Oscillator circuit

Abstract

Voltage-controlled oscillator (200), comprising: an oscillator circuit (100); and an operating point control circuit unit 202; wherein the oscillator circuit (100) comprises a transistor (T1) and first and second coupled transmission lines (102, 104), wherein the oscillator circuit (100) is configured to provide a variable load impedance at a first end (114) of a signal line (106) of the first transmission line (102), such that a variable inductance between the first and second ends (116, 118) of a signal line (110) of the second transmission line (104) is provided depending on the variable load impedance, wherein the oscillator circuit (100) is configured to provide a variable inductance between the first and second ends (116, 118) of a signal line (110) of the second transmission line (104).118) the variable inductance provided of the signal line (110) of the second transmission line (104) by adjusting the variable load impedance provided at the first end (114) of the signal line (106) of the first transmission line (102), wherein the variable inductance provided between the first and second ends (116, 118) of the signal line (110) of the second transmission line (104) forms a frequency-determining element of the oscillator circuit (100); wherein the second transmission line (104) is connected in series between a base of the transistor (T1) and a node (VB2) of the operating point control circuit unit (202).

Description

Technical FieldEmbodiments of the present invention relate to an oscillator circuit for a voltage controlled oscillator. Some embodiments relate to a voltage controlled oscillator comprising the oscillator circuit and a method for operating the oscillator circuit. Some embodiments relate to an electrically variable inductor for a wide VCO frequency tuning range (VCO=Voltage Controlled Oscillator).BACKGROUND OF THE INVENTIONFor next generation automotive radar applications (e.g., 77 GHz long range radar and 79 GHz short range radar applications) and very high data rate communications, a VCO tuning range of around 5 GHz over a temperature operating range of -40 to 125°C is required. In the VCO designs, an additional 3.5 GHz should be added to compensate for the VCO frequency temperature response and also ensure high production yield. The VCO should therefore have an overall electrically tunable range of around 8.5 GHz while also meeting the very demanding phase noise requirement. Further, a VCO tuning voltage range of 0.3 to 2.8 V is required due to the low cost requirement by using only a 3.3 V supply voltage.Actual VCO architectures cannot meet this 5 GHz tuning range over the full temperature range with the required phase noise specification. A transition solution is discussed or examined. One idea is to have two or more VCO designs on a chip. This idea, however, requires more chip area and also requires a complex algorithm for selecting between the various VCO operations during operation of the system. Another idea is to use a switchable capacitance for selecting the VCO frequency bands. However, this idea degrades the VCO continuous tuning range (perhaps unable to meet the requirement of a large electrical tuning range for short range radar application) and also degrades the VCO phase noise.WO 98 / 52 277 A1 shows a varactor adjustable resonator having two coupled microstrip lines and a voltage controlled oscillator using said resonator.BRIEF DESCRIPTION OF THE INVENTIONEmbodiments of the present invention provide an oscillator circuit for a voltage controlled oscillator, the oscillator circuit comprising first and second coupled transmission lines, the oscillator circuit being configured to provide a variable load impedance at a first end of a signal line of the first transmission line such that a variable inductance is provided between first and second ends of a signal line of the second transmission line depending on the variable load impedance, the oscillator circuit being configured to adjust the variable inductance provided between the first and second ends of the signal line of the second transmission line by adjusting the variable load impedance provided at the first end of the signal line of the first transmission line, the variable inductance provided between the first and second ends of the signal line of the second transmission line forming a frequency determining element of the oscillator circuit.By further embodiments, a voltage controlled oscillator is provided comprising an oscillator circuit comprising first and second coupled transmission lines, the oscillator circuit being configured to provide a variable load impedance at a first end of a signal line of the first transmission line such that a variable inductance is provided between first and second ends of a signal line of the second transmission line depending on the variable load impedance, the