Voltage-controlled oscillator

Abstract

Voltage-controlled oscillator (VCO) that includes: a VCO core (202) having a plurality of transistors; and a varactor circuit (204) coupled to emitter terminals of the VCO core (202), the varactor circuit (204) comprising a first capacitor (228) having a first terminal coupled to an emitter node of the VCO core (202), a first transmission line element (232) having a first terminal coupled to a second terminal of the first capacitor (228), a first varactor diode (230) having a first terminal coupled to a second terminal of the first transmission line element (232) and a second terminal coupled to a tuning node, and an RF choke circuit coupled between a second terminal of the first capacitor (228) and a second reference node.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a voltage controlled oscillator (VCO). BACKGROUND

[0002] Document US 2006 / 0 049 880 A1 proposes a VCO with output buffer.

[0003] Document US 2004 / 0032303 A1 proposes an oscillator transmission circuit.

[0004] Document US 2011 / 0267147 A1 proposes an oscillator circuit.

[0005] Document US 2012 / 0319787 A1 proposes a VCO having a resonator circuit with a phase noise filter.

[0006] Applications in the millimeter-wave frequency range have gained significant interest in the past few years due to rapid advances in low-cost semiconductor technologies such as silicon germanium (SiGe) and fine-geometry complementary metal-oxide-semiconductor (CMOS) processes. The availability of high-speed bipolar and metal-oxide-semiconductor (MOS) transistors has led to an increasing demand for integrated circuits for mm-wave applications at 60 GHz, 77 GHz, and 80 GHz, and even beyond 100 GHz. Such applications include, for example, automotive radar and multi-gigabit communication systems.

[0007] In some radar systems, the distance between the radar and a target is determined by transmitting a frequency-modulated signal, receiving a reflection of the frequency-modulated signal, and determining a distance based on a time delay and / or frequency difference between the transmission and reception of the frequency-modulated signal. The resolution, accuracy, and sensitivity of the radar system may depend in part on the phase noise performance and frequency agility of the radar's frequency-generating circuitry, which generally includes an RF oscillator and circuitry that controls the frequency of the RF oscillator.

[0008] However, as the operating frequencies of RF systems continue to rise, generating signals at such high frequencies presents a significant challenge. Oscillators operating at high frequencies can suffer from poor phase noise performance caused by 1 / f and thermal noise in the devices that comprise the VCO. Phase noise can be further affected by nonlinear effects of self-bias conditions, which affect the VCO's gain and tuning characteristics. SUMMARY OF THE INVENTION

[0009] There is a need to provide an improved concept for a voltage controlled oscillator.

[0010] Such a need can be met by the subject matter of the claims.

[0011] Some embodiments relate to a VCO having a VCO core with a plurality of transistors; and a varactor circuit coupled to emitter terminals of the VCO core, the varactor circuit comprising a first capacitor having a first terminal coupled to an emitter node of the VCO core, a first transmission line element having a first terminal coupled to a second terminal of the first capacitor, a first varactor diode having a first terminal coupled to a second terminal of the first transmission line element and a second terminal coupled to a tuning node, and an RF choke circuit coupled between a second terminal of the first capacitor and a second reference node.

[0012] Optionally, the RF choke circuit comprises a second transmission line element having a first terminal coupled to the first terminal of the first transmission line element, a third transmission line element having a first node coupled to a second terminal of the second transmission line element, and a second capacitor coupled between the first node of the first transmission line element and a second reference node.

[0013] Furthermore, the first transmission line element optionally comprises a length of at least 100 µm.

[0014] Furthermore, the RF choke circuit optionally has a quarter wavelength of approximately twice the operating frequency of the VCO.

[0015] Optionally, the VCO further includes a resistor coupled between a common supply node of the VCO core and a power supply input terminal of the VCO.

[0016] Furthermore, the resistor optionally has a resistance value between approximately 1 ohm and approximately 20 ohms.

