Method and apparatus for voltage controlled oscillator with distributed active transformer core

Abstract

Embodiments of the present disclosure relate to methods and apparatus for voltage controlled oscillators comprising a distributed active transformer core. Embodiments of the present disclosure relate to voltage controlled oscillators comprising a plurality of oscillator cores, magnetically coupled in series.

Description

[0001] Cross Reference to Related Applications

[0002] This application claims the benefit of French application number 1908586, filed on July 29, 2019, which is hereby incorporated by reference herein. TECHNICAL FIELD

[0003] The present disclosure relates generally to the field of voltage controlled oscillators (VCOs), and in particular to distributed active transformer VCOs. BACKGROUND

[0004] Radio frequency wireless communications often rely on VCOs to implement local oscillators at transmitters and / or receivers, for example for generating carrier frequencies, clock generation, bit recovery, etc. The high growth in wireless communications, and the scarcity and cost of spectrum reservations, have increased the demand for the number of available frequency channels, especially for mobile communications. This demand has placed increasingly stringent requirements on the phase noise of local oscillators. Indeed, phase noise strongly limits the spectral efficiency of communications over wireless links.

[0005] Moreover, the importance of reducing the phase noise of voltage controlled oscillators is not limited to the field of wireless communications, but also applies to other fields, such as integrated circuit clock generation, sensing and radar applications.

[0006] Phase noise generally corresponds to short-term random frequency fluctuations of the sinusoidal waveform generated by a VCO.

[0007] There is a need for VCOs that can be implemented using silicon-based semiconductor technologies, such as complementary metal-oxide-semiconductor (CMOS) or bipolar CMOS (BiCMOS) technologies, and that have relatively low phase noise. SUMMARY

[0008] It is an object of the embodiments described herein to at least partially address one or more needs in the art.

[0009] According to one embodiment, a voltage controlled oscillator is provided, comprising a plurality of oscillator cores magnetically coupled in series. For example, the oscillator cores are coupled in series by electromagnetic induction (also referred to as inductive coupling), for example via a common inductive loop. For example, each oscillator core comprises a first inductive portion and a second inductive portion, the first inductive portion and the second inductive portion of each of the oscillator cores are magnetically coupled to the common inductive loop, a first end of each of the first conductive portion and the second conductive portion is coupled to an excitation circuit of the oscillator core, and a second end of each of the first conductive portion and the second conductive portion is coupled to a power supply rail.

[0010] According to one embodiment, the plurality of oscillator cores comprises M oscillator cores, where M is equal to 4 or more.

[0011] According to one embodiment, each oscillator core comprises a first inductive portion and a second inductive portion, the first inductive portion and the second inductive portion of each of the oscillator cores being magnetically coupled to a common inductive loop.

[0012] According to one embodiment, the voltage controlled oscillator further comprises a first output line coupled to one end of the common inductive loop and a second output line coupled to another end of the common inductive loop.

[0013] According to one embodiment, the common inductive loop forms a regular polygon shape having at least four sides, and the first inductive portion and the second inductive portion are positioned adjacent to a side of the common inductive loop.

[0014] According to one embodiment, the oscillator core positions its first inductive portion adjacent to a first side of the common inductive loop and positions its second inductive portion adjacent to a second side of the common inductive loop, the first end of each of the first and second conductive portions being coupled to the excitation circuit of the oscillator core.

[0015] According to one embodiment, the common inductive loop comprises a common mode voltage connection at a midpoint along a first side of the sides of the common inductive loop, or at a vertex between two of the sides, and a first differential output line and a second differential output line coupled to a second side opposite the first side of the sides of the common inductive loop.

[0016] According to one embodiment, each oscillator core comprises a varactor and a differential amplifier having a first output node coupled to one end of the first inductive portion and a second output node coupled to one end of the second inductive portion, the varactor being coupled between the first output node and the second output node.

[0017] According to one embodiment, each oscillator core is selectively activatable.