oscillator circuit being configured to adjust the variable inductance provided between the first and second ends of the signal line of the second transmission line by adjusting the variable load impedance provided at the first end of the signal line of the first transmission line, the variable inductance provided between the first and second ends of the signal line of the second transmission line forming a frequency determining element of the oscillator circuit.By further embodiments, a method of operating an oscillator circuit for a voltage controlled oscillator is provided, the oscillator circuit comprising first and second coupled transmission lines, the method comprising:providing a variable load impedance at a first end of a signal line of the first transmission line so as to provide a variable inductance between first and second ends of a signal line of the second transmission line depending on the variable load impedance; andadjusting the variable inductance provided between the first and second ends of the signal line of the second transmission line by adjusting the variable load impedance provided at the first end of the signal line of the first transmission line, wherein the variable inductance provided between the first and second ends of the signal line of the second transmission line constitutes a frequency-determining element of the oscillator circuit.Brief Description of the DrawingsEmbodiments of the present invention will be described herein with reference to the accompanying drawings, in which: FIG. 1 is an explanatory view of an oscillator circuit for a voltage controlled oscillator according to an embodiment; FIG. 2 is a schematic view showing a simulation configuration of an oscillator circuit for a voltage controlled oscillator according to an embodiment; FIG. 3 is a schematic view of a simulation structure of a voltage controlled oscillator according to an embodiment; FIG. 4 is a graph showing simulation results of an inductance provided by first and second coupled transmission lines plotted against a fine tuning voltage; FIG. 5 ashows in a diagram simulation results of an oscillator frequency plotted over the fine tuning voltage for a plurality of different coarse tuning voltages; FIG. 5 bshows in a diagram simulation results of the oscillator frequency plotted against the fine tuning voltage for a coarse tuning voltage of 1.5 V; FIG. 5 cshows in a diagram simulation results of a sensitivity versus the fine tuning voltage of an oscillation frequency plotted for a plurality of different coarse tuning voltages; FIG. 5 d is a graph showing simulation results of sensitivity of the oscillation frequency plotted against the fine tuning voltage for a coarse tuning voltage of 1.5 V; FIG. 6 is a schematic view of a VCO including an oscillator circuit; FIG. 7 is a schematic view of a part of the oscillator circuit of the VCO shown in FIG. 6; FIG. 8 shows a three-dimensional view of the first and second coupled transmission lines; and FIG. 9 shows a flow chart of a method for operating an oscillator circuit for a voltage controlled oscillator.Identical or equivalent elements or elements having the same or equivalent functionality are denoted by identical or equivalent reference numerals in the following description.DETAILED DESCRIPTION OF THE INVENTIONFIG. 1 is an explanatory view of an oscillator circuit 100 for a voltage controlled oscillator. The oscillator circuit 100 includes first and second coupled transmission lines 102 and 104.First and second coupled transmission lines 102 and 104 may each include a signal line and a ground line. In detail, the first transmission line 102 may include a signal line 106 and a ground line 108, where the second transmission line 104 may include a signal line 110 and a ground line 112. Further, in some embodiments, first and second coupled transmission lines 102 and 104 may include a common ground line 108, 112, as exemplarily shown in FIG. 1.The oscillator circuit 100 may be configured to provide a variable load impedance Z_load at a first end 114 of the signal line 106 of the first transmission line 102 such that a variable inductance L_eff is provided between first and second ends 116 and 118 of the signal line 110 of the second transmission line 104 depending on the variable load impedance Z_load. Further, the oscillator circuit 100 may be configured to adjust the variable inductance L_effprovided between the first and second ends 116 and 118 of the signal line 110 of the second transmission line 104 by adjusting the variable load impedance Z_loadprovided at the first end 114 of the signal line 106 of the first transmission line 102.For example, the oscillator circuit 100 may comprise a variable load impedance element configured to provide the variable load impedance Z_load at the first end 114 of the signal line 106 of the first transmission line 102. The