Voltage-controlled oscillator

Abstract

Voltage-controlled oscillator (200; 270; 280), comprising: • an oscillator core (202) comprising a plurality of transistors (212); and • a varactor circuit (204) of which a first end is coupled to emitter terminals of the oscillator core (202) and a second end is coupled to a tuning terminal, wherein the varactor circuit (204) comprises a varactor diode (230) which increases with increasing voltage applied to the tuning terminal with respect to the emitter terminals of the oscillator core (202), wherein the varactor circuit (204) comprises: • a first capacitor (228) of which a first terminal is coupled to a first of the emitter terminals of the oscillator core (202); • a first varactor diode (230) of which a cathode is coupled to a second terminal of the first capacitor (228) and an anode is coupled to the tuning terminal;and • a high-frequency choke circuit (234) coupled between the second terminal of the first capacitor (228) and a varactor reference terminal (235); and • a voltage reference circuit (260, 272) coupled to the varactor reference terminal (235), wherein the voltage reference circuit (260, 272) comprises: - a first resistor (268) coupled between a first reference terminal and the varactor reference terminal (235), the first reference terminal being coupled to collector terminals of the oscillator core (202); and - a diode (262, 264, 266) coupled between the varactor reference terminal (235) and a second reference terminal.

Description

ARTThe present invention relates generally to an electronic device, and more particularly to a voltage controlled oscillator (VCO).BACKGROUNDMillimeter wave frequency range applications have gained significant importance in recent years due to the rapid advancement of low cost semiconductor technologies such as silicon germanium (SiGe) and fine geometry complementary metal oxide semiconductor (CMOS) methods. The availability of high speed bipoloar 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 also above 100 GHz. Such applications include, for example, automotive radar and multi-gigabit communication systems.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 transmitting and receiving 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 frequency generation circuitry of the radar, which generally includes a radio frequency (RF) oscillator (also referred to as RF; these terms are used interchangeably in this application) and circuitry that controls the frequency of the RF oscillator.However, as the operating frequencies of RF systems continue to increase, generating signals at such high frequencies presents a great challenge. Oscillators operating at high frequencies may suffer from poor phase noise performance caused by pink noise (1 / f) and thermal noise in the devices comprising the VCO.US 2012 / 0 049 967 A1 discloses a voltage-controlled oscillator having an oscillator core with transistors and a varactor circuit. This varactor circuit is similar in behavior to the varactor circuit described later with reference to Figure 1c, i.e., as tuning voltage increases, the capacitance of varactor diodes of the circuit decreases and the oscillation frequency increases.A circuit with such a behavior is also known from DE 10 2014 114 215 A1 of the applicant.US 2004 / 0 032 303 A1 discloses a further voltage-controlled oscillator with varactor diodes. In this variant varactor diodes are coupled to collector terminals of capacitors of the voltage controlled oscillator.It is therefore an object to provide improved oscillators and corresponding methods, in particular for high frequencies.SUMMARYAccording to the invention, there are provided voltage controlled oscillators as claimed in claim 1, 4 or 11. The dependent claims define further embodiments.According to an embodiment, a voltage controlled oscillator (VCO) includes a VCO core having a plurality of transistors and a varactor circuit having a first end (e.g., a first terminal) coupled to emitter terminals of the VCO core and a second end (e.g., a second terminal) coupled to a tuning terminal, wherein the varactor circuit specifically provides a capacitance between the first and second terminals). The varactor circuit includes a capacitance that increases with an increasing voltage applied to the tuning terminal relative to the emitter terminals of the VCO core.a first capacitor having a first terminal coupled to a first of the emitter terminals of the oscillator core;a first varactor diode having a cathode coupled to a second terminal of the first capacitor and an anode coupled to the tuning terminal; anda high frequency choke circuit coupled between a second terminal of the first capacitor and a varactor reference terminal.The voltage controlled oscillator further comprises a voltage reference circuit coupled to the varactor reference terminal.In a variant, the voltage reference circuit comprises a first resistor coupled between a first reference terminal and the varactor reference terminal, the first reference