[0017] Optionally, the VCO further comprises a fourth transmission line element coupled between the power supply input terminal and the resistor, the fourth transmission line element having a quarter wavelength of about twice an operating frequency of the VCO.

[0018] According to one embodiment, a voltage-controlled oscillator (VCO) includes a VCO core with multiple transistors, a bias resistor coupled between collector terminals of the VCO core and a first supply node, and a varactor circuit coupled to emitter terminals of the VCO core. The bias resistor is configured to limit a self-bias condition of the VCO core. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings in which: Fig. 1a-d illustrate the operation of an example automotive radar system, a diagram of a conventional VCO, and a performance of the conventional VCO; Fig. 2a-b show diagrams of embodiment VCOs; Fig. 3a-f show performance results of embodiment VCOs; Fig. 4 is a block diagram of an embodiment method; and Fig. 5 represents an embodiment radar system.

[0020] Corresponding numbers and symbols in the various figures generally refer to corresponding parts unless otherwise noted. The figures have been drawn to clearly illustrate the relevant aspects of the preferred embodiments and are not necessarily drawn to scale. To more clearly illustrate particular embodiments, a letter indicating variations of the same structure, material, or process step may follow a figure number. DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0021] The practice and use of the presently preferred embodiments are discussed in detail below. However, it should be recognized that the present invention provides many applicable inventive concepts that may be embodied in a wide variety of specific contexts. The specific embodiments discussed merely illustrate specific ways of making and using the invention and do not limit the scope of the invention.

[0022] The present invention will be described with reference to preferred embodiments in a specific context with a system and method for a radar system, such as an automotive radar system. The invention can also be applied to other systems and applications that utilize RF oscillators, such as general radar systems and wireless communication systems.

[0023] In embodiments of the present invention, a low-phase-noise VCO utilizes a feedback resistor to reduce the effect of self-bias during operation. This reduction in self-bias also mitigates the distortion of the varactor tuning characteristic caused by varactor modulation during VCO operation, preventing an unexpected change in VCO gain that can increase phase noise within a frequency generation system such as a phase-locked loop (PLL) or reduce the locking range of the same PLL. Further phase noise reduction can be achieved by decoupling VCO core harmonics from the varactor.

[0024] Fig. 1a illustrates an example automotive radar scenario 100 in which a motor vehicle 102 includes an automotive radar system 104. The automotive radar system 104 transmits and receives, for example, a frequency-modulated continuous wave (FMCW) signal and detects reflections of this transmitted signal to determine a distance between the automotive radar system 104 and other vehicles or objects on the road. In the illustrated scenario, a large vehicle 106, such as a truck, is closer to the motor vehicle 102 than a small vehicle 108, such as a motorcycle. Under normal operating conditions, the echo or reflection from the large vehicle 106 has a higher amplitude than the reflection from the small vehicle 108 because the large vehicle 106 is both larger and closer than the small vehicle 108.

[0025] Fig. Figure 1b shows a graph 120 of the level of the received signal as a function of the received frequency for the scenario of Fig. 1a. The signal level versus frequency curve 122 corresponds to the received reflection from the large vehicle 106, and the frequency f1 of the signal level peak 130 corresponds to the distance between the automotive radar system 104 and the large vehicle 106. Likewise, the signal level versus frequency curve 126 corresponds to the received reflection from the small vehicle 108, and the frequency F2 of the signal level peak 132 corresponds to the distance between the automotive radar system 104 and the small vehicle 108. Consequently, the distance between the large vehicle 106 and the small vehicle 108 is proportional to the distance between the frequencies F1 and F2.

[0026] Along with the desired output signal, the phase noise of the radar transmitter is also transmitted and reflected. The phase noise reflected by the large vehicle 106 is shown as a dashed line 124. As can be seen in graph 120, the phase noise 124 affects the radar's ability to receive signals reflected by the small vehicle 108. The signal-to-noise ratio between the signal level peak 132 due to the small vehicle 108 and the corresponding noise floor due to phase noise reflected by the large vehicle 106 is shown as a length 134. From the graph of Fig. Figure 1B shows that phase noise affects the ability of automotive radar system 104 to detect small and distant objects. The higher the radar transmitter's phase noise, the less the radar system can detect small and distant objects.

