Inductor-capacitor oscillator with embedded second harmonic filter and associated dual core oscillator

An inductor-capacitor (LC) oscillator with an embedded second harmonic filter and an associated dual core oscillator are provided. The LC oscillator includes a first transistor, a second transistor, a first part-one inductor, a second part-one inductor, a part-one capacitor, a part-two inductor and at least one part-two capacitor. A first end of the first part-one inductor and a first end of the second part-one inductor are coupled to gate terminals of the second transistor and the first transistor, respectively. The part-one capacitor is coupled between the first end of the first part-one inductor and the first end of the second part-one inductor. The part-two inductor is coupled between a second end of the first part-one inductor and a second end of the second part-one inductor. The at least one part-two capacitor is coupled to drain terminals of the first transistor and the second transistor.

BACKGROUND

The present invention is related to inductor-capacitor (LC) oscillator, and more particularly, to an LC oscillator with an embedded second harmonic filter and an associated dual core oscillator.

In general, an oscillator not only generates a fundamental frequency which is determined by a main resonant tank, but also generates unwanted second harmonic frequency which may cause noise up-conversion. An extra tank may be adapted to filter the second harmonic frequency in order to block or weaken the second harmonic frequency in related arts. Some disadvantages still exist in this architecture, however. For example, the main resonant tank and the extra tank may be independent and apart from each other, and process variation of components such as inductors and capacitors within the main resonant tank and the extra tank may degrade the performance of second harmonic filtering since their variations are typically also independent. Furthermore, the extra tank is usually designed to have low-quality factor (low-Q) because of the area limitation, and therefore has poor noise-related performance.

Thus, there is a need for a novel architecture of an LC oscillator, which has less sensitivity regarding the process variation and better noise-related performance (e.g. less phase noise) in comparison with the related arts.

SUMMARY

An objective of the present invention is to provide an inductor-capacitor (LC) oscillator with an embedded second harmonic filter and an associated dual core oscillator, which have less sensitivity regarding the process variation and better noise-related performance (e.g. less phase noise) in comparison with the related arts.

At least one embodiment of the present invention provides an LC oscillator with an embedded second harmonic filter. The LC oscillator may comprise a first transistor, a second transistor, a first part-one inductor, a second part-one inductor, a part-one capacitor, a part-two inductor and at least one part-two capacitor. A first end of the first part-one inductor and a first end of the second part-one inductor are coupled to gate terminals of the second transistor and the first transistor, respectively. The part-one capacitor is coupled between the first end of the first part-one inductor and the first end of the second part-one inductor. The part-two inductor is coupled between a second end of the first part-one inductor and a second end of the second part-one inductor. The at least one part-two capacitor is coupled to drain terminals of the first transistor and the second transistor.

At least one embodiment of the present invention provides a dual core oscillator, wherein the dual core oscillator may comprise a first LC oscillator and a second LC oscillator identical to each other. Each of the first LC oscillator and the second LC oscillator may comprise a first transistor, a second transistor, a first part-one inductor, a second part-one inductor, a part-one capacitor, a part-two inductor and at least one part-two capacitor. A first end of the first part-one inductor and a first end of the second part-one inductor are coupled to gate terminals of the second transistor and the first transistor, respectively. The part-one capacitor is coupled between the first end of the first part-one inductor and the first end of the second part-one inductor. The part-two inductor is coupled between a second end of the first part-one inductor and a second end of the second part-one inductor. The at least one part-two capacitor is coupled to drain terminals of the first transistor and the second transistor. More particularly, the part-two inductor of the first LC oscillator is coupled to the part-two inductor of the second LC oscillator.

At least one embodiment of the present invention provides a dual core oscillator. The dual core oscillator may comprise an LC oscillator, wherein the LC oscillator may comprise a first transistor, a second transistor and a first LC tank. The first LC tank may comprise a first part-one inductor, a second part-one inductor, a part-one capacitor, a part-two inductor and at least one part-two capacitor. A first end of the first part-one inductor and a first end of the second part-one inductor are coupled to gate terminals of the second transistor and the first transistor, respectively. The part-one capacitor is coupled between the first end of the first part-one inductor and the first end of the second part-one inductor. The part-two inductor, coupled between a second end of the first part-one inductor and a second end of the second part-one inductor. The at least one part-two capacitor, coupled to drain terminals of the first transistor and the second transistor. The dual core oscillator may further comprise a second LC tank, which comprises at least one inductor and at least one capacitor. More particularly, the part-two inductor of the first LC oscillator is coupled to the at least one inductor of the second LC tank.