terminal coupled to collector terminals of the oscillator core; and a diode coupled between the varactor reference terminal and a second reference terminal.In another variation, the voltage reference circuit includes a voltage regulator coupled between the varactor reference terminal and a first reference terminal.BRIEF DESCRIPTION OF THE DRAWINGSFor a more complete understanding of the present invention and its advantages, reference will now be made to the following descriptions, taken in conjunction with the accompanying drawings, in which: FIG. 1 includes FIGS. 1 aand 1 b illustrating operation of an exemplary automotive radar system, FIG. 1 c illustrating a schematic illustration of a conventional VCO, FIG. 1 d illustrating performance of the conventional VCO, and FIGS. 1 eand 1 f being block diagrams of embodiment frequency generation systems; FIG. 2 illustrates a schematic illustration of an embodiment VCO; FIG. 3 illustrates a schematic illustration of a VCO of another embodiment; FIG. 4 illustrates a schematic illustration of a VCO of another embodiment; FIG. 5 illustrates a frequency-over-tuning voltage graph for an embodiment VCO; FIG. 6 illustrates a block diagram of a method by which embodiments are operable; and FIG. 7 illustrates an embodiment radar system.Like reference numerals and symbols in different figures generally refer to the same parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the preferred embodiments and are not necessarily drawn to scale. To more clearly illustrate certain embodiments, a number may be followed by a letter indicating variations in the same structure, material, or process step.DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTSThe manufacture and use of the preferred embodiments herein is discussed in detail below. It should be understood, however, that the present invention provides many applicable inventive concepts that may be embodied in a variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention and do not limit the scope of the invention.The present invention will be described with respect to preferred embodiments in a specific context, system, and method for a radar system, such as an automotive radar system. The invention can also be applied to other systems and applications using RF oscillators, such as general radar systems and wireless communication systems.In embodiments of the present invention, the tuning characteristic of the VCO is configured such that the frequency of the VCO decreases as the voltage applied to a varactor circuit of the VCO increases. By forming the tuning characteristic of the VCO such that the VCO decreases as the voltage increases, the range in which the VCO operates with the lowest Kvco and phase noise corresponds to a low or minimum voltage. Accordingly, embodiment VCOs can operate high power and low noise operating ranges with low control voltages. Such embodiments are suitable for operation in low supply voltage systems, for example.FIG. 1 a illustrates an example automotive radar scenario 100 in which the automobile 102 includes an automotive radar system 104. For example, the automotive radar system 104 transmits and receives 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 automobile 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, as the large vehicle 106 is both larger and closer to the small vehicle 108.FIG. 1 b illustrates a graph 120 of a received signal level versus a received frequency for the scenario of FIG. 1 a. Signal level versus frequency curve 122 corresponds to the received reflection from large vehicle 106, and frequency f1of signal level peak 130 corresponds to the distance between automotive radar system 104 and large vehicle 106. Similarly, the signal level versus frequency curve 126 corresponds to the received reflection from the small vehicle 108, and the frequency f2of the signal level peak 132 corresponds to the distance between the automotive radar system 104 and the small vehicle 108. Accordingly, the distance between the large vehicle 106 and the small vehicle 108 is proportional to the distance between the frequencies f 1 and f 2.Together with the desired output signal, the phase noise of the radar transmitter is also transmitted and reflected. The phase noise reflected from the large vehicle 106 is shown as dashed line 124. As can be seen from the graph 120, the phase noise 124 affects the ability of the radar to receive signals reflected from 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 the phase noise reflected from the large vehicle 106 is shown as length 134. From the graph of FIG. 1 b, it can be seen that the phase noise affects the ability of the automotive radar system 104 to detect small and remote objects. The higher the phase noise of the radar transmitter, the lower the ability of the radar system to detect small and remote objects.FIG. 1 c illustrates a conventional VCO 150 according to a push-push architecture. The VCO includes a VCO core 151 having transistors 153 and inductors 