[0027] Fig. Figure 1c illustrates a conventional VCO 150 according to a "common-mode" architecture. The VCO includes a VCO core 151 with transistors 153 and inductors 154, matching networks 152, varactors 158, and a current source 160. The transistors 153 are biased according to the bias voltage Vbias, and the capacitance of the varactors 158 is tuned according to the tuning voltage Vtune. The oscillation frequency of the VCO 150 is approximately: fOSC=12πL154C158, where L 154 is the inductance of the inductor 154 and C 158 is the capacitance of the varactor 158. The output of the VCO 150 is taken as Vout, which creates an output frequency of twice f-osc.

[0028] Varactor 158 can be implemented as a diode capacitance that is inversely proportional to the voltage applied across its terminals. This reduction in applied voltage may be due to the increase in the width of the depletion region in the reverse-bias diode, causing a corresponding reduction in its capacitance. An example relationship between the DC varactor capacitance C varaktor in relation to the tuning voltage is shown as curve 170 in Fig. 1d. As shown, C varaktordecreases with increasing voltage. However, during VCO operation, the actual voltage across varactor 158 varies with time during each VCO oscillation cycle, therefore causing the capacitance to vary during each VCO oscillation cycle. Curves 174, 176a, 176b, and 176c represent the variance of the applied tuning voltage across varactor 158 due to the change in the VCO output over one cycle. Curves 176a-c represent a small VCO amplitude, and curve 174 represents a large VCO amplitude. As shown, when the VCO produces a large amplitude, represented by curve 174, the capacitance of varactor 158 changes from the capacitance value C1, corresponding to point 173 on curve 170, to the capacitance value C2, corresponding to point 175 on curve 170. The practical net effect of this large-signal behavior causes a change in the effective tuning characteristics of the varactor 158.These effective tuning characteristics are shown as C. eff in Fig. 1c. Curve 178 represents the effective tuning characteristic corresponding to the small amplitude curves 176a-c, and curve 180 represents the effective tuning characteristic corresponding to the large amplitude curve 174. As shown, curve 180, which corresponds to C eff for a large signal amplitude, exhibits a "kink" in its tuning characteristics. This "kink" can cause reduced VCO gain, which can affect the ability of a PLL to achieve frequency locking over the VCO tuning range, and can increase phase noise when the VCO is operating at high K VCO -tuning range works.

[0029] Fig. Figure 2a illustrates the VCO 200 according to one embodiment of the present invention, including a VCO core 202, a varactor circuit 204 including varactors 230, and a bias circuit 210. In one embodiment, the VCO core includes transistors 212, capacitors 214, and transmission line elements 216. In one embodiment, the VCO is configured to oscillate at a frequency between about 5 GHz and about 40 GHz, for example, about 20 GHz. However, in alternative embodiments, other oscillation frequency ranges may be used. The transmission line elements 216 are sized to create an inductive impedance at the bases of the transistors 212. Bias voltages for the bases of transistors 212 are provided by bias circuit 210, which is coupled to VCC via transmission line element 222.In one embodiment, transmission line element 222 is sized to have a quarter wavelength of twice the oscillation frequency of VCO 200. In some embodiments, the bias voltage VBIAS is filtered via a bias filter network 207 including transmission line element 240 and capacitor 242. In some embodiments, transmission line element 240 has a quarter wavelength of approximately four times the oscillation frequency of VCO 200.

