Oscillator circuit

Abstract

A differential Colpitts oscillator circuit is described which provides a larger tuning range, has better phase noise and uses less power than conventional differential Colpitts oscillator circuits. The circuit is characterized by a capacitive ladder in which only variable capacitor is used for tuning the circuit. In some embodiments, a variable capacitor can be used for fine tuning.

Claims

1. A circuit comprising: a first transistor and a second transistor; a first inductor connected in a first line between a first gate of the first transistor and a connecting point; a third transistor and a fourth transistor; a second inductor connected in a second line between a second gate of the third transistor and the connecting point; a first pair of capacitors connected between the first gate and a first source of the first transistor; a second pair of capacitors connected between the second gate and a second source of the third transistor; and a variable capacitor connected to the first pair of capacitors and the second pair of capacitors, wherein a fourth gate of the fourth transistor is connected to a first center-tap point of the first inductor and a third gate of the second transistor is connected to a second center-tap point of the second inductor.

2. The circuit according to claim 1, the variable capacitor being adjustable to adjust a tuning range of the circuit.

3. A circuit comprising: a first transistor comprising a first terminal and a second terminal that is a gate terminal; a second transistor comprising a third terminal and a fourth terminal; a third transistor comprising a fifth terminal and a sixth terminal that is a gate terminal; a fourth transistor comprising a seventh terminal and an eighth terminal; a first inductor comprising a ninth terminal and a tenth terminal; a second inductor comprising an eleventh terminal and a twelfth terminal; a resistor comprising a thirteenth terminal; and a capacitive ladder comprising a fourteenth terminal, a fifteenth terminal, a sixteenth terminal, and a seventeenth terminal, wherein the second terminal, the fifteenth terminal, and the ninth terminal form a first node, wherein the tenth terminal and the eleventh terminal form a second node, wherein the twelfth terminal, the seventeenth terminal, and the sixth terminal form a third node, wherein the first terminal, the fourteenth terminal, and the third terminal form a fourth node, wherein the fifth terminal, the sixteenth terminal, and the seventh terminal form a fifth node, and wherein the fourth terminal, the eighth terminal, and the thirteenth terminal form a sixth node, wherein the capacitive ladder comprises: a first capacitor comprising an eighteenth terminal; a second capacitor comprising a nineteenth terminal; a third variable capacitor comprising a twentieth terminal and a twenty-first terminal; a fourth capacitor comprising a twenty-second terminal; and a fifth capacitor comprising a twenty-third terminal, wherein the eighteenth terminal, the nineteenth terminal, and the twentieth terminal form a seventh node, and wherein the twenty-first terminal, the twenty-second terminal, and the twenty-third terminal form an eighth node.

4. The circuit according to claim 3, further comprising a sixth variable capacitor comprising a twenty-fourth terminal and a twenty-fifth terminal, wherein the twenty-fourth terminal additionally forms the fourth node, and wherein the twenty-fifth terminal additionally forms the fifth node.

5. The circuit according to claim 4, wherein the first inductor further comprises a first center-tap terminal, wherein the second inductor further comprises a second center-tap terminal, wherein the second transistor further comprises a twenty-sixth terminal that is a gate terminal, wherein the fourth transistor further comprises a twenty-seventh terminal that is a gate terminal, wherein the twenty-seventh terminal and the first center-tap terminal form a ninth node, and wherein the twenty-sixth terminal and the second center-tap terminal form a tenth node.

6. The circuit according to claim 5, wherein the sixth variable capacitor is adjustable to adjust a tuning range of the circuit.

7. The circuit according to claim 5, wherein the third variable capacitor is adjustable to adjust a tuning range of the circuit.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The above, as well as additional, features will be better understood through the following illustrative and non-limiting detailed description of example embodiments, with reference to the appended drawings.

(2) FIG. 1 is a schematic illustration of a conventional single-ended Colpitts oscillator circuit, according to an example embodiment.

(3) FIG. 2 is a schematic illustration of a conventional differential Colpitts oscillator circuit, according to an example embodiment.

(4) FIG. 3 is a schematic illustration of another conventional differential Colpitts oscillator circuit, according to an example embodiment.

(5) FIG. 4 illustrates voltage swing at gates of transistors M3 and M4 of the circuit shown in FIG. 3, according to an example embodiment.