variable load impedance element may be connected in series between the first end 114 of the signal line 106 of the first transmission line 102 and a first end 126 of a ground line 108 of the first transmission line 102. The variable load impedance element may include, for example, a varactor, a PIN diode, or a MOS device for adjusting the variable load impedance Z_loadof the variable load impedance element to adjust the variable inductance L_effprovided between the first and second ends 116 and 118 of the signal line 110 of the second transmission line 104.The variable inductance L_eff provided between the first and second ends 116 and 118 of the signal line 110 of the second transmission line 104 constitutes a frequency determination element of the oscillator circuit 100. In other words, an oscillator frequency of the oscillator circuit depends on the variable inductance L_effprovided between the first and second ends 116 and 118 of the signal line 110 of the second transmission line 104.As shown in FIG. 1, the first and second coupled transmission lines 102 and 104 may be arranged parallel to each other and extend along a longitudinal direction 122 (e.g., along a geometric line).Further, a length of the first and second coupled transmission lines 102 and 104 should be less than a quarter-wave length (λ / 4) to have inductive behavior, where λ is a wavelength of a signal propagating along the signal line 106 of the first transmission line 102 or the signal line 110 of the second transmission line 104.The oscillator circuit 100 may be configured to provide a reference potential for a second end 120 of the signal line 106 of the first transmission line 102. The reference potential can be, for example, a ground potential, a high-frequency ground potential or a high-frequency clock range ground potential.For example, the second end 120 of the signal line 106 of the first transmission line 102 and a second end 124 of the ground line 108 of the first transmission line 102 may be connected to each other.FIG. 2 is a schematic view showing a simulation configuration of an oscillator circuit for a voltage controlled oscillator according to an embodiment.More specifically, in FIG. 2, the signal lines 106 and 110 of the first and second coupled transmission lines 102 and 104 are simulated or modeled by first and second impedance elements 130 and 132. In particular, the signal line 106 of the first transmission line 102 is simulated or modeled by the first impedance element 130, wherein the signal line 110 of the second transmission line 104 is simulated or modeled by the second impedance element 132.The first impedance element 130 may be connected in series between the nodes 114 and 120 corresponding to the ends 114 and 120 of the signal line 106 of the first transmission line (see FIG. 1 ). Similarly, the second impedance element 132 may be connected in series between the nodes 116 and 118 corresponding to the ends 116 and 118 of the signal line 110 of the second transmission line 104.As shown in FIG. 2, the node 114 (corresponding to the first end 114 of the signal line 106 of the first transmission line 102) is connected to a variable load impedance element 136 configured to provide the variable load impedance Z_load. More specifically, the variable load impedance element 136 may be connected in series between the node 114 and a reference potential 138, e.g., a ground potential or a high frequency ground potential. By adjusting the variable load impedance Z_loadprovided by the variable load impedance element 136, the variable inductance L_effprovided between the nodes 116 and 118 (corresponding to the first and second ends 116 and 118 of the signal line 110 of the second transmission line 104) may be adjusted.Further, the node 120 (corresponding to the signal line 106 of the first transmission line 102 in the second end 120) may be connected to the reference potential 138, e.g., a ground potential or a high-frequency ground potential.In contrast to the commonly used method of varying varactor capacitance, the concept disclosed herein utilizes an electrically tunable inductance that can also vary the VCO oscillation frequency. In this way, the VCO performance is not degraded and a large chip area is not required either.The variable inductance may be implemented by a tunable load impedance 136, which may be controlled by a control voltage. As shown in FIG. 2, the effective inductance L_eff of a coupled transmission line 102 and 104 is also determined by the inductive coupling factor M. If the load impedance Z_load is zero, then the return path current or coupling factor is maximized. If the load impedance Z_load is infinite, then the return path current will be zero and the coupling factor will be minimized. In this way, the effective L_eff is changed with the load impedance Z_load. The variable load impedance Z_load