[0030] The collectors of transistors 212 are coupled to VCC via transmission line elements 218, a feedback resistor 220, and a transmission line element 222. In one embodiment, transmission line elements 218 are sized to maximize signal swing. Feedback resistor 220, in some embodiments, mitigates the self-biasing effect of high VCO amplitudes, which distort the tuning curve of varactors 230. For example, under corner conditions of low temperature and / or a fast process, the tendency of transistors 212 to increase gain and current, resulting in a larger signal swing in the VCO and varactor node, is offset by the effect of feedback resistor 220. An increase in bias current and / or signal swing, resulting in increased current, causes a corresponding voltage drop across feedback resistor 220.This voltage drop reduces the signal swing of the VCO core 202 and therefore reduces the effects of the self-bias effect on the VCO tuning characteristics. On the other hand, under corner conditions of high temperature and / or a slow process, the tendency of the gain and current of the transistors 212 to decrease limits the occurrence of the self-bias, so that the additional voltage drop across the feedback resistor 220 is negligible and / or does not cause a noticeable decrease in the VCO amplitude. In some embodiments, the resistance of the feedback resistor is between about 5 Ω and about 10 Ω for a bias current of about 20 mA. Alternatively, bias currents and other resistance values ​​can be used for the feedback resistor 220.

[0031] The varactor circuit 204 includes varactor elements 230, AC coupling capacitors 228, series transmission line elements 232, and RF choke circuits including transmission line elements 234 and 236 and a capacitor 238. In some embodiments, the tuning voltage V TUNEfiltered via a bias filter network 208 including a transmission line element 244 and a capacitor 246. In some embodiments, the transmission line element 240 has a quarter wavelength of approximately four times the oscillation frequency of the VCO 200. The combination of each RF choke circuit and the transmission line element 232 can form an inductive voltage divider. In one embodiment, AC coupling capacitors 228 allow the varactors 230 to be biased based on the applied tuning voltage Vtune and a reference voltage Vn1. The series transmission line elements 232 and the AC coupling capacitors 228 form a series resonant circuit that allows the fundamental frequency of the oscillator to pass through to the varactors while attenuating the harmonics of the VCO 200.In some embodiments, the series transmission line elements 232 may be implemented using a transmission line with a length of approximately 400 μm in one example. In another example, the length of the series transmission line elements 232 may be between approximately 100 μm and approximately 500 μm. Of course, however, the length of the series transmission line elements 232 may be outside this range depending on the embodiment and its particular specifications. In some alternative embodiments, the series transmission line elements 232 may be implemented using an inductive element.

[0032] In one embodiment, the RF choke circuit, including transmission line elements 234, 236, and capacitor 238, creates a high impedance to the emitters of transistors 212 at approximately twice the oscillation frequency of VCO 200 and provides a lower impedance at other harmonics of the oscillation frequency. By providing a lower impedance to the oscillation harmonics across the series transmission line element 232 and the RF choke circuit, phase noise can be improved due to reduced nonlinear behavior of the varactor.

[0033] Exit V OUTThe VCO 200 is coupled to the emitters of transistors 212 via transmission line elements 224 and 226, which isolate the VCO core from the output, forcing the VCO's fundamental signal to remain within the VCO core. This also improves the resonator's Q factor and results in better phase noise performance. The tail current for transistors 212 is provided by transmission line element 248 and resistor 250. In one embodiment, transmission line element 248 has a quarter wavelength of twice the oscillation frequency of VCO 200.

[0034] Of course, in some embodiments, the sizing of the transmission elements within the VCO 200 may vary from the lengths and corresponding wavelengths described above, depending on the particular embodiment and its specifications.

[0035] Fig. Figure 2b illustrates the VCO 260 according to another embodiment of the present invention. The VCO 260 is similar to the one shown in Fig. 2a, with the addition of an output filter 274 and various alternative implementation details. In one embodiment, transmission line element 222 is split into series transmission line elements 222a and 222b, transmission line element 248 is split into series transmission line elements 248a and 248b, and transmission line elements 216 are split into series transmission line elements 216a and 216b. Likewise, the emitters of transistors 212 are coupled to common node N2 via transmission line elements 266a-d. In varactor circuit 204, transmission line elements 236 are coupled to ground via resistor 262 and capacitor 264.

