part of the range of capacitance variation of another of the varactors. The total capacitance of the circuit arrangement  100  is equivalent to the sum of the capacitance of each of the varactors.  
         [0042]      FIG. 4  shows the capacitance versus voltage characteristic of the circuit arrangement  100  of  FIG. 2  superimposed on the equivalent curve of a circuit having a typical single MOSvar, as shown in  FIG. 1   a.  It can be seen that the curve of the circuit arrangement  100  is more linear than that of the MOSvar. This fact is more clearly seen in  FIG. 5  which illustrates the first derivative dC/dV of the characteristic of  FIG. 3 . It can be see that the circuit arrangement  100  produces less variation in the dC/dV characteristic over the operational control voltage range compared with that of the dC/dV characteristic of the typical MOSvar, as evidenced by the approximately flat character of the relevant part of the curve.  
         [0043]     To summarize, the circuit arrangement  100  has a capacitive network made up of a number of varactors connected in parallel, each varactor being inherently non-linear over its operating range, and yet the network as a whole having the advantage of a more linear capacitance versus control input response compared to that of the typical varactor implementation capable of capacitance variation over the same range.  
         [0044]     The circuit arrangement  100  has many different applications. However, it is of particular benefit in RF tuning circuits such as voltage controlled oscillators, filters and tuned amplifiers.  
         [0045]     Referring now to  FIG. 6  of the drawings, the capacitance part of a voltage controlled oscillator  200  in accordance with the invention includes a modulator  220 . Tuning of the oscillator is accomplished by a network of parallel varactors coupled to a control input and respective offset sources as described above with reference to  FIG. 2 . The modulator  220  comprises a varactor  222  connected effectively in parallel with the varactors  110 ,  112 ,  114  of the tuning network. The modulator varactor is connected at one of its electrodes to a DC offset bias voltage source V 4  and at its other electrode to a modulation input  223  for receiving a modulation signal V mod . The varactor  222  is coupled to the circuit arrangement  100  via a DC blocking capacitor  224 , thereby isolating the modulation input  223  from the control input  116 .  
         [0046]     Use of an additional varactor  222  specifically for frequency modulation of the VCO output signal, the modulation being applied to this varactor directly from a modulation input which is isolated from the control input  116 , has the advantage that the sensitivity of the modulation process can be set substantially independently of the VCO tuning frequency. That is to say, the variations in capacitance produced by the modulation signal applied to the modulation input  226  do not vary significantly in magnitude for a given modulation voltage amplitude as the VCO operating frequency alters. Accordingly, the depth of modulation remains substantially constant.  
         [0047]     Referring to  FIG. 7 , an emitter coupled LC oscillator  300  in accordance with the invention has a cross-coupled transistor pair Q 0 , Q 1  arranged as a voltage controllable oscillator with a differential output across the collectors of the transistors Q 0 , Q 1 .  
         [0048]     The frequency of the oscillator  300  is determined by the inductive and capacitive components connected to the collectors of the cross-coupled transistor pair Q 0 , Q 1  and the virtual ground formed by a bias block  302  which incorporates a plurality of offset voltage sources producing varactor bias voltages V 1 , V 2  and V 3 . In this circuit, the frequency-determining components are inductors L 1  and L 2 , capacitors C 0  and C 1  and varactors C 10 , C 11 , C 12 , C 13 , C 14  and C 15 .  
         [0049]     Each varactor is connected to a respective DC offset voltage source V 1 , V 2  or V 3  in the bias block  302  and the total capacitance of the varactors is adjusted by varying the value of the control voltage, V control .  
         [0050]     Accordingly, the connections between the voltage bias block  302  and the varactors connected to bias voltages sources V 1 , V 2  and V 3  can be considered to be an RF ground. Therefore, the varactors C 10 , C 1  and C 13 , located on the left hand side (LHS) of the circuit, are effectively connected in parallel at radio frequencies. The total capacitance of the frequency-determining components on the LHS of the circuit is the capacitance resulting from the connection of capacitor C 0  in series with the total capacitance of the parallel-connected varactors C 10 , C 1  and C 13 . Similarly, the capacitance of the frequency-determining components on the right-hand side of the circuit comprises capacitor C 0  in series with the parallel combination of the varactors C 13 , C 14  and C 15 . The total capacitance of the frequency-determining components in the oscillator  300  is equal to the overall capacitance of the frequency determining capacitances (C 1 , C 13 , C 14 , C 15 ) on the RHS in series with the overall capacitance of the frequency determining capacitances (C 0 , C 10 , C 11 , C 12 ) on the LHS.  
         [0051]     The total inductance of the inductive frequency-determining components in the oscillator  300  is equal to the inductance of inductor L 1  in series with that of the inductor L 2 .  
         [0052]     The transistors Q 0 , Q 1  are connected at their bases to a bias voltage source VB via resistors R 3  and R 4  respectively so as to forward bias their base-emitter junctions.  
         [0053]     The transistors Q 0 , Q 1  are capacitively cross-coupled. Specifically, coupling capacitors C 2  and C 3  couple the signals generated at the collectors of transistors Q 1  and Q 0  to the bases of the transistors Q 0 , Q 1  respectively to cause oscillation in a well-known manner. The varactor pairs C 10 , C  13 ; C 10 , C 14 ; and C 12 , C 15  are selected such that the varactors of each pair have the same CN characteristic. However, the CN characteristic of each pair may be selected to have a different characteristic and in particular different capacitance ranges. In a preferred embodiment, the CN characteristic of the complete set of varactors follows a square law curve in order to achieve a linearized frequency/voltage characteristic for the voltage controllable oscillator. This can be achieved, for example, by use of a varactor associated with the highest offset voltage which has a characteristic having a steeper CN curve and extends over a larger capacitive range.  
         [0054]     Variations may be made without departing from the scope of the invention. For example, the control means may comprise a plurality of control sources connected to the plurality of variable capacitance elements; or a common offset bias and a plurality of different value control sources connected to the capacitance elements. Furthermore, the circuit arrangement  100  may be used for a tunable filter or any other application requiring a linearized variable capacitance.  
         [0055]     A presently preferred embodiment of the subject invention is shown in  FIG. 2 .  
         [0056]     While at least one presently preferred embodiment of the invention has been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.