[0018] According to another aspect, there is provided a communication device comprising an antenna circuit for driving one or more antennas, and the voltage controlled oscillator described above. BRIEF DESCRIPTION OF DRAWINGS

[0019] The foregoing features, advantages, and other aspects of the present disclosure are described in greater detail below in the detailed description of specific embodiments, which should be considered in connection with the accompanying drawings, wherein:

[0020] Figure 1 is a plan view of a VCO according to an example embodiment of the present disclosure;

[0021] Figure 2 is a circuit diagram of a Gm block of an oscillator core according to an example embodiment of the present disclosure; Figure 1 is a circuit diagram of a Gm block of an oscillator core according to an example embodiment of the present disclosure;

[0022] Figure 3 is a circuit diagram of a differential amplifier corresponding to a Gm block of an oscillator core according to an example embodiment of the present disclosure; Figure 2 is a circuit diagram of a differential amplifier corresponding to a Gm block of an oscillator core according to an example embodiment of the present disclosure;

[0023] Figure 4 schematically illustrates a VCO according to a further example embodiment of the present disclosure;

[0024] Figure 5 schematically illustrates a wireless communication device comprising a VCO according to an example embodiment of the present disclosure;

[0025] Figure 6 is a graph representing phase noise as a function of carrier offset frequency;

[0026] Figure 7 schematically illustrates a VCO according to a further example embodiment of the present disclosure; and

[0027] Figure 8 schematically illustrates a VCO according to a further example embodiment of the present disclosure. DETAILED DESCRIPTION

[0028] In the various drawings, like features will be denoted by like reference numerals. In particular, structural and / or functional features common among the various embodiments can have the same reference numerals, and can be arranged with consistent structural, dimensional, and material properties.

[0029] Unless otherwise noted, where reference is made to two elements being connected together, it means directly connected with nothing intervening therebetween other than a conductor; whereas where reference is made to two elements being linked or coupled together, it means that the two elements can be connected via one or more other elements, or can be linked or coupled.

[0030] In the following disclosure, unless otherwise noted, where reference is made to an absolute positional qualifier (such as the terms "front", "back", "top", "bottom", "left", "right", etc.), or a relative positional qualifier (such as "over", "under", "higher", "lower", etc.), or an orientational qualifier (such as "horizontal", "vertical", etc.), reference is made to the orientation shown in the drawings, or to the orientation during normal use of the VCO.

[0031] Unless otherwise noted, the expressions "nearby", "about", "substantially", and "on the order of" mean within 10%, preferably within 5%.

[0032] It is proposed to reduce the phase noise in a VCO by coupling several oscillator cores in parallel to each other. Indeed, the phase noise of a VCO has been shown to be proportional to its resistance, and by providing a plurality of oscillator cores arranged in parallel, the resistance of the VCO can be reduced. However, it is difficult to couple several oscillator cores in parallel when the number of cores is greater than or equal to four, as this will cause layout problems that are particularly challenging, if not insurmountable.

[0033] Figure 1 is a plan view of a VCO 100 according to an example embodiment of the present disclosure and based on a distributed active transformer series-coupled oscillator core (SDAT).

[0034] In Figure 1 The VCO 100 in the example comprises four oscillator cores C1, C2, C3 and C4, which are magnetically series-coupled. For example, the oscillator cores are coupled in series through electromagnetic induction (also referred to as inductive coupling), for example via a common inductive loop.

[0035] For example, each oscillator core comprises a magnetizing loop 102 receiving a control signal Vctrl on an input line 104, and outputs 105 and 107. Furthermore, each oscillator core comprises an inductive part 106 coupled to the output 105 of the magnetizing loop 102, and an inductive part 108 coupled to the output 107 of the magnetizing loop 102. The inductive parts 106, 108 are positioned adjacent to edges of a common inductive loop 110. Thus, for example, the inductive parts 106, 108 of an oscillator core will be arranged to form components of a one-to-one transformer primary plate, while the common inductive loop forms a secondary plate of the one-to-one transformer.

[0036] In Figure 1 The common inductive loop 110 is square-shaped in the example, and each oscillator core has its inductive parts 106, 108 formed adjacent to different edges of the inductive loop 110.

[0037] For example, the inductive part 106A of the oscillator core C1 is formed adjacent to edge 110A of the common inductive loop 110, while the inductive part 108D of the oscillator core C1 is formed adjacent to edge 110D of the common inductive loop 110.