[0036] In one embodiment, bias circuit 210 provides a bias voltage V BIASvia the transmission line elements 292a-b, the resistor 296, the diode-coupled transistors 298a-c, and the capacitor 291. The emitters of the transistors 298a-c are coupled to ground via the resistor 299. An output filter 274 is connected between the node N2 and the output terminal V OUT coupled. In one embodiment, filter 274 is a bandpass filter with a center frequency of approximately twice the oscillation frequency of VCO 260. Alternatively, other filter types and center frequencies may be used. Filter 274 is implemented as an LC ladder circuit with capacitors 276, 280, 284, 286, and 290 and inductors implemented using transmission line elements 278, 282, and 288. In further alternative embodiments, the inductors may be implemented using discrete inductors or on-chip inductors.

[0037] Fig. Figure 3a illustrates a series of curves representing equivalent capacitance versus frequencies with respect to various embodiment architectures. Curve 302 represents the performance of a VCO with a feedback resistor 220, but without series transmission line elements 232 and without the RF choke consisting of transmission line elements 234, 236, and capacitor 238; curve 304 represents the performance of a VCO with a feedback resistor 220 and the RF choke consisting of transmission line elements 234, 236, and capacitor 238, but without series transmission line elements 232; and curve 306 represents the performance of a VCO with a feedback resistor 220, the RF choke consisting of the transmission line elements 234, 236 and the capacitor 238, and the series transmission line elements 232.As can be seen, curve 306 has a higher effective capacitance at higher frequencies and a lower effective capacitance at lower frequencies, thereby increasing the tuning range of the VCO.

[0038] Fig. 3b shows a measured spectral diagram of output V OUT of the VCO 200, which is Fig. 2a. Here, the output frequency of the VCO 200 is about 40 GHz, which is twice the VCO's oscillation frequency of 20 GHz. Fig. Figure 3c shows a phase noise diagram showing the phase noise performance of the VCO 200. As shown, the phase noise is approximately -82.76 dBc / Hz at an offset of 50 kHz, -90.66 dBc / Hz at an offset of 100 kHz, -110.48 dBc / Hz at an offset of 1 MHz, and -129.12 dBc / Hz at an offset of 10 MHz.

[0039] Fig. Figure 3d illustrates plots of oscillation frequency versus tuning voltage for an embodiment VCO. Curve 310 illustrates the oscillation frequency versus tuning voltage at 25°C, and curve 312 illustrates the oscillation frequency versus tuning voltage at 130°C. Fig. Figure 3e shows diagrams of the VCO gain (KVCO) with respect to the tuning voltage for the embodiment VCO shown in Fig. 3d. Curve 316 represents the VCO gain versus tuning voltage at 25 °C, and curve 314 represents the oscillation frequency versus tuning voltage at 130 °C. Fig. Figure 3f represents the oscillation frequency with respect to the supply voltage VCC over various tuning voltages at 25 °C. Curve 320 represents the oscillation frequency with a tuning voltage of approximately 0 V; curve 322 represents the oscillation frequency with a tuning voltage of approximately 2.5 V; and curve 324 represents the oscillation frequency with a tuning voltage of approximately 5.0 V. Of course, the diagrams of Fig. 3a-f illustrate the performance of example embodiments. Other embodiment VCOs may differ from what is shown in Fig. 3a-f, work differently.