154, matching networks 152, varactors 158, and a current source 160. Transistors 153 are biased according to a bias voltage Vbias, and the capacitance of varactor 158 is tuned according to a tuning voltage Vrune. The oscillation frequency of the VCO 150 is approximately: where L 154 is the inductance of the inverter 154, and C 158 is the capacitance of the varactor 158. The output of VCO 150 is taken at Vout, which provides an output frequency of twice f OSC.Varactor 158 may be implemented as a diode capacitance inversely proportional to the voltage applied across its terminals. As shown, the tuning voltage Vruneis coupled to the cathode of the varactor diodes forming varactor 158. As the tuning voltage Vruneincreases with respect to ground, the varactor diode is increasingly reverse biased and there is a corresponding decrease in capacitance of varactor 158. This decrease in capacitance versus applied voltage may be due to the increase in the width of the junction in the reverse biased diode as the voltage across the diode increases. Since the oscillation frequency f OSC of the VCO 150 is inversely proportional to C 158, of the capacitance of the varactor 158, the oscillation frequency f OSC increases with a corresponding increase in the tuning voltage Vrune.An exemplary relationship between oscillation frequency f OSC versus tuning voltage Vrune is shown as plot 170 in FIG. 1 d. Also shown in FIG. 1 dis curve 172 representing VCO gain Kvco vs. Vtune, and curve 174 representing phase noise PNssb vs. Vtune. As shown, the phase noise PNssb decreases as the applied tuning voltage Vtune increases and the VCO gain Kvco decreases.Since the region with the best phase noise performance corresponds to higher applied Vrunee voltages, it may be difficult to construct a low voltage system using such a VCO. For example, if the particular low voltage system, such as a PLL, is constrained to only provide a tuning voltage between about 0.2 V and about 2.0 V, the system may not be able to operate the VCO in the lowest phase noise operating ranges. This may present system design difficulties in ensuring that the available tuning voltage range maps the specified output frequency range that has sufficient phase noise performance.FIG. 1 eillustrates a block diagram of an embodiment RF system 180 having a front end circuit 182 that includes an RF oscillator 184 whose frequency is controlled by a digital-to-analog converter (DAC) 187 in a microcontroller unit (MCU) 188. As shown, the front end circuit 182 may be implemented in a separate package from the MCU 188 and / or on an integrated circuit die separate from the MCU 188. In one embodiment, front end circuit 182 may be implemented in a high-power RF method that implements RF transistors such as SiGe HPTs and other types of transistors. The MCU 188, on the other hand, may be implemented using a fine geometry CMOS process. In one embodiment, front end circuit 182 may include another DAC 186 to drive an additional tuning terminal. In some embodiments, the maximum supply voltage that may be applied to the MCU 188 is limited by the particular semiconductor process in which the MCU 188 is fabricated. In some cases, these maximum supply voltages may be about 1.2 V, but different semiconductor methods may be capable of tolerate different maximum supply voltages. In one example, the usable tuning voltage range that may be provided by the DAC may be between about 0.2 V and about 1 V. This tuning range may be modified, for example, using a level shifter circuit 185.In one embodiment, the MCU 188 may be used to implement a digital and / or software-based PLL in addition to performing other functions for the particular RF system being implemented. Software PLLs can be used in embodiment radar or communication systems to take advantage of the low phase noise at higher offset frequencies (i.e., 1 MHz offset) for a free-running VCO as compared to a PLL.According to another embodiment, both the RF front-end circuitry 182 and the MCU functions of the system may be implemented on a single MCU integrated circuit 190 as shown in FIG. 1 f. MCU integrated circuits include DACs 186 and 187 for tuning oscillator 184.In one embodiment, the tuning characteristic of the VO is reversed such that lower tuning voltages correspond to operating ranges with lower phase noise. FIG. 2 illustrates embodiment VCO 200 having such tuning characteristics. As shown, VCO 200 includes VCO core 202, varactor circuit 204 including varactors 230, biasing circuit 210, and voltage reference circuit 260. The VCO core 202 includes transistors 212, capacitors 214, and transmission line elements 216. The transmission line elements 216, as well as other transmission line elements used in the VCO 200, may be implemented using microstrip structures and / or other transmission line structures known in the art. 