[0038] Similarly: the inductor portion 106B of the oscillator core C2 is formed adjacent to the edge 110B of the common inductor loop 110, while the inductor portion 108A of the oscillator core C2 is formed adjacent to the edge 110A of the common inductor loop 110; the inductor portion 106C of the oscillator core C3 is formed adjacent to the edge 110C of the common inductor loop 110, while the inductor portion 108B of the oscillator core C3 is formed adjacent to the edge 110B of the common inductor loop 110; and the inductor portion 106D of the oscillator core C4 is formed adjacent to the edge 110D of the common inductor loop 110, while the inductor portion 108C of the oscillator core C4 is formed adjacent to the edge 110C of the common inductor loop 110.

[0039] For example, each of the inductor portions 106, 108 has an inductance in the range 10 to 100 pH (and for example in the range 20 to 30 pH) for an oscillation frequency around 50 GHz.

[0040] For example, each pair of inductor portions 106A, 108A; 106B, 108B; 106C, 108C; and 106D, 108D formed adjacent to the same edge 110A to 110D of the square-shaped common inductor loop 110 has one end coupled to a corresponding excitation circuit 102, and the other end coupled to the power supply rail at the point 114, thereby providing a virtual AC ground. In Figure 1 In examples, the point 114 is coupled to the power supply rail VDD. In some embodiments, each pair of inductor portions is formed by a continuous strip of conductive track, the end of each strip being coupled to the output of the corresponding excitation circuit 102 of the corresponding oscillator core, while the point 114 (e.g. the mid-point of each strip) is coupled to a power supply rail, such as VDD. For example, each pair of inductor portions 106, 108 formed by a continuous strip of conductive track has a total length of between 50 and 150 pm (e.g. around 80 pm). The advantage of coupling the point 114 to the power supply rail is that this provides a simple way of biasing each oscillator core, avoiding the use of DC decoupling capacitors.

[0041] For example, the common inductive loop 110 comprises a strip of square-shaped conductive tracks that are continuous except at an edge 110A along which one end 116 of the strip of square-shaped conductive tracks is coupled to an output line 118 of the VCO 100, while the other end 120 of the strip of square-shaped conductive tracks is coupled to an output line 122 of the VCO 100, the output lines 118, 122 providing differential output signals OUT+, OUT- of the VCO 100, respectively. For example, the ends 116, 120 of the inductive loop 110 are separated from each other by a gap 124 near the midpoint of the edge 110A of the common inductive loop. Moreover, for example, a point 126 near the midpoint of an opposite edge 110C of the common inductive loop 110 is coupled to a common-mode supply voltage Vcm that imposes a common-mode level on the differential output signals OUT+, OUT-.

[0042] For example, each of the oscillator cores C1-C4 receives the same control voltage Vctrl generated by a control circuit (CTRL) 128. For example, the control circuit 128 forms part of a control loop and thus receives a feedback signal FB based on an estimate of the differential output signals OUT+, OUT- and generates the control voltage Vctrl in dependence on the feedback signal FB.

[0043] Although in the example of Figure 1 the excitation circuits 102 of the oscillator cores C1-C4 are positioned at the corners of the square-shaped common inductive loop 110 and the inductive portions 106, 108 are formed adjacent to different edges of the square-shaped common inductive loop 110, other arrangements are possible as will now be described with reference to Figure 7 .

[0044] Figure 7 is a plan view of a VCO 700 according to another example embodiment of the present disclosure and based on a distributed active transformer series-coupled oscillator core (SDAT). The VCO 700 has many features in common with the VCO 100 of Figure 1 and the same features will be denoted with the same reference numerals and will not be described again in detail. In the VCO 700, the excitation circuits 102 of the oscillator cores C1-C4 are positioned near the midpoints of the edges of the square-shaped common inductive loop and the inductive portions 106, 108 of a given oscillator core are formed adjacent to the same edge of the square-shaped common inductive loop 110. In this arrangement, for example, the output lines 118, 122 are positioned at one corner of the common inductive loop 110 and, for example, the common-mode connection 126 is positioned at an opposite corner of the common inductive loop 110.

[0045] Moreover, it would be possible to remove theFigure 1 and Figure 7 one or more of the oscillator cores in the embodiments of For example, a solution based on only two oscillator cores can be implemented by removing cores C1 and C2, and including their corresponding inductance parts 106A, 108D, 108A and 106B, or by removing cores C3 and C4, and including their corresponding inductance parts 106C, 108B, 108C and 106D.