(6) FIG. 5 illustrates voltages across transistors M3 and M4 of the circuit shown in FIG. 3, according to an example embodiment.

(7) FIG. 6 is a schematic illustration of a differential Colpitts oscillator circuit with low power supply voltage having a low power supply, according to an example embodiment.

(8) FIG. 7 illustrates voltage swing at gates of transistors for the circuit shown in FIG. 6, according to an example embodiment.

(9) FIG. 8 illustrates voltages across transistors for the circuit shown in FIG. 6, according to an example embodiment.

(10) FIG. 9 is a schematic illustration of a differential Colpitts oscillator circuit, according to an example embodiment.

(11) FIG. 10 is a schematic illustration of a single-ended equivalent capacitive ladder or voltage divider forming part of the circuit of FIG. 9, according to an example embodiment.

(12) FIG. 11 illustrates a comparison in the performance of circuits shown in FIGS. 2, 3 and 7 in a cadence simulation, according to an example embodiment.

(13) All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary to elucidate example embodiments, wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION

(14) Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings. That which is encompassed by the claims may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example. Furthermore, like numbers refer to the same or similar elements or components throughout.

(15) A conventional single-ended Colpitts oscillator circuit 10 is shown in FIG. 1 and comprises first and second capacitors C1, C2, a resistor R, and an inductor L connected between a supply voltage VDD and ground 12. A transistor 14 is provided for applying a bias 16 to the circuit 10. A capacitive voltage divider is formed by the first and second capacitors C1 and C2 which provides a positive feedback path. The inductor L and the capacitive voltage divider formed by the first and second capacitors C1 and C2 together form an oscillation tank.

(16) The single-ended Colpitts oscillator circuit 10 generally has lower phase noise than most other oscillator topologies or circuits and its use is common in implementations where phase noise is an issue. However, the circuit 10 is typically sensitive to single-ended parasitics.

(17) FIG. 2 is a schematic illustration of a conventional differential Colpitts oscillator circuit 20. The circuit 20 can be considered to comprise two portions 20a, 20b corresponding to two identical single-ended Colpitts oscillators which are joined by a variable or tunable capacitor C2 at center-tap points 24a, 24b with cross-connections between transistors as described below.

(18) Portion 20a comprises transistors M1 and M3 where transistor M1 corresponds to transistor 14 in FIG. 1. A bias 26a is applied to transistor M1. As shown, a capacitor C1.sub.a is positioned across transistor M1 and is connected to the line joining center-tap points 24a, 24b containing the variable or tunable capacitor C2. Inductor L.sub.a is connected to power supply VDD and to transistor M1. Transistor M3 is connected to ground 22.

(19) Similarly, portion 20b comprises transistors M2 and M4 where transistor M2 corresponds to transistor 14 in FIG. 1. A bias 26b is applied to transistor M2. As shown, a capacitor C1.sub.b is positioned across transistor M2 and is connected to the line joining center-tap points 24a, 24b containing the variable or tunable capacitor C2. Inductor L.sub.b is connected to power supply VDD and to transistor M2. Transistor M4 is connected to ground 22.

(20) The center-tap points 24a, 24b are located between respective ones of the transistor pairs M1, M3 and M2, M4. The variable or tunable capacitor C2 is connected to a line joining the center-tap points 24a, 24b.

(21) Connections are provided between the respective transistor pairs M1, M3 and M2, M4 and respective gates of transistor M4 and transistor M3 as shown. The connections are made at points 28a and 28b with respect to respective ones of the transistor pairs M1, M3 and M2, M4. A differential output is provided by coupling the two portions 20a, 20b by sharing variable or tunable capacitor C2.

(22) The differential Colpitts oscillator circuit 20 has the potential advantage of having low phase noise which is the same as the single-ended Colpitts oscillator circuit as shown in FIG. 1. Although the phase noise has not been increased, the circuit 20 is generally insensitive to common-mode parasitics and common-mode interferences.

(23) Another conventional differential Colpitts oscillator circuit 30 is shown in FIG. 3. This circuit is similar to that described by Kim et al. as discussed above. As before, the circuit 30 can be considered to comprise two portions 30a, 30b corresponding to two identical single-ended Colpitts oscillators which are joined by a variable or tunable capacitor C2 at tap points 36a, 36b with cross-connections between transistors as described below.