[0040] Fig. Figure 4 illustrates a block diagram 400 of an embodiment method for operating a VCO. In step 402, a supply voltage is applied to a VCO core via a resistor. In one embodiment, the VCO core and the feedback resistor may be connected using a circuit similar to the VCO core 202 and feedback resistor 220 shown in Fig. 2a. In step 404, a self-bias condition may be limited using this resistor. In some embodiments, limiting the self-bias condition may mitigate the effects of the self-bias on the effective capacitance of the VCO. In step 406, the VCO is tuned by applying a tuning voltage to a varactor, and in step 408, a signal path is provided between the VCO core and the varactor at the oscillation frequency. In some embodiments, the signal path is implemented using a series resonant circuit with an AC coupling capacitor and a transmission line element coupled between the VCO core and an operating node, for example, as in Fig. 2a with respect to the AC coupling capacitors 228 and the series transmission line elements 232. In step 410, VCO harmonics are shunted to a reference node such as ground, for example, using an RF choke circuit. By performing steps 408 and 410, VCO harmonics coupled to the varactor are attenuated, thereby improving phase noise performance in some embodiments.

[0041] Fig.5 illustrates a single-chip radar transmission system 500 including an upconverter 502, a power amplifier 504, and a frequency generation circuit 506. As shown, the upconverter 502 upconverts a baseband signal BB to a higher frequency signal, which is then amplified by the power amplifier 504 and output at a pin OUT. In some embodiments, the baseband signal BB may be a swept frequency or other signal type used in a radar system. The frequency generation circuit 506 generates a local oscillator LO signal based on a reference frequency at the pin REF, which may be generated, for example, using a crystal oscillator. In one embodiment, the frequency generation circuit 506 is implemented using a phase-locked loop (PLL) with a phase detector 512, a loop filter 510, a VCO 508, and a divider 514.VCO 508 may be implemented using embodiment VCOs described herein. It should be appreciated that system 500 is only one of many examples of embodiment systems that may use embodiment oscillators. Alternative systems may include, for example, wireless and wireline communication systems and other systems that utilize VCOs.

[0042] According to one embodiment, a voltage-controlled oscillator (VCO) includes a VCO core with multiple transistors, a bias resistor coupled between collector terminals of the VCO core and a first supply node, and a varactor circuit coupled to emitter terminals of the VCO core. The bias resistor is configured to limit a self-bias condition of the VCO core.

[0043] In one embodiment, the varactor circuit includes a first capacitor having a first terminal coupled to an emitter node of the VCO core, a first transmission line element having a first terminal coupled to a second terminal of the first capacitor, a first varactor diode having a first terminal coupled to a second terminal of the first transmission line and a second terminal coupled to a tuning node, and an RF choke circuit coupled between a second terminal of the first capacitor and a second reference node. The first transmission line may have a length of at least 100 µm, and the RF choke circuit may have a quarter wavelength of approximately twice an operating frequency of the VCO. In some embodiments, the first transmission line and the RF choke form an inductive voltage divider.

[0044] In one embodiment, the VCO further comprises a fourth transmission line element coupled between the tuning node and an input tuning terminal, a third capacitor coupled between the input tuning terminal and the second reference node, a fifth transmission line element coupled between a base bias node of the VCO core and an output node of a bias circuit, and a fourth capacitor coupled between the output node of the bias circuit and the second reference node. The fourth transmission line element may have a quarter wavelength of approximately four times an operating frequency of the VCO, and the fifth transmission line element may have a quarter wavelength of approximately four times the operating frequency of the VCO.

[0045] In one embodiment, the RF choke circuit includes a second transmission line element having a first terminal coupled to the first terminal of the first transmission line element, a third transmission line element having a first node coupled to a second terminal of the second transmission line element, and a second capacitor coupled between the first node of the first transmission line and the second reference node. The VCO may have an output node coupled to the emitter terminals of the VCO core. In one embodiment, the VCO has an operating frequency between about 10 GHz and about 30 GHz.

[0046] According to another embodiment, a voltage-controlled oscillator (VCO) comprises a VCO core having a plurality of transistors and a varactor circuit coupled to emitter terminals of the VCO core. The varactor circuit includes a first capacitor having a first terminal coupled to an emitter node of the VCO core, a first transmission line element having a first terminal coupled to a second terminal of the first capacitor, a first varactor diode having a first terminal coupled to a second terminal of the first transmission line and a second terminal coupled to a tuning node, and an RF choke circuit coupled between a second terminal of the first capacitor and a second reference node.

