Push-push voltage controlled oscillator for obtaining differential signals

Disclosed is a push-push voltage controlled oscillator which obtains output signals having a frequency two times the fundamental resonance frequency of an LC resonator with differential signals having the same amplitude and opposite phases, creating the advantage of high frequency differential outputs which are obtained using such a voltage controlled oscillator, and thus reducing current consumption.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a voltage controlled oscillator for a Radio Frequency (RF) integrated circuit and, more particularly, to a push-push voltage controlled oscillator that can obtain output signals having a frequency twice as high as the fundamental resonance frequency of an LC resonator with differential signals having the same amplitude and opposite phases.

2. Description of the Related Art

Recently, markets for mobile communication terminals that not only operate in several frequency bands but also have various functions have developed domestically and abroad.

Therefore, mobile communication terminals are being miniaturized in order to satisfy the demand of consumers, the parts of mobile communication terminals are being miniaturized, and the various parts of the RF system are also being miniaturized.

Particularly, in order to implement a transceiver for high frequency communication, an oscillator having a high output frequency, low phase noise and low power consumption is essential. Generally, inductors and capacitors, which are components constituting the LC resonator, must be physically miniaturized in order to make an oscillator with an LC resonator to oscillate in a high (radio) frequency band. Since it is known that, as elements become smaller, the errors of the elements increase, and there is a strong possibility that the error between a design frequency value and a manufacturing frequency value increases as the operating frequency of an oscillator increases.

In order to solve this problem a method of using a separate frequency doubler or a separate frequency multiplier in a voltage controlled oscillator is used for acquiring a high output frequency. However, this method has a problem in that an additional circuit is required and current consumption increases.

Another method of increasing an output frequency is based on the structure of a push-push voltage controlled oscillator. This structure is constructed so as to combine two outputs from two balanced oscillators at one point and thus obtain a doubled output frequency.

Meanwhile, it is known that differential signal processing is superior to single-ended signal processing in order to efficiently eliminate common-mode noise in an RF/analog Integrated Circuit (RFIC) and, thus, achieving signal processing resistant to noise. Therefore, RF/analog integrated circuit designers demand circuit structures capable of generating or processing differential signals. However, a conventional push-push voltage controlled oscillator generally generates only single ended output.

Accordingly, in order to acquire differential signals, an additional circuit, such as a single-to-differential converter, must be further included.

As illustrated inFIG. 1, a frequency doubler, which generates differential signals, includes a power supply (VDD), transistors M1, M2, M3and M4, impedance elements LPand LM, a non-inverted input signal Vin+, an inverted input signal Vin−, and a ground terminal. Using this, differential signals composed of fundamental frequency components, which are generated by a voltage-controlled oscillator, can be converted into differential signals each having a doubled frequency. However, there is a problem in that an additional circuit, called a differential frequency doubler, is required which results in an increase in circuit complexity and power consumption.

Furthermore, in order to acquire a high frequency within a voltage-controlled oscillator without additionally using a frequency doubler, a push-push voltage-controlled oscillator may be used. The push-push voltage-controlled oscillator has an efficient structure capable of obtaining a frequency twice as high as a fundamental operating frequency. This has advantages in that current consumption is less than that in a general structure, and when it is applied to a direct conversion type RF transceiver, the undesired pulling problem experienced with a voltage-controlled oscillator decreases. As a result, the push-push voltage-controlled oscillator structure is widely used in a Si RF integrated circuit or a compound semiconductor Monolithic Microwave Integrated Circuit (MMIC). As illustrated inFIG. 2, the structure of a conventional push-push voltage-controlled oscillator includes an LC resonator which includes an inductor Ltankand variable capacitance diodes Ctank, and determines a fundamental oscillation frequency, and a transistor pair M1and M2which generates negative resistance in order to compensate for the loss of the LC resonator, thereby outputting a doubled frequency at a common source terminal M. In this case, in order to increase the amplitude of the output signal, an impedance element is provided to the common source terminal M. However, the structure has a problem in that there is only one output signal. As described above, a single signal is very sensitive to external noise, so that it is difficult to apply it directly to an RF integrated circuit.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a push-push voltage-controlled oscillator which can decrease current consumption by not using an additional circuit for acquiring a high frequency, and generate differential signals having a frequency twice as high as a fundamental resonance frequency.