(24) Portion 30a comprises transistors M1 and M3 and a capacitor C1a connected between line 42a extending between a bias voltage node 40 and gate aa of transistor M1 and line 44 joining center-tap points 36a, 36b containing the variable or tunable capacitor C2. Transistor M1 is connected to voltage supply VDD as shown with transistor M3 connected to ground 32 via resistor 34. Inductor L.sub.a is connected in the line 42a between the bias voltage node 40 and gate aa of transistor M1. Gate a of transistor M3 is connected to line 42b at tap point 38b as shown.

(25) Similarly, portion 30b comprises transistors M2 and M4 and a capacitor C1b between line 42b extending between the bias voltage node 40 and gate bb of transistor M2 and line 44 joining center-tap points 36a, 36b containing the variable or tunable capacitor C2. Transistor M2 is connected to voltage supply VDD as shown with transistor M4 connected to ground 32 via resistor 34. Inductor L.sub.b connected in the line 42b between the bias voltage node 40 and gate bb of transistor M2. Gate b of transistor M4 is connected to line 42a at tap point 38a as shown.

(26) The tap points 36a, 36b are located between respective ones of the transistor pairs M1, M3 and M2, M4. The variable or tunable capacitor C2 is connected to a line joining the center-tap points 36a, 36b.

(27) A differential output is provided by coupling the two portions 30a, 30b by sharing variable or tunable capacitor C2.

(28) However, by cross-coupling the transistors M3 and M4, it was found that the minimum supply voltage VDD was generally equal to the sum of the drain/source voltage Vds for transistor M1 and the threshold voltage Vth for transistor M3. Typically, the minimum power supply voltage is about 0.7 V. Moreover, transistors M1 and M2 generally need to be large so that they can be fully switched by the bias voltage applied at bias voltage node 40.

(29) The conventional way of realizing the tuning range is to tune the variable or tunable capacitor C2. When the capacitance of C2 increases, the oscillation frequency is reduced as the total capacitance C.sub.total increases. The oscillation frequency f can be expressed as:

(30) $f=\frac{1}{2\sqrt{C_{\mathrm{total}}L}}$$\mathrm{where}$$C_{\mathrm{total}}=\frac{2C_1{C}_2}{C_1+2C_2}$

(31) and C1, C2 and L respectively correspond to the capacitance value of C1, the capacitance value of C2 and the inductance of the inductor L.

(32) The feedback voltage V2 can be expressed as:

(33) $\begin{array}{cc}{V}_2=\frac{C_1{V}_1}{C_1+2C_2}& \left(1\right)\end{array}$

(34) where V1 is the power supply voltage VDD, and C1 and C2 are the capacitance values of the capacitors C1 and C2 respectively.

(35) If the feedback voltage V2 decreases below a threshold value, start-up requirements for the oscillator generally cannot be met. The minimum power supply voltage VDD is typically limited by the sum of Vth (from transistor M3) and Vds (from transistor M1). The minimum bias voltage is typically limited by the sum of Vth (from transistor M1), Vth (from transistor M3) and Vds (from the resistor 34).

(36) The limited tuning range can be a significant disadvantage for Colpitts oscillator implementation, and, one problem is how to extend the tuning range without compromising on area, design complexity, and phase noise.

(37) When the capacitance C2 increases, the oscillation frequency and the feedback voltage V2 decrease, and the positive feedback generally cannot sustain the oscillation. One challenge is to maintain the positive feedback voltage while tuning the oscillation frequency.

(38) FIG. 4 illustrates plots of the voltage at gate aa of transistor M1 (line 50), the voltage at gate a of transistor M3 (line 52), and the bias voltage (line 54) applied to the bias node 40 for circuit 30 shown in FIG. 3. It will be appreciated that the voltages are the same for gate bb of transistor M2 and gate a of transistor M4 as transistors M2 and M4 are identical to transistors M1 and M3.