[0046] By providing magnetically series coupled oscillator cores as shown in Figure 1 and Figure 7 the resistance of the cores will be increased. However, the phase noise will still be reduced. In fact, it can be shown that the phase noise is proportional to the resonance tank resistance, and inversely proportional to the square of the amplitude of the output signal. By magnetically series coupling multiple oscillator cores, the oscillations of the multiple oscillator cores will add to each other, increasing the amplitude and allowing the phase noise to be reduced.

[0047] Figure 2 is a circuit diagram of an oscillator core C1 according to an example embodiment. Figure 1 and Figure 7 The other oscillator cores C2 to C4 can for example be implemented by similar circuits.

[0048] The excitation circuit 102 for example comprises a varactor 202 controlled by a signal Vctrl, which is for example an analog control signal. The varactor 202 is coupled between the output lines 105, 107 of the excitation circuit 102. The circuit 102 also comprises for example a differential amplifier 206 (Gm) having a positive differential output and a negative differential output coupled to the output lines 105 and 107, respectively. Furthermore, the positive output of the amplifier 206 is coupled to the negative input of the amplifier 206, and the negative output of the amplifier 206 is coupled to the positive input of the amplifier 206. In Figure 2 The resistor 204 shown by the dashed line in Figure 1 and Figure 7 the resistance of the cores is presented in series.

[0049] Although the varactor 202 is shown connected between the output lines 105, 107, according to an example embodiment the varactor 202 can be connected between the input lines 103, 104 of the excitation circuit 102. Figure 2As shown in the variant 210, the terminals of the varactor 202 (i.e. the output lines 105, 107) can be shorted to VDD via switches 212, 214 coupled between the output line 105 and the VDD power rail and between the output line 107 and the VDD power rail, respectively. The switches 212, 214 are for example implemented by relatively small MOS transistors and are for example controlled by an enable signal EN, allowing to selectively activate the oscillator core.

[0050] Figure 3 a circuit diagram of a differential amplifier 206 of an oscillator core according to an example embodiment of the present disclosure. In Figure 2 Figure 3 In an example, the differential amplifier 206 is implemented by a differential pair 304, 306 (for example implemented by MOS or bipolar transistors), each differential pair of the differential pair 304, 306 being coupled by its main current conduction node between the output line 105 and the ground voltage rail GND and between the output line 107 and the ground voltage rail GND, respectively. In some embodiments, the emitter or source of the transistors 304, 306 can also be shorted to a common node, and this common node is connected to the ground rail via a resistor or a current source. The control nodes of the transistors 304, 306 are coupled together via resistors 308, 310. In addition, the control node of the transistor 304 is coupled to the output line 107 for example via a capacitor 312, while the control node of the resistor 306 is coupled to the output line 105 for example via a capacitor 314.

[0051] Of course, Figure 3 Only one example implementation of the differential amplifier 206 is provided, there are many alternative circuits that can be used. For example, in alternative embodiments, the capacitors 312 and 314 can be replaced by short circuits.

[0052] Although Figure 1 and Figure 7 The example of the VCO 400 is based on four oscillator cores, but the shape of the common inductive loop can also be adapted to the case of more than four oscillator cores, as will now be described with reference to Figure 4

[0053] Figure 4 schematically illustrates a VCO 400 also based on distributed active transformer series-coupled oscillator cores (SDAT) according to another example embodiment of the present disclosure. Figure 8 The example of the VCO 400 comprises eight oscillator cores C1 to C8 magnetically coupled in series, each oscillator core being implemented by a circuit similar to the one of the VCO 100 of Figure 2 The oscillator cores C1 to C8 are coupled in series for example by electromagnetic induction (also referred to as inductive coupling), for example via a common inductive loop.​​

[0054] Each oscillator core C1 to C8 of the VCO 400 has an inductor portion 406 coupled to the output line 105 of the excitation circuit 102, and an inductor portion 408 coupled to the output line 107 of the excitation circuit 102. Inductor portions 406 and 408 are both positioned adjacent to the side of the common inductor loop 410.

[0055] exist Figure 4 In the example, the common inductor circuit 410 is octagonal in shape, and each oscillator core C1 to C8 has inductor portions 406, 408 formed adjacent to different sides of the inductor circuit 410. Therefore, the inductor portions 406, 408 of the oscillator cores are arranged, for example, as components forming the primary plate of a one-to-one transformer, while the common inductor circuit forms the secondary plate of the one-to-one transformer.