In order to accomplish the above objectives, the present invention provides a push-push voltage controlled oscillator, including an LC resonance unit for determining a fundamental oscillation frequency; a first negative resistance generation means connected to the LC resonance unit and configured to generate negative resistance and deliver an output signal to a first common source terminal; a first impedance element for controlling an output signal of the first common source terminal; a second negative resistance generation means connected to the LC resonance unit and configured to generate negative resistance and deliver an output signal to a second common source terminal; and a second impedance element for controlling an output signal of the second common source terminal.

The LC resonance unit according to the present invention determines a fundamental oscillation frequency, and may be implemented using any device that performs this task, preferably inductors and variable capacitance diodes.

The first negative resistance generation means according to the present invention is connected to the LC resonance unit, generates negative resistance, and delivers an output signal to the first common source terminal. The negative resistance generation means may be any device that performs this task, preferably transistors, more preferably MOS FETs manufactured through a Si CMOS process, more particularly a differential transistor pair in which PMOS FET pairs are cross-connected.

The first impedance element according to the present invention is connected to the first common source terminal, controls the output signal of the first common source terminal, and may be implemented using any output signal control means used in the art, preferably a resistor, a transistor, an inductor or the combination of two or more of them, more preferably an inductor.

The second negative resistance generation means according to the present invention is connected to the LC resonance unit, generates negative resistance, and delivers an output signal to the second common source terminal. The negative resistance generation means may be any resistance generation means that performs this task, preferably transistors, more preferably MOS FETs manufactured through Si CMOS process, more particularly a differential transistor pair in which NMOS FET pairs are cross-connected.

The second impedance element according to the present invention is connected to the second common source terminal, controls the output signal of the second common source terminal, and may be implemented using a resistor, a transistor, an inductor or the combination of two or more of them, preferably the same as the first impedance element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3A and 3Bare diagrams illustrating the structure of a push-push voltage controlled oscillator according to the present invention,FIGS. 4A and 4Bare diagrams illustrating conceptual operation and simulated signal waveforms when the tank swing of the push-push voltage controlled oscillator according to the present invention is small, respectively,FIGS. 5A and 5Bare diagrams illustrating conceptual operation and simulated signal waveforms when the tank swing of the push-push voltage controlled oscillator according to the present invention is large, respectively,FIG. 6is a diagram illustrating the amplitude mismatch and phase mismatch of differential output signals due to the impedance element of the push-push voltage controlled oscillator according to the present invention,FIGS. 7A and 7Bare diagrams illustrating the concept of waveforms of 2nd harmonic differential output signals according to the characteristic of the fundamental oscillation frequency signal of the push-push voltage controlled oscillator according to the present invention,FIG. 8is a diagram illustrating an example of implementing the impedance adjustment unit of the push-push voltage controlled oscillator according to the present invention,FIGS. 9A and 9Bare diagrams illustrating the effects of the adjustment of capacitors of the push-push voltage controlled oscillator according to the present invention on the waveforms of differential outputs, respectively,FIGS. 10A and 10Bare diagrams illustrating the circuit of the push-push voltage controlled oscillator according to the present invention and the simulation waveforms thereof, respectively; andFIG. 11is a diagram illustrating an example of implementing an RF transceiver using the push-push voltage controlled oscillator according to the present invention, with reference to which the present invention is described below.

As illustrated inFIGS. 3A, a push-push voltage controlled oscillator according to the present invention includes an LC resonance unit12for determining a fundamental oscillation frequency, a first negative resistance generation means2connected to the LC resonance unit12and configured to generate negative resistance and deliver an output signal to a first common source terminal22, a first impedance element18for controlling the output signal of the first common source terminal22, a second negative resistance generation means6connected to the LC resonance unit12and configured to generate negative resistance and deliver an output signal to a second common source terminal24, and a second impedance element20for controlling the output signal of the second common source terminal24.