(39) FIG. 5 illustrates plots of the transistor voltages for transistors M1 and M3 for the circuit 30 shown in FIG. 3. The transistor voltages for transistors M2 and M4 are identical to respective ones of those for transistors M1 and M3. The gate/source voltage Vgs and the gate/drain voltage Vds for transistor M3 are shown by lines 60 and 62 respectively. The gate/source voltage Vgs and the gate/drain voltage Vgd for transistor M1 are shown by lines 64 and 66 respectively. It can be seen that the Vgs for transistor M3 is twice that of the Vgs for transistor M1 but the Vgd voltages are approximately the same for transistors M1 and M3. As a result of the difference in Vgs for transistors M3 and M1, transistor M3 can easily be damaged by a high voltage swing.

(40) A schematic illustration of a differential Colpitts oscillator circuit 100 is shown in FIG. 6 which illustrates how a low power supply voltage may be implemented to address the issues with circuit 30 shown in FIG. 3 and described with reference to FIGS. 4 and 5 above. The circuit 100 is similar to that shown in FIG. 3 and comprises two portions 110, 120. Each portion 110, 120 comprises its own power supply VDD with variable or tunable capacitor C2 being shared therebetween as before. Portion 110 includes transistor M1 and transistor M3 connected in series with a resistor R to ground 130, and inductor L.sub.a connected to gate aa of transistor M1. The inductor L.sub.a is connected between the gate aa of transistor M1 and point 140 to which a bias voltage Vbias can be applied as shown.

(41) Similarly, portion 120 includes transistor M2 and transistor M4 connected in series with the resistor R to ground 130 and inductor L.sub.b connected to gate bb of transistor M2. The inductor L.sub.b is connected between gate bb of transistor M2 and the point 140 to which the bias voltage Vbias can be applied as shown.

(42) Variable or tunable capacitor C2 is connected between each of the transistor pairs M1, M3 and M2, M4 as shown at tap points 150, 160. Capacitor C1.sub.a in portion 110 is connected to line 170 between inductor L.sub.a and gate aa of transistor M1 at one end and to line 180 between tap points 150, 160, including the variable or tunable capacitor C2, at the other end. Similarly, capacitor C1.sub.b in portion 120 is connected to line 190 between inductor L.sub.b and gate bb of transistor M2 at one end and to line 180 between tap points 150, 160 at the other end.

(43) Gate a of transistor M4 is connected to a center-tap point A of the inductor L.sub.a in the other portion 110 of the circuit 100 and gate b of transistor M3 is connected to a center-tap point B of the inductor L.sub.b in the other portion 120 of the circuit 100, that is, the inductors L.sub.a, L.sub.b are cross-coupled with the gates a, b of transistors M3, M4 as shown.

(44) The potential advantages of having the center-tapped connection to points A and B of respective inductors L.sub.a and L.sub.b are shown in FIGS. 7 and 8.

(45) FIG. 7 is similar to FIG. 4 but illustrates the plots for the voltages at gate aa of transistor M1 (line 70), at gate a of transistor M4 (line 72), at gate b of transistor M3 (line 74) and the bias voltage applied to point 140 (line 76). As will readily be understood, the plots of the voltage at gate bb of transistor M2 will be identical to that shown by line 70.

(46) FIG. 8 is similar to FIG. 5 illustrating the voltages for transistors M1 and M3. As before, the transistor voltages for transistors M2 and M4 are identical to respective ones of those for transistors M1 and M3. The gate/source voltage Vgs and the gate/drain voltage Vds for transistor M3 are shown by lines 80 and 82 respectively. The gate/source voltage Vgs and the gate/drain voltage Vgd for transistor M1 are shown by lines 84 and 86 respectively. It can be seen that both the Vgs and Vgd values for transistor M3 are now similar to those for transistor M1 and transistor M3 will no longer be damaged by high voltage swings. This means that the size of transistors M1 and M3 can be the same thereby saving area.

(47) The minimum bias voltage applied to point 140 is now no longer dependent on threshold voltage Vth and is now determined by the sum of Vds (for transistor M1) and Vds (for transistor M3). This can provide an improvement of around 100 mV to 200 mV. The voltage swing at gates aa, bb of respective transistors M1, M2 is much higher than that at sources of the associated transistors. As a result, the size of the transistors M3, M4 can be reduced, and an improved layout is achieved for the transistors M1, M2, M3, and M4.

(48) Tuning can be improved using a Colpitts oscillator circuit 200 shown in FIG. 9. The circuit 200 is similar to that shown in FIG. 3 but includes a capacitive ladder or voltage divider 300. As before, the circuit 200 comprises two portions 210, 220 each comprising its own power supply VDD with a variable capacitor C2 shared therebetween as before.