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 generate inductive impedance at the bases of the transistors 212. Bias voltages are supplied to the bases of transistors 212 by bias circuit 210 coupled to VCC via transmission line element 222. In one embodiment, the transmission line element 222 is dimensioned to be one quarter wave length at twice the oscillation frequency of the VCO 200. In some embodiments, bias voltage VBIASis filtered via bias filtering network 207 that includes transmission line element 240 and capacitor 242. In some embodiments, the transmission line element 240 has a quarter wave length of about four times the oscillation frequency of the VCO 200.The collectors of the transistors 212 are coupled to the VCC via transmission line elements 218, a feedback resistor 220, and a transmission line element 222. In one embodiment, the transmission line elements 218 are sized to maximize signal swing. Feedback resistor 220 attenuates the self-bias effect of high VCO amplitudes, which distort the tuning curve of varactors 230, in some embodiments, as described in U.S. Patent Application Serial No. 14 / 041,931, filed September 30, 2013, which is incorporated herein by reference in its entirety. In some embodiments, the resistance of the feedback resistor is between about 5 Q and about 10 Ω for a bias current of about 20 mA. Alternatively, bias currents and other resistance values may be used for the feedback resistor 220.Varactor circuit 204 includes varactor elements 230, AC coupling capacitors 228, series transmission line elements 232, and RF choke circuits including transmission line element 234. In some embodiments, a bias voltage is applied to the varactor circuit via the transmission element 234. A node 235 providing this bias voltage may be referred to as a varactor reference terminal. As shown, the anodes of varactor elements 230 are coupled to tuning voltage Vrune. In some embodiments, the tuning voltage V TUNE is filtered by the bias filtering network 208 that includes the transmission line element 244 and the capacitor 246. In some embodiments, the transmission line element 240 has a quarter wave length of about four times the oscillation frequency of the VCO 200. The combination of each RF choke circuit and transmission line element 232 may form an inductive voltage divider. In one embodiment, AC coupling capacitors 228 enable varactors 230 to be biased based on applied tuning voltage Vrune and 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 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 having a length of about 400 μ, in one example. In another example, the length of the series transmission line elements 232 may be between about 100 μ and about 500 μ. However, it should be appreciated that the length of the series transmission line elements 232 may be outside this range depending on the embodiment and its specific specifications. In some alternative embodiments, the series transmission line elements 232 may be implemented using an inductive element.In one embodiment, the RF choke circuit comprising transmission line elements 234, 236 and capacitor 238 generates a high impedance for the emitters of transistors 212 at about 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 may be improved due to the reduced non-linear behavior of the varactor.Voltage reference circuit 260 provides a bias voltage to the cathodes of varactor 230. In one embodiment, voltage reference circuit 260 includes a resistor 268 coupled to VCC via transmission line element 222 and diodes 262, 264, and 266. In alternative embodiments of the present invention, voltage reference circuit 260 may include more or less than the three diodes 262, 264, and 266 shown in FIG. 2. In one embodiment, the voltage across diodes 262, 264, and 266 defines the voltage range of Vrune. For example, the VCO 200 may include a tuning voltage range between about 0 V and about three diode drops. When diodes 262, 264 and 266 are silicon diodes having a forward voltage of about 0.7V, the input tuning voltage range is between 0V and about 2.1V.In one embodiment, the sensitivity of the varactor capacitance to the supply voltage VCCis reduced as a function of the use of the resistor 268. For example, as the supply voltage VCC decreases, the current through resistor 268 decreases, causing a corresponding decrease in the voltage across resistor 268. This decrease in voltage across resistor 268 reduces the voltage drop across varactor 230.The output V OUT of the VCO 200 is coupled to the emitters of the transistors 212 via the transmission line elements 224 and 226 that isolate the VCO core from the output, thereby forcing the base signal of the VCO to remain in the VCO core. Therefore, the output frequency of the VCO 200 is twice the oscillation frequency of the VCO core. This also improves the quality factor of the resonator and results in better phase noise performance. The residual current for transistors 212 is provided by a transmission line element 248 and a bias resistor (bias resistor) 250. In one embodiment, the transmission line element 248 has a quarter wave length at twice the oscillation frequency of the VCO 200.It should be