In more detail, the push-push voltage controlled oscillator includes the LC resonance unit12for determining a fundamental oscillation frequency, the first negative resistance generation means2connected to the first terminal10and second terminal10′ of the LC resonance unit12and configured to generate negative resistance and deliver an output signal to the first common source terminal22; the first impedance element18connected at one end thereof to the common source terminal22of the first negative resistance generation means and at the other end thereof to the power supply VDD, the second negative resistance generation means6connected to the first terminal10and second terminal10′ of the LC resonance unit12connected in series to the first negative resistance generation means2and configured to generate negative resistance and deliver an output signal to the second common source terminal24, and the second impedance element20connected at one end thereof to the common source terminal24of the second negative resistance generation means6and at the other end thereof to a ground terminal.

The LC resonance unit12according to the present invention, which determines a fundamental oscillation frequency, includes an inductor14connected between the first terminal10and the second terminal10′ which connect the first negative resistance generation means2to the second negative resistance generation means6, and first and second variable capacitance diodes16and16′ connected between the first terminal10and the second terminal10′ and connected in parallel to the inductor14. In this case, a terminal formed between the first and second variable capacitance diodes16and16′ is connected to a frequency tuning voltage15to allow the voltage to be varied by an external device.

The first negative resistance generation means2according to the present invention includes PMOS FETs4and4′ cross-connected to generate negative resistance in order to compensate for the loss of the LC resonator12, and receives oscillation signals from the first terminal10and the second terminal10′ of the LC resonator12and delivers an output signal to the first common source terminal22. Preferably, the first negative resistance generation means2is constructed such that the drain of the first PMOS FET4is connected to the gate of the second PMOS FET4′ and the drain of the second PMOS FET4′ is connected to the gate of the first PMOS FET4.

The second negative resistance generation means6according to the present invention includes NMOS FETs8and8′ cross-connected to generate negative resistance in order to compensate for the loss of the LC resonator12, and receives oscillation signals from the first terminal10and the second terminal10′ of the LC resonator12and delivers an output signal to the second common source terminal24. Preferably, the second negative resistance generation means6is constructed such that the drain of the first NMOS FET8is connected to the gate of the second NMOS FET8′ and the drain of the second NMOS FET8′ is connected to the gate of the first NMOS FET8.

The first impedance element18according to the present invention is connected at one end thereof to the common source terminal22of the first negative resistance generation means2and at the other end thereof to the power supply VDD, controls the output signal of the first common source terminal22, and is implemented using an inductor.

The second impedance element20according to the present invention is connected at one end thereof to the common source terminal22of the first negative resistance generation means2and at the other end thereof to the ground terminal, controls the output signal of the second common source terminal24, and is implemented using an inductor.

In this case, as illustrated inFIG. 3B, the first impedance element18and the second impedance element20can be combined to each other in a transformer form such that the corresponding signals are coupled in opposite phase, so that the output signals from the respective impedance elements can become more accurate differential signal.

When the voltage controlled oscillator according to the present invention operates, differential outputs are realized at the first terminal10and the second terminal10′. At this time, the upper and lower common source terminals (the first common source terminal of the PMOS FETs is referred to as a ‘P terminal’, while the second common source terminal of the NMOS FETs is referred to as an ‘M terminal’) of the PMOS FETs4and4′ and the NMOS FETs8and8′ acquire output signals, that is, a first output voltage signal (hereinafter referred to as “V2fp”) and a second output voltage signal (hereinafter referred to as “V2fm”), which are the 2nd harmonics of a first oscillation voltage signal (hereinafter referred to as “Vfp”) and a second oscillation voltage signal (hereinafter referred to as “Vfm”). In this case, V2fpand V2fm, generated in the P terminal22and the M terminal24, are differential signals, the phase mismatch between which is 180 degrees and the amplitudes of which are the same.

There are two modes in the generation of the 2ndharmonic signal at the P terminal22and the M terminal24depending on the operating mechanism thereof. In this case, since the PMOS FET-related signal waveform of the voltage controlled oscillator is based on the same principle as the NMOS FET-related signal waveform, and is symmetrical to the other waveform, the following description is given only in conjunction with the NMOS FET.