[0056] For example, the inductor portion 406A of the oscillator core C1 is formed as a side 410A adjacent to the common inductor loop 410, while the inductor portion 408H of the oscillator core C1 is formed as a side 410H adjacent to the common inductor loop 410. Similarly, the inductor portions 406B to 406H of the oscillator cores C2 to C8 are formed as sides 410B to 410H adjacent to the common inductor loop 410, respectively, while the inductor portions 406A to 406G of the oscillator cores C2 to C8 are formed as sides 410A to 410G adjacent to the common inductor loop 410, respectively.

[0057] For example, for an oscillation frequency around 50 GHz, each inductor portion of inductor portions 406 and 408 has an inductance in the range of 10 to 100 pH (e.g., in the range of 20 to 30 pH).

[0058] For example, each pair of inductor portions 406A, 408A to 406H, 408H of the corresponding sides 410A to 410H of the common inductor loop 410, which is formed in a near-octagonal shape, has one end coupled to the corresponding excitation circuit 102 and the other end coupled to the power rail at point 414, thereby providing a virtual AC ground. Figure 4 In the example, point 414 is coupled to the power rail VDD. In some embodiments, each pair of inductor portions is formed by a continuous strip of conductive rails, the end of each strip being coupled to the corresponding output of the corresponding excitation circuit 102 of the corresponding oscillator core, while point 414 (e.g., the midpoint of each strip) is coupled to the power rail, such as VDD. Each continuous strip of conductive rails forming a pair of inductor portions 406, 408 has, for example, a length between 50 and 150 μm (e.g., a length around 80 μm). The advantage of coupling point 414 to the power rail is that this provides a simple way to bias each oscillator core, avoiding the use of DC decoupling capacitors.

[0059] The common inductive loop 410 for example comprises an octagonal shaped strip of continuous conductive track except along a side 410A where one end 416 of the octagonal shaped strip of continuous conductive track is coupled to an output line 418 of the VCO 400 and the other end 420 is coupled to an output line 422 of the VCO 400, the output lines 418, 422 providing the output signals OUT+, OUT- of the VCO 400 respectively. The ends 416, 420 of the inductive loop 110 are for example separated from each other by a gap 424 near the midpoint of the side 410A of the common inductive loop. Furthermore, a point 426 near the midpoint of the opposite side 410E of the common inductive loop 410 is for example coupled to a common mode supply voltage Vcm which imposes a common mode level on top of the differential output signals OUT+, OUT-.

[0060] Although not shown in Figure 4 , each of the oscillator cores CI to C8 receives the same control voltage Vctrl which is generated by a control circuit similar to Figure 1 the circuit 128.

[0061] Similar to the embodiments of Figure 7 , the excitation circuits 102 of the oscillator cores CI to C8 of the VCO 400 can instead be positioned near the midpoints of the sides of the octagonal shaped common inductive loop 410, while the inductive parts 406, 408 of a given oscillator core can be formed adjacent to the same side of the octagonal shaped common inductive loop 410. In such an arrangement, the output lines 418, 422 would for example be positioned at one of the vertices of the common inductive loop 410 and the common mode connection 426 would for example be positioned at the opposite vertex, for example just central to the common inductive loop 410.

[0062] Furthermore, it would be possible to remove one or more of the oscillator cores from the VCO 400. For example, by removing cores CI and C2, a solution based on six oscillator cores can be achieved.

[0063] Although Figure 1 , Figure 4 and Figure 7 the embodiments illustrate examples with four and eight oscillator cores respectively, more generally there can be M cores, where M is equal to two or more. In some embodiments, M can be an odd number. The gain in terms of reduced phase noise can be shown to be equal to 10*Log10M dB when all cores are activated. Thus in Figure 1 , Figure 4 and Figure 7 the example with M=4 has a gain of for example 6dB of reduced phase noise, while Figure 4The gain of the example with M=8 is for example a reduction of the phase noise by 9 dB. In the example of the odd number of cores, the common mode connection is for example coupled to the vertex opposite to the edge at which the output line of the VCO is formed (and in particular in the center of the inductive loop).