(49) Portion 210 includes transistor M1 and transistor M3 connected in series with a resistor R to ground 230, and inductor L.sub.a connected to gate aa of transistor M1. The inductor L.sub.a is connected between gate aa of transistor M1 and point 240 to which a bias voltage Vbias can be applied as shown.

(50) Similarly, portion 220 includes transistor M2 and transistor M4 connected in series with the resistor R to ground 230 and inductor L.sub.b connected to gate bb of transistor M2. The inductor L.sub.b is connected between gate bb of transistor M2 and the point 240 to which the bias voltage Vbias can be applied as shown.

(51) Gate a of transistor M4 is connected to a center-tap point A of the inductor L.sub.a in the other portion 210 of the circuit 200 and gate b of transistor M3 is connected to a center-tap point B of the inductor L.sub.b in the other portion 220 of the circuit 200, that is, the inductors L.sub.a, L.sub.b are cross-coupled with the gates a, b of transistors M3, M4 as shown.

(52) Variable capacitor C2 is connected between each of the transistor pairs M1, M3 and M2, M4 as shown at tap points 250, 260. Instead of having a single capacitor C1.sub.a, C1.sub.b in each portion 210, 220 of the circuit 200, the single capacitor C1.sub.a, C1.sub.b is replaced by capacitors C1.sub.a, C1.sub.b and C3.sub.a, C3.sub.b arranged in series between line 270 and line 280 in portion 210 and between line 290 and line 280 in portion 220 as shown.

(53) The capacitive ladder or voltage divider 300 comprises capacitors C1.sub.a, C1.sub.b and C3.sub.a, C3.sub.b in each of portions 210, 220 and a tunable capacitor Ctune connected between tap point 310 in portion 210 and tap portion 320 in portion 220 of circuit 200. The capacitance of capacitors C1.sub.a, C1.sub.b and C3.sub.a, C3.sub.b are C1 and C3 respectively (as capacitors C1.sub.a and C1.sub.b are identical, and, capacitors C3.sub.a and C3.sub.b are identical) with the capacitance of capacitor Ctune as Ctune/2 and the capacitance of capacitor C2 as C2/2. As there are now two variable or tunable capacitors C2 and Ctune in parallel, the capacitance value of each capacitor C2 and Ctune is now halved.

(54) By using the center-tapped configuration of FIG. 6 with the capacitive ladder or voltage divider 300, an increased tuning range can generally be achieved while maintaining a low power supply voltage and protecting the transistors as the threshold voltage Vth is no longer relevant for determining the power supply voltage.

(55) FIG. 10 illustrates the effect of the capacitive ladder or voltage divider 300 where the effective capacitance thereof is Ctotal. Only one end of the capacitive ladder is shown which, with the other end, provides a total effective capacitance of Ctotal.

(56) In contrast to FIG. 6, the capacitive voltage divider 300 of FIG. 9 provides positive feedback for the circuit 200.

(57) The total equivalent capacitance Ctotal of the capacitive voltage divider 300 can be expressed as:

(58) $\begin{array}{cc}\mathrm{Ctotal}=\frac{C_1{C}_2{C}_3}{C_1{C}_2+C_1{C}_3+C_2{C}_3+C_2{C}_{\mathrm{tune}}+C_3{C}_{\mathrm{tune}}}+\frac{C_1{C}_2{C}_{\mathrm{tune}}}{C_1{C}_2+C_1{C}_3+C_2{C}_3+C_2{C}_{\mathrm{tune}}+C_3{C}_{\mathrm{tune}}}+\frac{C_1{C}_3{C}_{\mathrm{tune}}}{C_1{C}_2+C_1{C}_3+C_2{C}_3+C_2{C}_{\mathrm{tune}}+C_3{C}_{\mathrm{tune}}}& \left(2\right)\end{array}$

(59) As can be seen from equation (2), the capacitance values C2 and Ctune have equal impact on the total capacitance Ctotal. In equation (2) above, C2 and Ctune correspond to C2 and Ctune respectively as shown in FIG. 9.