appreciated that in some embodiments, the dimensioning 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.FIG. 3 illustrates a VCO 270 according to another embodiment of the present invention. The VCO 270 is similar to the VCO 200 shown in FIG. 2, except that the cathodes of the varactor 230 are referenced using a low voltage drop voltage regulator (LDO) 272 instead of the voltage reference circuit 260. The LDO 272 may be implemented, for example, using low voltage drop voltage regulators known in the art, for example, a linear regulator using a series pass transistor. Alternatively, other known voltage regulator circuits may be used. In some embodiments, the LDO 272 is implemented using low noise circuitry. The LDO 272 may be implemented on the same integrated circuit as the VCO 270 or may be implemented off the VCO 270 in some embodiments.FIG. 4 illustrates a VCO 280 according to another embodiment of the present invention. The VCO 280 is similar to the VCO 270 shown in FIG. 3 except that the LDO 282 is coupled between VCC and the transmission line element 222. As shown, the cathodes of varactors 230 are connected to transmission line element 222 via the RF coupling circuits implemented by transmission elements 234. In embodiments, the LDO 282 may be implemented on the same integrated circuit as the VCO 280, or outside of the integrated circuit on which the VCO 280 is located.FIG. 5 illustrates a graph of oscillation frequency versus applied tuning voltage Vrunefor an embodiment VCO. As shown, the oscillation frequency decreases as the applied tuning voltage Vrune increases. In the particular tuning curve shown, an oscillation frequency range of about 60.25 GHz to about 65.25 GHz may be tuned with an applied tuning voltage of between about 0.5 V and about 2.5 V. In many embodiments, this voltage range is supported using DACs implemented in various standard CMOS semiconductor methods.FIG. 6 shows a block diagram 400 of a method of operating an embodiment VCO that includes a VCO core having a plurality of transistors and a varactor circuit having a first end coupled to emitter terminals of the VCO core and a second end coupled to a tuning terminal. As discussed in accordance with present embodiments, the varactor circuit includes a capacitance that increases with increasing voltage applied to the tuning terminal with respect to the emitter terminals of the VCO core.In step 402, the frequency of the VCO is increased by decreasing a voltage applied to the tuning terminal with respect to the emitter terminals of the VCO core. In step 404, the frequency of the VCO is lowered by increasing the voltage applied to the tuning terminal with respect to the emitter terminals of the VCO core. By performing steps 402 and 404, the oscillation frequency can be tuned. In one embodiment, step 402, in which the frequency of the VCO is increased by lowering the tuning voltage of the VCO, allows the VCO to operate in a lower phase noise operating range.FIG. 7 illustrates a single-chip radar transmission system 500 that includes a boost converter 502, a power amplifier 504, and a frequency generation circuit 506. As shown, the upconverter 502 upconverts the baseband signal BBto a higher frequency signal which is then amplified by the power amplifier 504 and output at pin OUT. In some embodiments, the baseband signal BB may be a sweep frequency or other type of signal used in a radar system. The frequency generation circuit 506 generates the local oscillator signal LO based on a reference frequency on the pin REF, which may be generated using, for example, a crystal oscillator. In one embodiment, the frequency generation circuit 506 is implemented using a phase locked loop (PLL) that includes a phase detector 512, a loop filter 510, a VCO 508, and a divider 514. VCO 508 may be implemented using the embodiment VCOs described herein. In some embodiments, the function of phase detector 512, loop filter 510 may be performed digitally using digital circuits and systems known in the art, as well as performed using analog circuitry. For example, these functions may be implemented using customized digital logic, standard cell digital logic, and / or may be implemented in software running on a processor, microcontroller, or digital signal processor. Such processors may include, for example, a processor core, a memory coupled to the processor core, and one or more input / output ports. Alternatively, other circuits and systems known in the art may be used to implement these functions. 