[0064] Figure 8 An example of a VCO 800 based on five cores magnetically coupled in series via an inductive loop 110 shaped as a pentagon is illustrated. Figure 8 The same features as in Figure 1 and 7 have been denoted with the same reference signs as in Figure 1 and Figure 7 and will not be described again. However, in Figure 8 there are also additional reference signs 106E, 108E and 110E denoting parts forming the fifth side of the pentagon shape.

[0065] Furthermore, it will be possible that in embodiments of Figure 1 and 4 or more generally in embodiments with M oscillator cores magnetically coupled in series, only certain oscillator cores are selectively activated. For example some or all of the oscillator cores, including Figure 2 a variant 210 of Figure 5 will now be described in more detail with reference to the attached drawings.

[0066] Figure 5 A wireless communication device 500 comprising a VCO 502 is schematically illustrated according to an example embodiment of the present disclosure. The VCO 502 is for example implemented by the circuit of Figure 1 , Figure 4 , Figure 7 or Figure 8 with M oscillator cores, wherein at least some of the oscillator cores can be selectively activated. The VCO 502 receives for example a control voltage Vctrl from a control circuit (CTRL) 504 and also receives an enable signal EN to enable each core to be selectively activated. An output signal from the VCO 502 is for example provided to an antenna circuit 56 coupled to an antenna 58. For example, via the antenna 508, the signal provides a carrier frequency for RF transmission.

[0067] In operation, initially, all oscillator cores of the VCO 502 are active, e.g. so that a signal with a relatively low phase noise is transmitted using the antenna 508. Then, the control circuit 504 evaluates the channel quality, e.g. One or more frames or packets known to the receiver are transmitted to the receiver, so that the receiver is able to evaluate the channel and return the obtained channel estimate to the transmitter, and in particular to the control circuit 504. If the quality is relatively high, the transmission continues, e.g. based on the activation of all oscillator cores, so that a relatively high data transmission rate can be obtained. However, if the channel quality is found to be relatively poor, one or more oscillator cores are deactivated, e.g. In practice, this can allow to reduce the power consumption without the reduced phase noise significantly improving the channel bandwidth.

[0068] Figure 6 is a plot representing the single sideband phase noise spectral density in dBc / Hz as a function of the offset from the carrier frequency in Hz, where dBc refers to the phase noise amplitude level of the carrier per 1 Hz bandwidth. The curve 602 illustrates an example based on a VCO with one core, while the curve 604 illustrates an example based on a VCO with eight oscillator cores like the one in Figure 4 Figure 6 The two cases are VCOs centered around the 60 GHz frequency range and using BiCMOS transistor technology. It can be seen that a reduction of the phase noise of 9 dB is obtained at 1 MHz offset, where the usual silicon-based oscillators just start to experience the constant thermal noise effect (the curve of Figure 6 is almost parallel).

[0069] An advantage of the embodiments described herein is that a layout feasible circuit arrangement is provided while the VCO phase noise is reduced. Furthermore, the addition of the oscillator core output signals results in a differential output signal with a relatively high amplitude, which means that in some cases amplification is not needed. In some cases, attenuators can be used to adjust the amplitude for a given application while advantageously maintaining a low phase noise.

[0070] Various embodiments and variants have been described. It will be appreciated by the skilled person that certain features of the embodiments can be combined, and that other variants will readily occur to the skilled person. In particular, while in the described embodiments the differential amplifier of the excitation circuit is connected or coupled to the ground rail, and the inductive part of the oscillator core is connected or coupled to the positive power supply rail, in alternative embodiments the power supply voltage of the oscillator core can be reversed.

[0071] Furthermore, while the described embodiments have been described in connection with Figure 1 , Figure 4 , Figure 7 andFigure 8 VCO embodiments based on transformers having the shape of regular polygons with 4, 5 and 8 sides are described, but the principles described in relation to these figures can be extended to transformers having any regular polygon shape.

[0072] Furthermore, while examples based on BiCMOS transistor technology have been described, it will be apparent to those skilled in the art that other transistor technologies (such as pure bipolar transistors, or CMOS transistors) can be used in alternative embodiments.

Claims

1. A voltage controlled oscillator, comprising: a plurality of oscillator cores magnetically coupled in series, wherein: each oscillator core includes a first inductive portion and a second inductive portion magnetically coupled to a common inductive loop; a first end of each of the first inductive portion and the second inductive portion is coupled to an excitation circuit of the oscillator core; and a second end of each of the first inductive portion and the second inductive portion is coupled to a voltage supply rail; and the common inductive loop includes a common mode voltage connection at a midpoint along a first side of the common inductive loop, or at a first vertex between two sides of the common inductive loop.