A first mode is a source follow operation mode. As illustrated inFIG. 4A, transistors8and8′ alternately operate as a source follow amplifier or are turned off depending on the input voltage of a gate. At this time, the signal waveform of “V2fm” following the half-cycle +regions of “Vfp” and “Vfm” is generated depending on a transistor operating as a source follow transistor, and “V2fp” is generated from the PMOS FETs4and4′.

In order to prove this, a 3 GHz voltage controlled oscillator circuit was designed using a 0.18 μm CMOS process as illustrated inFIG. 3A, and SPICE simulation was performed.FIG. 4Bis a diagram illustrating simulated signal waveforms which represent the signal waveform of V2fm, following the half-cycle +regions of Vfpand Vfmand, in contrast, represent the signal waveform of V2fpfollowing the half-cycle −regions of Vfpand Vfm. At this time, it must be noted that the signal waveforms illustrated inFIG. 4Boccur when the amplitudes of Vfpand Vfmare lower than that of the gate-source bias voltage of each transistor. In this case, V2fpand V2fmare signals similar to differential signals.

A second mode according to the present invention is performed when the amplitude of an LC tank increases and then the amplitudes of Vfpand Vfmare higher than that of the gate-source bias voltage. V2fmis represented as a doubled operating frequency because part of the−regions of Vfpand Vfmalternatively appear at the terminal M24as illustrated inFIG. 5A.FIG. 5Bis a diagram illustrating the signal waveforms resulting from the simulation of Vfp, Vfm, V2fpand V2fmfor the 3 GHz voltage controlled oscillator circuit designed using 0.18 μm CMOS process as inFIG. 3A. In this drawing, +regions and −regions of Vfpand Vfmare represented in V2fpand V2fm. At this time, V2fpand V2fm, are very precise differential signals.

ComparingFIG. 4BwithFIG. 5B, it is appreciated that relatively precise differential signals are generated in the second mode according to the present invention rather than the first mode. The reason for this is that the PMOS FETs4and4′ and NMOS FETs8and8′ generate V2fpand V2fm, using a relatively unbalanced source follow operation in the first mode, but differential Vfpand Vfmgenerated by the voltage controlled oscillator directly form V2fpand V2fmin the second mode.

As described above, the push-push voltage controlled oscillator generates differential signals, for example, V2fpand V2fmthat are two times Vfp) and Vfm, using the same.

The common source terminals22and24according to the present invention are used as terminals for generating an output signal. Generally, since the common source terminals22and24act as a virtual ground when being used in a differential circuit, low output signals are generated at the common source terminals22and24. As a result, in order to increase the low outputs generated at the P terminal22and M terminal24, a device for increasing impedances thereof must be included, and, preferably, the first and second impedance elements18and20may be implemented using inductors.

Meanwhile, the impedances of the first and second output voltages vary depending on inductance values, and, therefore, the amplitudes of V2fpand V2fmand phase mismatch therebetween may vary. In order to examine this, amplitude mismatch and phase mismatch due to the first and second impedance elements18and20was simulated, so that the inductor of the first impedance element18is set to 1.8 nH, and the inductor of the second impedance element20is set from 3.2 nH to 4.0 nH, the resultant values of which are shown inFIG. 6. That is,FIG. 6illustrates the tendency in which amplitude mismatch and phase mismatch vary depending on the values of the first and second impedance elements.

Generally, a passive component is known to have a process tolerance rate of ±5%. Therefore, when the inductor of the second impedance element20has a tolerance of 3.2 nH to 4.0 nH according to the tolerance rate, the phase thereof varies by about ±5 degrees, and the amplitude mismatch thereof is maintained within 1.0 dB

As described above, the push-push voltage controlled oscillator according to the present invention obtains differential signals of the first and second output voltages using differential signals of the first and second oscillation voltages at the common source terminals22and24of the voltage controlled oscillator. At this time, in order to be perfect differential signals for the first and second output voltages, the first and second oscillation voltages must be formed such that the signal waveforms of Vfpand Vfmare symmetrical with each other.FIG. 7Aillustrates the signal waveform of Vf, composed of differential Vfpand Vfm, and signal waveforms of V2fpand V2fmobtained from this. A doubled frequency, generated by combining half-cycles of Vfpand Vfmis divided into a positive region V2fpand a negative region V2fm. As a result, it will be appreciated that V2fpand V2fmare perfect differential signals formed so as to be symmetrical with each other.