The LC oscillator provided by embodiments of the present invention has an LC tank with an embedded second harmonic filter therein. All components (e.g. inductors and capacitors) within the LC tank can be laid out together, which greatly reduce sensitivity to the process variation in comparison with using a standalone second harmonic filter tank. In addition, embodiments of the present invention will not greatly increase overall costs. Thus, the present invention can solve the problem of the related art without introducing any side effect or in a way that is less likely to introduce side effects.

DETAILED DESCRIPTION

FIG. 1Ais a diagram illustrating an inductor-capacitor (LC) oscillator10with an embedded second harmonic filter according to an embodiment of the present invention. As shown inFIG. 1A, the LC oscillator10may comprise a first transistor such as a P-type transistor MP (e.g. P-type Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)), a second transistor such as an N-type transistor MN (e.g. N-type MOSFET), a first part-one inductor such as an inductor L11, a second part-one inductor such as an inductor L12, a part-one capacitor such as a capacitor C1, a part-two inductor such as an inductor L2, and at least one part-two capacitor C2. A first end of the inductor L11and a first end of the inductor L12are coupled to gate terminals of the N-type transistor MN and the P-type transistor MP, respectively. The capacitor C1is coupled between the first end of the inductor L11and the first end of the inductor L12. The inductor L2is coupled between a second end of the inductor L11and a second end of the inductor L12. The at least one part-two capacitor is coupled to drain terminals of the first transistor and the second transistor. For example, the capacitor C2is coupled between the drain terminals of the P-type transistor MP and the N-type transistor MN. Source terminals of the P-type transistor MP and the N-type transistor MN are coupled to a supply voltage terminal (e.g. a terminal providing the highest fixed voltage level within the LC oscillator10) and a ground voltage terminal (e.g. a terminal providing the lowest fixed voltage level within the LC oscillator10), respectively.

As shown inFIG. 1A, the LC oscillator10may further comprise a first part-three inductor such as an inductor L31and a second part-three inductor such as an inductor L32, where the inductor L31is coupled between the second end of the inductor L11and the drain terminal of the P-type transistor MP, and the inductor L32is coupled between the second end of the inductor L12and the drain terminal of the N-type transistor MN.

For better illustration, inductance and capacitance of corresponding components are indicated by italic of the same/similar symbols of the corresponding components. For example, capacitance of the capacitor C1and capacitance of the capacitor C2are represented by C1and C2, respectively, inductance of each of the inductors L11and L12is represented by L1, inductance of the inductor L2is represented by L2, inductance of each of the inductors L31and L32is represented by L3, where a symbol “s” may represent a variable associated with frequency and phase. As for the architecture which has the inductors L31and L32shown inFIG. 1A, at least the inductors L11and L12, the inductor L2and the capacitor C1constitute a fundamental frequency resonant tank. In detail, an impedance Zin1at a fundamental frequency fo may be illustrated as follows:

Zi⁢n⁢1@fo=s(L1+L22[1+s2⁡(L3⁢2⁢C2)1+s2⁡(L3+L22)⁢(2⁢C2)])⁢1s⁡(2⁢C1)≈s⁡(L1+L22)⁢1s⁡(2⁢C1)
Furthermore, at least the inductor L2, the capacitor C2, the inductors L31and L32constitute a second harmonic filter to block or weaken second harmonic signals of the LC oscillator10. In detail, an impedance Zin2at a second harmonic frequency 2fo may be illustrated as follows:

In some embodiments, the inductors L31and L32may be omitted, e.g. the second end of inductor L11and the second end of the inductor L12may be directly connected to the drain terminals of the P-type transistor MP and the N-type transistor MN, respectively. Under this condition, at least the inductor L2and the capacitor C2constitute a second harmonic filter to block or weaken the second harmonic signals. In detail, the impedance Zin2under the condition where the inductors L31and L32are omitted may be illustrated as follows:

Zin⁢⁢2@2⁢fo=s(L22[1+s2⁡(L1⁢2⁢C1)1+s2⁡(L1+L22)⁢(2⁢C1)])⁢1s⁡(2⁢C2)*α≈s⁡(L22)⁢1s⁡(2⁢C2*α)
As the inductors L31/L32do not greatly affect the impedance Zin1, related details are omitted here for brevity. The symbol α may represent a positive value, showing a larger C2multiplied by a factor for given 2fo, which is for illustrative purpose only, and is not meant to be a limitation of the present invention.