could be embodied in various ways, e.g. a varactor, a PIN diode or a MOS device with a control voltage.In general, the oscillation frequency f_ osc of a VCO may be determined by f_ osc=1 / (2*pi*sqrt(L*C).In a usual VCO, the capacitance C is realized by a varactor and the oscillation frequency f_osc is tuned by changing the varactor capacitance.In contrast, according to the concept disclosed herein, the oscillator circuit 100 comprises first and second coupled transmission lines 102 and 104, wherein the oscillator circuit is configured to adjust the variable inductance L_effprovided between the first and second ends 116 and 118 of the signal line 110 of the second transmission line 104 by adjusting the variable load impedance Z_loadprovided at the first end 114 of the signal line 106 of the first transmission line 102.Thereby, L_eff can be determined by L_eff = L1 + i2 / il*M, where L_eff is changed by changing the load impedance of Z_load. As already mentioned, at Z_load=0, the coupling of the two lines is the highest (maximum return path current) and the effective inductance L_eff is the smallest. When Z_load= ∞, the coupling between the two lines is the lowest (no return path current) and the inductance L_eff is the highest.The coupling structure can be designed as a coupled transmission line, strip line or overlapped suspended transmission line. The variable load impedance Z_loadmay be embodied by any impedance tunable device (e.g., a varactor, a PIN diode, a heterojunction bipolar transistor, or a MOS device).An advantage of the variable inductance is that the inductance value can be linearly varied with the control voltage. In this way, a linear VCO tuning curve can be achieved. Furthermore, the design of a highly linear PLL (phase-locked loop) is very facilitated.FIG. 3 shows a schematic view of a simulation structure 140 of a VCO according to an embodiment. The simulation setup 140 includes a VCO core 142 and a VCO buffer 144.The VCO core 142 includes a positive supply input VDD, a first coarse tuning voltage input v_tunel, a second coarse tuning voltage input v_tune2, a bias current input i_vcobias, a fine tuning voltage input SWL_P1, a current input SWL_P2, a negative supply input VSS, and differential oscillator outputs RF_OUT and RF_OUTN.Furthermore, the simulation structure 114 comprises a coarse tuning voltage source (VT_Var: VTC) 146 configured to provide a coarse tuning voltage for the first and second coarse tuning voltage inputs v_tunel and v_tune2 of the VCO core 142, a fine tuning voltage source (VT_IND: VTF) 148 configured to provide a fine tuning voltage for the fine tuning voltage input SWL_P1 of the VCO core 142, a first current source 150 configured to provide a bias current for the bias current input i_vcobias of the VCO core 142, a second current source 152 configured to provide a (constant) current for the current input SWL_P2 of the VCO core 142, and a current sensor 154, configured to sense a current flowing into the supply input VDD of the VCO core 142. The above-mentioned elements may be direct current elements (DC = direct current).The VCO buffer 146 includes differential inputs Pin and PinN, a buffer current input ibuf, a negative supply input VSS, a positive supply input VCC, differential outputs Out and OutN.Furthermore, the simulation setup 140 comprises a third current source 156 configured to provide a current to the buffer current input ibufof the VCO buffer 146 and a current sensor 158 configured to sense a current flowing into the supply input VCCof the VCO buffer 146. The above-mentioned elements may be direct current (DC = direct current).FIG. 4 shows a graph of simulation results of an inductance provided between first and second ends 116 and 118 of the signal line 110 of the second transmission line 104, plotted against a fine tuning voltage VT_IND. Here, the ordinate denotes the inductance L_eff in pH, and the abscissa denotes the fine tuning voltage VT_IND in V.Here, the variable load impedance element was realized by a varactor having a control voltage.Further, the inductance was changed by changing the control voltage by about 25% (from 85 pH to 68 pH).As shown in FIG. 4, the inductance curve plotted against fine tuning voltage is highly linear.FIG. 5 ashows in a diagram simulation results of an oscillator frequency plotted against the fine tuning voltage for a plurality of different coarse tuning voltages. Here, the ordinate denotes the oscillator frequency f_ osc in Hz, and the abscissa denotes the fine tuning voltage VT_IND in V.FIG. 5 bshows in a diagram simulation results of the oscillator frequency plotted against the fine tuning voltage for a coarse tuning voltage of 1.5 V. In this case, the ordinate denotes the oscillator frequency f_ osc in Hz, the abscissa denotes the fine tuning voltage VT_IND in V.FIG. 5 cshows in a diagram simulation results of a sensitivity