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 using VCOs.According to one embodiment, a voltage controlled oscillator (VCO) includes a VCO core having a plurality of transistors and a varactor circuit having a first end coupled to emitter terminals of the VCO core and a second end coupled to a tuning channel. The varactor circuit includes a capacitance that increases with increasing voltage applied to the tuning terminal with respect to the emitter terminals of the VCO core.Implementations may include one or more of the following features. In one embodiment, the varactor circuit includes: a first capacitor having a first terminal coupled to a first of the emitter terminals of the VCO core; a first varactor diode having a cathode coupled to a second terminal of the first capacitor and an anode coupled to the tuning terminal; and an RF choke circuit coupled between a second terminal of the first capacitor and a varactor reference terminal. The VCO may further include a voltage reference circuit coupled to the varactor reference terminal. In some embodiments, the voltage reference circuit includes: a resistor coupled between a first reference terminal and the varactor reference terminal, the first reference terminal coupled to collector terminals of the VCO core; and a diode coupled between the varactor reference terminal and a second reference terminal. This diode may comprise a plurality of diodes and / or the second reference terminal may be a ground terminal.In one embodiment, the voltage reference circuit includes a voltage regulator coupled between the varactor reference terminal and a first reference terminal. The voltage regulator may include, for example, a low voltage drop voltage regulator (LDO). The first reference terminal may be coupled to collector terminals of the VCO core and / or the collector terminals of the VCO core may be coupled to the varactor reference terminal. The varactor reference terminal may be coupled to the collector terminals of the VCO core via a second resistor.In some embodiments, the VCO includes an output node that may be coupled to the emitters of the VCO core. The VCO may have an operating frequency between about 10 GHz and about 30 GHz.According to another embodiment, a VCO includes a VCO core having a plurality of transistors and a varactor circuit coupled to emitters of the VCO core. The varactor circuit includes a first capacitor having a first terminal coupled to a first of the emitter terminals 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 cathode coupled to the second terminal of the first transmission line element and an anode coupled to a tuning terminal, and an RF choke circuit coupled between a second terminal of the first capacitor and a varactor reference terminal. The VCO further includes a feedback resistor coupled to a first reference terminal and the VCO core and a voltage reference circuit having an output node coupled to the varactor reference terminal.Implementations may include one or more of the following features. The voltage reference circuit includes: a first resistor coupled between the first reference terminal and the varactor reference terminal; and at least one diode coupled between the varactor reference terminal and a second reference terminal. The VCO may further include a series resistor coupled between the emitter terminals of the VCO core and the second reference terminal.In some embodiments, the voltage reference circuit includes a voltage regulator that may be coupled between the first reference terminal and the varactor reference terminal. In some implementations, the voltage regulator is coupled between the first reference terminal and the feedback resistor, and the varactor reference terminal is coupled to the feedback resistor.According to another embodiment, a method of operating a VCO includes increasing a frequency of the VCO by decreasing a voltage applied to a tuning terminal with respect to emitter terminals of the VCO core; and decreasing the frequency of the VCO by increasing the voltage applied to the tuning terminal with respect to emitter terminals of the VCO core. The VCO includes a VCO core that includes a plurality of transistors and a varactor circuit having a first end coupled to the emitters of the VCO core and a second end coupled to a tuning terminal, the varactor circuit including a capacitance that increases with increasing voltage applied to the tuning terminal with respect to the emitters of the VCO core.Implementations may include one or more of the following features. The method in which the varactor circuit comprises a first capacitor having a first terminal coupled to a first of the emitter terminals of the VCO core, a first varactor diode having a cathode coupled to a second terminal of the first capacitor and an anode coupled to the tuning terminal, and an RF choke circuit coupled between a second terminal of the first capacitor and a varactor reference terminal. In some embodiments, the method further comprises biasing the varactor reference terminal. Biasing the varactor reference terminal may include applying an output of a voltage reference circuit to the varactor reference terminal.Advantages of embodiments of the present invention include the ability to generate a frequency that has very low phase noise. Another advantage includes, for example, a wide VCO tuning range.Although this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will become apparent to those skilled in the art upon reference to the specification.