In general, a voltage gain AVfrom V2to V1is preferably as high as possible, where the voltage gain AVmay be illustrated as follows:

R⁢p=QL⁢QCQL+QC⁢L2C2
Assume that QL=QC, where QLrepresents a quality factor of the inductor L2, and QCrepresents a quality factor of the capacitor C2. To increase Zin(more particularly, to increase Rp) without changing a resonant frequency (e.g. 2fo) of the second harmonic filter, L2needs to be increased and C2needs to be reduced. Meanwhile, the voltage gain Av will be decreasingly reached to unit. With the inductors L31and L32, it is preferably to increase L3rather than increase L2, in order to increase the impedance Zin2without sacrificing the voltage gain AV, and thereby optimizing overall performance of the LC oscillator10.

In comparison with the related art, the LC oscillator10shown inFIG. 1Acan effectively increase the impedance Zin2at the second harmonic frequency 2fo, and therefore improve the noise-related performance (e.g. less phase noise) and efficiency (e.g. figure of merit (FOM), which is related to power consumption, noise and signal swing). Another advantage of the LC oscillator10shown inFIG. 1Ais that the second harmonic filter tracks variation caused by the fundamental frequency resonant tank. For example, when the capacitance C1varies and the resonant frequency of the fundamental frequency resonant tank is accordingly reduced (e.g. fo is reduced), the resonant frequency of the second harmonic filter will be reduced as well, substantially tracking the second harmonic frequency (e.g. tracking 2f0). In addition, as both the fundamental frequency resonant tank and the second harmonic filter will not be separated by transistors (e.g. the P-type transistor MP and the N-type transistors MN), all of the inductors L11, L12, L2, L31and L32can be laid out together, and more particularly, as shown inFIG. 1B, the inductors L11, L12, L2, L31and L32can be implemented by a continuous metal layer without segmentation. Even if the inductors L31and L32are omitted in some embodiments, the inductors L11, L12and L2also have the similar advantage of being implemented by a continuous metal layer without segmentation. Under this condition, impacts of process variation upon these components can be quite similar or substantially identical to each other, and mismatch (e.g. relative difference) between these components due to process variation can be minimized, which can minimize performance sensitivity to the process variation.

It should be noted that the first transistor and the second transistor are not limited to utilizing different types of transistor.FIG. 2is a diagram illustrating an LC oscillator20according to an embodiment of the present invention. The LC oscillator20is quite similar to the LC oscillator10shown inFIG. 1A. The main difference is both of the first transistor and the second transistor are implemented by P-type transistors MP1and MP2, the at least one capacitor is implemented by capacitors C21and C22, where the source terminal of the P-type transistors MP1and MP2are coupled to the supply voltage terminal, the capacitor C21is coupled between a drain terminal of the P-type transistor MP1and the ground voltage terminal, the capacitor C22is coupled between a drain terminal of the P-type transistor MP2and the ground voltage terminal, and a center tap of the inductor L2is coupled to the ground voltage terminal. The advantages and performance of the LC oscillator20are similar to that of the LC oscillator10, and related details are not repeated here for brevity.

FIG. 3is a diagram illustrating an LC oscillator30according to an embodiment of the present invention. The LC oscillator30is quite similar to the LC oscillator20shown inFIG. 2. The main difference is both of the first transistor and the second transistor are implemented by N-type transistors MN1and MN2, where the source terminal of the N-type transistors MN1and MN2are coupled to the ground voltage terminal, the capacitor C21is coupled between a drain terminal of the N-type transistor MN1and the supply voltage terminal, the capacitor C22is coupled between a drain terminal of the N-type transistor MN2and the supply voltage terminal, and the center tap of the inductor L2is coupled to the supply voltage terminal. The advantages and performance of the LC oscillator30are similar to that of the LC oscillator10, and related details are not repeated here for brevity.