of the oscillation frequency K_vco plotted against the fine tuning voltage for a plurality of different coarse tuning voltages. Here, the ordinate denotes the sensitivity of the oscillation frequency in Hz / V, and the abscissa denotes the fine tuning voltage VT_IND in V.FIG. 5 dshows in a diagram simulation results of a sensitivity of the oscillation frequency K_vco plotted against the fine tuning voltage for a coarse tuning voltage of 1.5 V. In this case, the ordinate denotes the sensitivity of the oscillation frequency in Hz / V, wherein the abscissa denotes the fine tuning voltage VT_IND in V.As shown in Figs. 5a to 5d, the total VCO tuning range is around 9.3 GHz. The sensitivity of the oscillation frequency K_VCO for the selected frequency band is almost constant.Further, as shown in Figs. 5a to 5d, another advantage of using this variable inductance is that the obtained oscillation frequency f_osc curve is extremely linear and the design of a highly linear PLL is very facilitated.FIG. 6 shows a schematic view of a VCO 200 with an oscillator circuit 100 (Colpitts oscillator).The VCO 200 includes a first operating point control circuit unit 202, a second operating point control circuit 204, and a balanced oscillator circuit 100 having a positive oscillator circuit part 208 and a negative oscillator circuit part 210.The first operating point control circuit 202 includes a resistor R 6, a resistor R 5, a transistor T 4, and a resistor R 5 connected in series between a positive supply node VCC and a negative supply node VEE.The second operating point control circuit 204 includes a resistor R 3, a transistor T 3, and a resistor R 2 connected in series between the positive supply node VCC and the negative supply node VEE. Further, the second operating point control circuit 204 includes a transmission line LE 2, a transistor T 2, and a resistor R 1 connected in series with the oscillation circuit 100 and a transmission line LC 3 between the positive supply node VCC and the negative supply node VEE.The positive oscillator circuit 208 includes a transmission line LC 2, a transmission line LC 1, a transistor T 1, and a transmission line LE connected in series between the transmission line LC 3 and the transmission line LE 2 of the second operating point control circuit 204. Further, the positive oscillator circuit 208 includes a capacitor CC and a varactor Cvar1connected in series between an emitter of the transistor T1and a coarse tuning voltage node Vrune1. Further, the positive oscillator circuit 208 includes a capacitor CC and a varactor Cvar2connected in parallel between a base of the transistor T1and a second coarse tuning voltage node Vrune2. Further, the positive oscillator circuit 208 includes a capacitor C 2 connected in parallel between the emitter and the base of the transistor T 1. In addition, the base of transistor T1 is connected to an LB network 211 comprising the first and second coupled transmission lines 102 and 104, which will be described in detail later with reference to FIG. 7.The negative oscillator circuit 210 includes a transmission line LC 2N, a transmission line LC 1N, a transistor T 1N, and a transmission line LEN connected in series between the transmission line LC 3 and the transmission line LE 2 of the second operating point control circuit 204. Further, the negative oscillator circuit 210 includes a capacitor CCN and a varactor Cvar1N connected in series between an emitter of the transistor T1N and a coarse tuning voltage node Vrune1. Further, the negative oscillator circuit 210 includes a capacitor CCN and a varactor Cvar2N connected in series between a base of the transistor T1N and the second coarse tuning voltage node Vrune2. Further, the negative oscillator circuit 210 includes a capacitor C 2N connected in series between the emitter and the base of the transistor T 1N. In addition, the base of transistor T1N is connected to an LB network 211 comprising first and second coupled transmission lines 102 and 104, which will be described in detail later with reference to FIG. 7.FIG. 7 is a schematic view of the LB network 211 shown in FIG. 6. The LB network 211 includes first and second coupled transmission lines 102 and 104 (LBC and LB). The second transmission line 104 (LB) is connected in series between the nodes VB 1 and VB 2, and thus in series between a base of the transistor T 1 (see FIG. 6 ) and a node between the resistors R 5 and R 6 of the first operating point control circuit 202.Since the LB network 211 shown in FIG. 7 is symmetric, the LB network 211 further includes third and fourth coupled transmission lines 102N and 104N (LBN and LBNC). The fourth transmission line 104N (LBNC) is connected between the nodes VB 1N and VB 2 and thus in series between the base of the transistor T 1N (see FIG. 6 ) and the node between the resistors R 5 and R 6 of the first operating