Meanwhile, when the two signals of the fundamental frequency Vfare asymmetrical because the rising times and falling times are different and, therefore, the peak times thereof are not matched with each other, as illustrated by the fundamental signal VfinFIG. 7B, V2fp, representing the positive region of the fundamental signal Vf, and V2fm, representing the negative region of the fundamental signal Vf, do not form differential signals. The reason for this is that the peak times of the signals Vfpand Vfmare different, so that the peak times of the signals V2fpand V2fmare different. That is, in order to be perfect differential signals for V2fpand V2fm, the signal waveform of the fundamental frequency Vfmust be a perfect differential signal as illustrated inFIG. 7A. As a result, in order to obtain differential signals V2fpand V2fm, according to the present invention, it is important to construct a circuit such that V2fpand V2fmare balanced.

As described above, impedance values vary depending on the values of the first and second impedance elements18and20, and errors of amplitudes and phases of V2fpand V2fmare generated. Therefore, the first and second impedance elements18and20are determined to minimize the errors of amplitude and phase. However, since the impedance values may vary depending on external conditions, such as capacitance and resistance due to a parasitic component and/or process tolerance, the errors can be corrected by adjusting the inductance values of the first and/or second impedance elements18and20. For this purpose, as illustrated inFIG. 8, an impedance adjustment unit26may be connected to the first impedance element18.

The impedance adjustment unit26compares amplitude and phase mismatch at the first common source terminal22with those at the second common source terminal24, and adjusts the impedance of the first or second impedance element18or20or overall impedances thereof The impedance adjustment unit26is connected in series or, preferably, in parallel to the first impedance element18. Furthermore, the impedance adjustment unit26may be constructed to automatically compare amplitudes and phase mismatchs and adjust the impedances.

In this case, the impedance adjustment unit26includes one or more capacitors28and switches30connected to the capacitors28to selectively turn on or off the capacitors28. The capacitors28are connected in series, in parallel, or in an arrangement having both serial and parallel connections. At this time, the values of the capacitors28connected in parallel are changed by operating switches30, so that the equivalent impedance value at the terminal22is also changed, thereby adjusting the amplitude and phase of V2fp.

As a result, the signals of V2fpand V2fmare adjusted to be perfect differential signals. Furthermore, when the impedance adjustment unit26is connected to the second impedance element20, the impedance at the terminal24can be adjusted, like the first impedance unit18.

That is, the impedance adjustment unit26is connected to the first impedance element18, the second impedance element19, or both of them, so that the amplitudes and phase errors of V2fpand V2fmcan be eliminated.

FIG. 9Ais a diagram illustrating the change in the signal waveform V2fin the case in which the capacitors28are not adjusted, andFIG. 9Bis a diagram illustrating the change in the signal waveform V2fin the case in which the capacitors28are adjusted. That is, it will be appreciated that the amplitude and phase error of V2fpchange depending on the values of the capacitors28.

Therefore, the change in impedance which is generated by process tolerance or other conditions are corrected by adjusting the capacitors28, so that V2fpand V2fm, having a doubled frequency of a fundamental frequency, can be obtained as perfect differential signals at the common source terminals22and24.

FIG. 10Ais a circuit diagram illustrating a push-push voltage controlled oscillator obtaining differential output signals in COMS technology.

The push-push voltage controlled oscillator circuit is constructed such that output signals Vopand Vomare measured using a buffer amplifier provided to measure differential signals V2fpand V2fmfrom the push-push voltage controlled oscillator illustrated inFIG. 8.