[0047] In one embodiment, the RF choke circuit comprises a second transmission line element having a first terminal coupled to the first terminal of the first transmission line element, a third transmission line element having a first node coupled to a second terminal of the second transmission line element, and a second capacitor coupled between the first node of the first transmission line and a second reference node. The first transmission line may have a length of at least 100 µm, and the first transmission line element and the RF choke may form an inductive voltage divider. In some embodiments, the RF choke circuit has a quarter-wavelength of approximately twice the operating frequency of the VCO.

[0048] In one embodiment, the VCO further includes a resistor coupled between a common supply node of the VCO core and a power supply input terminal of the VCO. This resistor may have a resistance value between about 1 ohm and about 20 ohms. Alternatively, other values ​​may be used. In any embodiment, the VCO further includes a fourth transmission line element coupled between the power supply input terminal and the resistor, such that the fourth transmission line element has a quarter wavelength of about twice an operating frequency of the VCO.

[0049] According to another embodiment, the method of operating a voltage controlled oscillator (VCO) comprises applying a supply voltage to a VCO core via a resistor coupled to collector terminals of the VCO core, limiting a self-bias condition of the VCO core via the resistor, and tuning the VCO, which comprises applying a tuning voltage to a varactor circuit coupled to emitter terminals of the VCO core.

[0050] The method may further include providing a signal path at the oscillation frequency of the VCO between a tuning node and the emitter terminals of the VCO core using a first transmission line element, and coupling harmonics of the VCO to a reference node via an RF choke circuit coupled between the emitter terminals of the VCO core and the reference node. In some embodiments, the RF choke has a quarter wavelength of approximately twice the oscillation frequency of the VCO.

[0051] The method may further comprise filtering the tuning node using a second transmission line element coupled between a tuning terminal of the VCO and the tuning node. The second transmission line element may have a quarter wavelength of approximately four times the oscillation frequency of the VCO.

[0052] Advantages of embodiments of the present invention include the ability to generate a frequency with very low phase noise. Another advantage, for example, includes a wide VCO tuning range.

[0053] Although this invention has been described with reference to illustrative embodiments, this description is not intended to be limiting. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to those skilled in the art upon reference to the description.

Claims

[1] Voltage-controlled oscillator (VCO), which includes: a VCO core (202) having a plurality of transistors; and a varactor circuit (204) coupled to emitter terminals of the VCO core (202), the varactor circuit (204) comprising a first capacitor (228) having a first terminal coupled to an emitter node of the VCO core (202), a first transmission line element (232) having a first terminal coupled to a second terminal of the first capacitor (228), a first varactor diode (230) having a first terminal coupled to a second terminal of the first transmission line element (232) and a second terminal coupled to a tuning node, and an RF choke circuit coupled between a second terminal of the first capacitor (228) and a second reference node. [2] The VCO of claim 1, wherein the RF choke circuit comprises: a second transmission line element (234) having a first terminal coupled to the first terminal of the first transmission line element (232), a third transmission line element (236) having a first node coupled to a second terminal of the second transmission line element (234), and a second capacitor (238) coupled between the first node of the first transmission line element (232) and a second reference node. [3] The VCO of claim 1 or 2, wherein the first transmission line element (232) has a length of at least 100 µm. [4] A VCO according to any one of claims 1 to 3, wherein the RF choke circuit has a quarter wavelength of about twice an operating frequency of the VCO. [5] The VCO of any one of claims 1 to 4, further comprising a resistor (220) coupled between a common supply node of the VCO core (202) and a power supply input terminal of the VCO. [6] The VCO of claim 5, wherein the resistor (220) has a resistance value between about 1 ohm and about 20 ohms. [7] The VCO of claim 5 or 6, further comprising a fourth transmission line element (222) coupled between the power supply input terminal and the resistor (220), the fourth transmission line element (222) having a quarter wavelength of approximately twice an operating frequency of the VCO.

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