point control circuit 202.Further, the LB network 211 includes a temperature compensation circuit 212 including a transistor T 3 and a resistor R 7 connected in series between a node VB 3 and a negative supply node VEE, and a capacitor C 3 also connected in parallel between the node VB 3 and the negative supply node VEE. A base of the transistor T3 is also connected to the node VB3. Temperature compensation circuit 212 is biased by a current Iref1 which is applied to resistor R8 which is connected to node VB3.The LB network 211 further includes a variable load impedance element 136 and a negative variable load impedance element 136N, wherein the first transmission line 102 (LBC) and the variable load impedance element 136 are connected in series between the node VB 3 and the fine tuning voltage node Vtune 3, wherein the negative first transmission line 102N (LBN) and the negative variable load impedance element 136N are connected in series between the node VB 3 and the fine tuning voltage node Vtune 3.As shown in FIG. 7, a coupled transmission line is used to provide a variable inductance. As a result, a length of the coupled transmission line is scalable. This provides a flexible design and allows easy optimizations of the VCO design.Further, a differential varactor may be used for impedance variation. This leads to a better quality factor of the implemented inductance. Furthermore, this results in reduced sensitivity for common mode interference of supply and ground.Furthermore, FIG. 7 shows a particular temperature compensation circuit 212 biased on another side of the coupled transmission line. With a reference source (Iref1) that has a negative temperature slope, the potential at node VB3 will decrease with increasing temperature (greater back voltage across varactor Cvar3 and Cvar3N). This will cause a smaller inductance (see FIGS. 4 and 5 ) and will then increase the oscillator frequency f_osc.Varactors Cvar2and Cvar2N(see FIG. 6 ) may be used to further extend the VCO tuning ranges.FIG. 8 shows a three-dimensional view of the first and second coupled transmission lines 102 and 104. The first transmission line 102 includes a signal line 106 (TL: LBC) and a ground line 108. The second transmission line 104 includes a signal line 110 (TL: LB) and a ground line 112. As exemplarily shown in FIG. 8, the first and second coupled transmission lines may include a common ground line 108, 112 (GND).FIG. 9 shows a flow chart of a method 300 for operating an oscillator circuit 100 for a voltage controlled oscillator. The method comprises a step 302 of providing a variable load impedance at a first end of a signal line of the first transmission line so as to provide a variable inductance between first and second ends of a signal line of the second transmission line depending on the variable load impedance; and a step 304 of adjusting the variable inductance provided between the first and second ends of the signal line of the second transmission line by adjusting the variable load impedance provided at the first end of the signal line of the first transmission line, wherein the variable inductance provided between the first and second ends of the signal line of the second transmission line constitutes a frequency determination element of the oscillator circuit.Further embodiments provide a variable inductance controlled by a control voltage.Although some aspects have been described in connection with an apparatus, it is clear that these aspects also represent a description of the corresponding method, where a block or a device corresponds to a method step or a feature of a method step. Analogously, the aspects described in connection with a method step also represent a description of a corresponding block or object or feature of a corresponding device. Some or all of the method steps may be performed by (or by) a hardware device, such as a microprocessor, a programmable computer, or an electronic circuit. In some embodiments, one or more of the most important method steps may be performed by such a device.In some embodiments, a programmable logic device (e.g., a field programmable logic array) may be used to perform some or all of the functionalities of the methods described herein. In some embodiments, a field programmable logic array may cooperate with a microprocessor to perform any of the methods described herein. Generally, the methods are preferably performed by any hardware device.The embodiments described above are illustrative only of the principles of the present invention. It is to be understood that modifications and variations of the arrangements and details described herein will be apparent to others skilled in the art. The intention is, therefore, to be limited only by the scope of the appended claims, and not by the specific details set forth in the description and explanation of the embodiments herein.