In this case, the buffer amplifier has a cascode amplifier structure and includes a third NMOS FET transistor32, a fourth NMOS FET transistor34, a fifth NMOS FET transistor36, a sixth NMOS FET transistor38, capacitors40,41,42and43, a first inductor44, and a second inductor44′.

The drain of the fifth NMOS FET36is connected to the first inductor44connected to a power supply VDD, the source of the fifth NMOS FET36is connected to the drain of the third NMOS FET32, and the gate of the fifth NMOS FET36is connected to the first capacitor40connected to a ground terminal. Furthermore, the gate of the fifth NMOS FET36is connected to a first resistor48connected to the power supply VDD, and the drain of the fifth NMOS FET36is connected to the third capacitor42.

The gate of the third NMOS FET32is connected to a first coupling capacitor46connected to a first common source terminal22. The source of the third NMOS FET32is connected to the source of the fourth NMOS FET34. Furthermore, the common source terminal of the third and fourth NMOS FET32and34is connected to the ground terminal.

The gate of the fourth NMOS FET34is connected to a second coupling capacitor46′ connected to a second common source terminal24, and the drain of the fourth NMOS FET34is connected to the source of the sixth NMOS FET38.

The gate of the sixth NMOS FET38is connected to the second capacitor41connected to the ground terminal, the drain of the sixth NMOS FET38is connected to the second inductor44′ connected to the power supply VDD, and the drain of the sixth NMOS FET38is connected to a fourth capacitor43. Furthermore, a second resistor48′ connected to the power supply VDD is connected between the gate of the sixth NMOS FET38and the second capacitor41.

FIG. 10Bis a diagram illustrating the signal waveforms of V2fpand V2fmin the push-push voltage controlled oscillator circuit obtaining differential outputs by using RF CMOS technology, in which V2fpand V2fmare the signals after they pass through the coupling capacitors46and46′, and therefore the difference of the dc values of the signals V2fpand V2fmare removed.

As illustrated inFIG. 11, the push-push voltage controlled oscillator circuit can be used in a direct conversion type RF transceiver. In this case, the oscillation frequency of the push-push voltage controlled oscillator core is foand a frequency supplied to mixers for frequency conversion is 2fo. In the direct conversion type RF transceiver, the voltage controlled oscillator connected to a phase locked loop is connected to first and second mixers, the first and second mixers are respectively in a transmission unit50and a reception unit52, and the transmission unit50and reception unit52are connected to an antenna through a duplexer. In this case, the mixer refers to a circuit for frequency upcoversion or downconversion. Furthermore, the phase locked loop serves to precisely adjust the frequency of the voltage controlled oscillator, thereby adjusting the output frequency of the oscillator to a desired frequency.

The transmission unit50is constructed in a form such that a Variable Gain Amplifier (VGA), a mixer, and a Power Amplifier (PA), are connected and input signals are sequentially supplied thereto. The reception unit52is constructed in a form in which a Low Noise Amplifier (LNA), a mixer and a variable gain amplifier are sequentially connected.

In the direct conversion type RF transceiver, the first buffer amplifier for a local oscillator is connected to the mixer of the transmission unit, and the second buffer amplifier for a local oscillator is connected to the mixer of the reception unit.

Since intermediate frequencies are omitted in such a direct conversion manner, the structure of a transceiver is simplified. As a result, various types of Surface Acoustic Wave (SAW) filters and mixers can be omitted, so the reduction of manufacturing costs and the miniaturization of size, and one-chip integration of a system become possible. Furthermore, when a push-push voltage oscillator circuit is applied to the direct conversion manner, the operating frequency of the oscillator core is separated from radio frequencies, so that frequency interference therebetween decreases and, therefore, decreasing the pulling problem experienced with the voltage oscillator circuit, and thereby, channel selectivity in the receiver and the output spectrum quality in the transmitter is improved.

As described above, the present invention obtains output signals having a frequency two times the fundamental resonance frequency of an LC resonator in a form of differential signals having the same amplitude and opposite phases, so that advantages occur in that external noise can be effectively eliminated, high frequency differential outputs are obtained using a voltage controlled oscillator, and current consumption can be reduced because an additional circuit is not used.