(60) The positive feedback voltage can be calculated as:

(61) $V_2=\frac{C_1{C}_3{V}_1}{C_1{C}_2+C_1{C}_3+C_2{C}_3+C_2{C}_{\mathrm{tune}}+C_3{C}_{\mathrm{tune}}}$

(62) with the denominator being simplified to:
(C.sub.1+C.sub.3)C.sub.2+C.sub.1C.sub.3+C.sub.2C.sub.tune+C.sub.3C.sub.tune(3)

(63) where the capacitance C2 has the coefficient of the sum of capacitances C1+C3 and the capacitance Ctune has the coefficient of the capacitance C3 where C2 and Ctune in equation (3) correspond to C2 and Ctune respectively as shown in FIG. 9. This means that the capacitance Ctune has less impact on the feedback voltage. When comparing whether C2 and Ctune should be tuned to determine which one would be more effective, for example, if C2 is tuned 10% more, then the denominator as expressed in equation (3) would become
(C.sub.1+C.sub.3)(C.sub.2*1.1)+C.sub.1C.sub.3+(C.sub.2*1.1)C.sub.tune+C.sub.3C.sub.tune

(64) and, if, Ctune is tuned 10% more, then the denominator as expressed in equation (3) would become
(C.sub.1+C.sub.3)C.sub.2+C.sub.1C.sub.3+C.sub.2(C.sub.tune*1.1)+C.sub.3(C.sub.tune*1.1)

(65) As a result, positive feedback voltage changes when Ctune is tuned are smaller than those when C2 is tuned.

(66) Although the capacitor C2 is a variable capacitor, it is proposed to use only capacitor Ctune to effect the frequency tuning; but fine-tuning of the oscillator circuit can be achieved by tuning capacitor C2.

(67) The term fine-tuning as used herein refers to making small precise adjustments in order to achieve the best or desired performance. This is in contrast to the term tuning which provides larger less precise adjustments.

(68) Cadence simulations have been performed to verify the improvement of the tuning for the differential Colpitts oscillator circuit 200 as shown in FIG. 9. Identical cadence simulations have also been performed using a conventional differential Colpitts oscillator circuit 30 as shown in FIG. 3. The circuit 30 shown in FIG. 3 is similar to that shown in FIG. 9 but without the capacitive ladder or voltage divider 300 and center-tapped inductors.

(69) Cadence simulations indicated that the circuit 200 (FIG. 4) has improved phase noise performance at low frequencies. Table 1 illustrates the results of a simulation showing a comparison between a conventional tuning scheme and the tuning scheme of the present disclosure (proposed tuning scheme).

(70) TABLE-US-00001 TABLE 1 conventional tuning scheme proposed tuning scheme C1 (pf) 4.2 4.2 3.0 3.0 3.0 C2 (pf) 0.2 0.2 0.2 0.2 0.2 C3 (pf) 1.0 1.0 1.0 1.0 1.0 Ctotal (pf) 0.25 1.0 0.25 0.465 1.0 PN@1 115.8 115.7 115.2 115.4 112.6 MHz PN@10 66.5 65.9 69.2 66.3 59.6 kHz I.sub.supply 926 1270 899 1020 1200 (A) f.sub.0 (GHz) 4.51 4.12 4.54 4.12 3.60 Tuning 3.98% 11.5% range

(71) As shown, the tuning range has been extended from 3.98% to 11.5% for the differential Colpitts oscillator circuit as shown in FIG. 9.

(72) In FIG. 11, the results obtained for phase noise from the simulations for circuit 30 of FIG. 3 is indicated by line 400 and the circuit 200 of FIG. 9 by line 440. Additionally, for comparison, the results for a differential Colpitts oscillator circuit without noise shifting is shown by line 480.

(73) In effect, it was found that tuning capacitor Ctune instead of C2 provides a larger tuning range for the oscillator circuit with better phase noise and less power consumption.

(74) While some embodiments have been illustrated and described in detail in the appended drawings and the foregoing description, such illustration and description are to be considered illustrative and not restrictive. Other variations to the disclosed embodiments can be understood and effected in practicing the claims, from a study of the drawings, the disclosure, and the appended claims. The mere fact that certain measures or features are recited in mutually different dependent claims does not indicate that a combination of these measures or features cannot be used. Any reference signs in the claims should not be construed as limiting the scope.