Claims

A voltage controlled oscillator (200; 270; 280) comprising: • an oscillator core (202) comprising a plurality of transistors (212); and • a varactor circuit (204) having a first end coupled to emitter terminals of the oscillator core (202) and a second end coupled to a tuning terminal, the varactor circuit (204) comprising a varactor diode (230) that increases with increasing applied voltage at the tuning terminal relative to the emitter terminals of the oscillator core (202), the varactor circuit (204) comprising: • a first capacitor (228) having a first terminal coupled to a first of the emitter terminals of the oscillator core (202); • a first varactor diode (230) having a cathode coupled to a second terminal of the first capacitor (228) and an anode coupled to the tuning terminal; and • a high frequency choke circuit (234) coupled between the second terminal of the first capacitor (228) and a varactor reference terminal (235); and • a voltage reference circuit (260, 272) coupled to the varactor reference terminal (235), the voltage reference circuit (260, 272) comprising: - a first resistor (268) coupled between a first reference terminal and the varactor reference terminal (235), the first reference terminal coupled to collector terminals of the oscillator core (202); and - a diode (262, 264, 266) coupled between the varactor reference terminal (235) and a second reference terminal.The voltage controlled oscillator (200; 270; 280) of claim 1, wherein the diode comprises a plurality of diodes (262, 264, 266).The voltage controlled oscillator (200; 270; 280) of claim 1 or 2, wherein the second reference terminal is a ground terminal.A voltage controlled oscillator (200; 270; 280) comprising: • an oscillator core (202) comprising a plurality of transistors (212); and • a varactor circuit (204) having a first end coupled to emitter terminals of the oscillator core (202) and a second end coupled to a tuning terminal, the varactor circuit (204) comprising a varactor diode (230) that increases with increasing applied voltage at the tuning terminal relative to the emitter terminals of the oscillator core (202), the varactor circuit (204) comprising: • a first capacitor (228) having a first terminal coupled to a first of the emitter terminals of the oscillator core (202); • a first varactor diode (230) having a cathode coupled to a second terminal of the first capacitor (228) and an anode coupled to the tuning terminal; and • a high frequency choke circuit (234) coupled between the second terminal of the first capacitor (228) and a varactor reference terminal (235); and • a voltage reference circuit (260, 272) coupled to the varactor reference terminal (235), the voltage reference circuit (260, 272) comprising a voltage regulator (272) coupled between the varactor reference terminal (235) and a first reference terminal.The voltage controlled oscillator (200; 270; 280) of claim 4, wherein the voltage regulator comprises a low voltage drop voltage regulator (272).The voltage controlled oscillator (200; 270; 280) of claim 4 or 5, wherein the first reference terminal is coupled to collector terminals of the oscillator core (202).The voltage controlled oscillator (200; 270; 280) of any of claims 4-6, wherein collector terminals of the oscillator core (202) are coupled to the varactor reference terminal (235).The voltage controlled oscillator (200; 270; 280) of claim 7, wherein the varactor reference terminal (235) is coupled to the collector terminals of the oscillator core via a feedback resistor (220).The voltage controlled oscillator (200; 270; 280) of any of claims 1-8, wherein the oscillator (200; 270; 280) comprises an output node coupled to the emitter terminals of the oscillator core.The voltage controlled oscillator (200; 270; 280) of any of claims 1-9, wherein the oscillator (200; 270; 280) has an operating frequency between about 10 GHz and about 30 GHz.A voltage controlled oscillator (200; 270; 280) comprising: • an oscillator core (202) comprising a plurality of transistors; • a varactor circuit (204) coupled to emitter terminals of the oscillator core (202), the varactor circuit comprising: - a first capacitor (228) having a first terminal coupled to a first of the emitter terminals of the oscillator 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 cathode coupled to the second terminal of the first transmission line element (232) and an anode coupled to a tuning terminal, and - a high frequency choke circuit (234) coupled between a second terminal of the first capacitor and a varactor reference terminal (235); • a feedback resistor (220) coupled between a first reference terminal and the oscillator core (202); • a voltage reference circuit (260, 272) having an output node coupled to the varactor reference terminal (235), the voltage reference circuit (260, 272) comprising a voltage regulator (272).The voltage controlled oscillator (200; 270; 280) of claim 11, wherein the voltage reference circuit (260, 272) comprises: a first resistor (268) coupled between the first reference terminal and the varactor reference terminal (235); and at least one diode (262, 264, 266) coupled between the varactor reference terminal (235) and a second reference terminal.The voltage controlled oscillator (200; 270; 280) of claim 11, further comprising a series resistor (250) coupled between the emitter terminals of the oscillator core (202) and the second reference terminal.The voltage controlled oscillator (200; 270; 280) of any of claims 11 to 13, wherein the voltage regulator (272) is coupled between the first reference terminal and the varactor reference terminal (235).The voltage controlled oscillator (200; 270; 280) of any of claims 11 to 14, wherein: the voltage regulator (272) is coupled between the first reference terminal and the feedback resistor (220), and the varactor reference terminal (235) is coupled to the feedback resistor (220).

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