Claims

A voltage controlled oscillator (200) comprising: an oscillator circuit (100); and a working point control circuit unit 202; wherein the oscillator circuit (100) comprises a transistor (T1) and first and second coupled transmission lines (102, 104), wherein the oscillator circuit (100) is configured to provide a variable load impedance at a first end (114) of a signal line (106) of the first transmission line (102) such that a variable inductance is provided between first and second ends (116, 118) of a signal line (110) of the second transmission line (104) depending on the variable load impedance, wherein the oscillator circuit (100) is configured between the first and second ends (116, 116, 118) of the signal line (110) of the second transmission line (104) to adjust the variable load impedance provided at the first end (114) of the signal line (106) of the first transmission line (102), wherein the variable inductance provided between the first and second ends (116, 118) of the signal line (110) of the second transmission line (104) constitutes a frequency-determining element of the oscillator circuit (100); wherein the second transmission line (104) is connected in series between a base of the transistor (T1) and a node (VB2) of the operating point control circuit unit (202).The voltage controlled oscillator (200) according to the preceding claim, wherein the first and second coupled transmission lines (102, 104) are arranged parallel to each other and extend along a geometric line (122).The voltage controlled oscillator (200) of any preceding claim, wherein the first and second coupled transmission lines (102, 104) are microstrip lines, strip lines, or overlapped suspended transmission lines.The voltage controlled oscillator (200) of any of the preceding claims, wherein a length of the first and second coupled transmission lines (102, 104) is less than a quarter-wave length (λ / 4), wherein λ is a wavelength of a signal propagating along the signal line of the first or second transmission line (102, 104).The voltage controlled oscillator (200) of any preceding claim, wherein the first and second coupled transmission lines (102, 104) comprise a common ground line.The voltage controlled oscillator (200) according to any of the preceding claims, wherein the oscillator circuit (100) is configured to provide a reference potential to a second end (120) of the signal line (106) of the first transmission line (102).The voltage controlled oscillator (200) according to any one of the preceding claims, wherein a second end (120) of the signal line (106) of the first transmission line (102) and a second end (124) of a ground line (108) of the first transmission line (102) are connected to each other.The voltage controlled oscillator (200) according to any of the preceding claims, wherein the oscillator circuit (100) comprises a variable load impedance element (136) configured to provide the variable load impedance to the first end (114) of the signal line (106) of the first transmission line (102).The voltage controlled oscillator (200) of claim 8, wherein the variable load impedance element (136) is connected in series between the first end (114) of the signal line (106) of the first transmission line (102) and a first end (126) of a ground line (108) of the first transmission line (102).The voltage controlled oscillator (200) according to any one of claims 8 or 9, wherein the variable load impedance element (136) comprises at least one of a varactor, a PIN diode, and a MOS device for adjusting the variable load impedance of the variable load impedance element (136).A voltage controlled oscillator (200), wherein the oscillator circuit (100) is a balanced oscillator circuit (100) having a positive oscillator circuit part (208) and a negative oscillator circuit part (210); wherein the positive oscillator circuit part (208) comprises the transistor (T1) and the first and second coupled transmission lines (102, 104); wherein the negative oscillator circuit part (210) comprises a transistor (T1N) and third and fourth coupled transmission lines (102N, 104N), wherein the oscillator circuit (100) is configured to provide a variable load impedance at a first end of a signal line of the third transmission line (102N) such that a variable inductance is provided between first and second ends of a signal line of the fourth transmission line (104N) depending on the variable load impedance, wherein the oscillator circuit (100) is configured to:, the variable inductance provided between the first and second ends of the signal line of the fourth transmission line (104N) by adjusting the variable load impedance provided at the first end of the signal line of the third transmission line, wherein the variable inductance provided between the first and second ends of the signal line of the fourth transmission line constitutes a frequency-determining element of the oscillator circuit (100); wherein the fourth transmission line (104N) is connected between a base of the transistor (T1N) of the negative oscillator circuit part (210) and the node (VB2) of the operating point control circuit (202).A method (300) of operating a voltage controlled oscillator, the voltage controlled oscillator comprising an oscillator circuit and a working point control circuit unit, the oscillator circuit comprising a transistor and first and second coupled transmission lines, the second transmission line being connected in series between a base of the transistor and a node of the working point control circuit unit, the method comprising: providing (302) a variable load impedance at a first end of a signal line of the first transmission line such that a variable inductance is provided between first and second ends of a signal line of the second transmission line depending on the variable load impedance; and adjusting (304) the variable inductance provided between the first and second ends of the signal line of the second transmission line by adjusting the variable load impedance provided at the first end of the signal line of the first transmission line, wherein the variable inductance provided between the first and second ends of the signal line of the second transmission line constitutes a frequency-determining element of the oscillator circuit.

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