2. The voltage controlled oscillator of claim 1, wherein the plurality of oscillator cores includes M oscillator cores, where M is equal to four or more.

3. The voltage controlled oscillator of claim 1, further comprising: a first output line coupled to one end of the common inductive loop; and a second output line coupled to another end of the common inductive loop.

4. The voltage controlled oscillator of claim 3, wherein the common inductive loop forms a regular polygon shape having at least four sides, and the first inductive portion and the second inductive portion are positioned adjacent to the sides of the common inductive loop.

5. The voltage controlled oscillator of claim 4, wherein each oscillator core positions its first inductive portion adjacent to a first side of the common inductive loop, and positions its second inductive portion adjacent to a second side of the common inductive loop.

6. The voltage controlled oscillator of claim 4, wherein the common inductive loop includes: the first output line and the second output line are coupled to a second side or a second vertex of the common inductive loop opposite the first side or the first vertex, respectively.

7. The voltage controlled oscillator of claim 1, wherein each oscillator core includes a varactor and a differential amplifier having a first output node coupled to a first end of the first inductive portion, and a second output node coupled to a first end of the second inductive portion, the varactor being coupled between the first output node and the second output node.

8. The voltage controlled oscillator of claim 1, wherein each oscillator core is selectively activatable.

9. The voltage controlled oscillator of claim 1, wherein the common inductive loop forms a regular polygon shape having at least four sides, and the first inductive portion and the second inductive portion are positioned adjacent to the sides of the common inductive loop.

10. A communication device, comprising: an antenna circuit configured to drive one or more antennas; and a voltage controlled oscillator coupled to the antenna circuit, the voltage controlled oscillator comprising: a plurality of oscillator cores magnetically coupled in series, the plurality of oscillator cores including: each oscillator core includes a first inductive portion and a second inductive portion magnetically coupled to a common inductive loop; a first end of each of the first inductive portion and the second inductive portion is coupled to an excitation circuit of the oscillator core; and a second end of each of the first inductive portion and the second inductive portion is coupled to a voltage supply rail; and the common inductive loop includes a common mode voltage connection at a midpoint along a first side of the common inductive loop, or at a first vertex between two sides of the common inductive loop. a second end of each of the first inductive part and the second inductive part is coupled to a voltage supply rail; and the common inductive loop includes a common mode voltage connection at a midpoint along a first side of the common inductive loop, or at a first vertex between two sides of the common inductive loop.

11. The communication device of claim 10, wherein the plurality of oscillator cores comprises M oscillator cores, where M equals four or more.

12. The communication device of claim 10, further comprising: a first output line coupled to one end of the common inductive loop; and a second output line coupled to another end of the common inductive loop.

13. The communication device of claim 12, wherein the common inductive loop forms a regular polygon shape with at least four sides, and the first inductive part and the second inductive part are positioned adjacent to the sides of the common inductive loop.

14. The communication device of claim 13, wherein each oscillator core has its first inductive part positioned adjacent to a first side of the common inductive loop, and has its second inductive part positioned adjacent to a second side of the common inductive loop.

15. The communication device of claim 13, wherein the common inductive loop comprises: the first output line and the second output line are coupled to a second side of the common inductive loop opposite the first side.

16. The communication device of claim 10, wherein each oscillator core comprises a varactor and a differential amplifier having a first output node coupled to a first end of the first inductive part, and a second output node coupled to a first end of the second inductive part, the varactor being coupled between the first output node and the second output node.

17. The communication device of claim 10, further comprising a control circuit coupled to the voltage controlled oscillator, the control circuit configured to provide a respective enable signal to each oscillator core.

18. The communication device of claim 10, further comprising a control circuit coupled to the voltage controlled oscillator, the control circuit configured to provide a control voltage to each of the plurality of oscillator cores.

19. The communication device of claim 18, wherein the control circuit is further coupled to the antenna circuit.

20. The communication device of claim 10, wherein the common inductive loop forms a regular polygon shape with at least four sides, and the first inductive part and the second inductive part are positioned adjacent to the sides of the common inductive loop.

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