As the second harmonic filtering is aimed to filter common mode current, any additional filter can be further added on a path where the common mode current flows through, in order to build double second harmonic filtering. More specifically, a tail filter can be added on a path where common mode current flow through in the LC oscillator20shown inFIG. 2or the LC oscillator30shown inFIG. 3. Taking the LC oscillator20as an example, the tail filter (e.g. an inductor Ltailand a capacitor Ctailconnected in parallel) can be coupled to source terminals of the P-type transistors MP1and MP2, to configure an LC oscillator40as shown inFIG. 4, and an impedance Zcmhighat the second harmonic frequency 2fo on a common mode signal path of the LC oscillator40are labeled “Zcmhigh@2fo” inFIG. 4for brevity. Similarly, the tail filter (e.g. the inductor Ltailand the capacitor Ctailconnected in parallel) can be coupled to a center tap of the inductor L2, to configure an LC oscillator50as shown inFIG. 5, and the impedance Zcmhighat the second harmonic frequency 2fo on a common mode signal path of the LC oscillator50are labeled “Zcmhigh@2fo” inFIG. 5for brevity. In comparison with merely utilizing the tail filter, phase noise sensitivity of the LC oscillator40or50regarding capacitor variation can be greatly reduced. The configuration of adding the tail filter into the oscillator30may be deduced by analogy, and is therefore omitted here for brevity.

FIG. 6Ais a diagram illustrating a dual core oscillator60according to an embodiment of the present invention. In particularly, the dual core oscillator may comprise a first LC oscillator and a second LC oscillator identical to each other. For example, an upper half portion of the dual core oscillator60may be regarded as the first LC oscillator, and a lower half portion of the dual core oscillator60may be regarded as the second LC oscillator, where each of the first LC oscillator and the second LC oscillator may be implemented by the LC oscillator10shown inFIG. 1A, and the part-two inductor of the first LC oscillator is coupled to the part-two inductor of the second LC oscillator. It should be noted that the part-two inductor of the first LC oscillator and the part-two inductor of the second LC oscillator are implemented by inductors L21and L22, where a center tap of the inductor L21is coupled to a center tap of the inductor L22, and both of the inductors L21and L22have the inductance L2. More particularly, combination of an upper portion of the inductor L21and an upper portion of the inductor L22may be regarded as the part-two inductor of the first LC oscillator, and combination of a lower portion of the inductor L21and a lower portion of the inductor L22may be regarded as the part-two inductor of the second LC oscillator. Furthermore, all inductors (e.g. the inductor L11/L12and the inductors L31/L32within both of the first LC oscillator and the second LC oscillator, and the inductors L21/L22) can be implemented by a continuous metal layer without segmentation as shown in FIG.6B.

In some embodiments, alternative designs of the LC oscillator10may be applied to the dual core oscillator60, e.g. the inductors L31and L32within the first LC oscillator and the second LC oscillator of the dual core oscillator60may be omitted. Similarly, when the inductors L31and L32within the first LC oscillator and the second LC oscillator may be omitted, all inductors (e.g. the inductor L11/L12within both of the first LC oscillator and the second LC oscillator, and the inductors L21/L22) within the dual core oscillator60can also be implemented by a continuous metal layer without segmentation. Design considerations of the dual core oscillator60with/without the inductors L31and L32may refer to the description related to the embodiment ofFIG. 1A, and are not repeated here for brevity.

In some embodiments, the P-type transistor MP and the N-type transistor MN within the second LC oscillator shown inFIGS. 6A and 6Bmay be omitted. For example, the second LC oscillator can be replaced with a second LC tank (e.g. without transistor therein), as shown by the lower half portion of a dual core oscillator70shown inFIG. 7A. As the only difference between the dual core oscillator60and the dual core oscillator70is whether to configure the P-type transistor MP and the N-type transistor MN in the second LC oscillator, other details of the dual core oscillator70are not repeated here for brevity. Similarly, all inductors (e.g. the inductor L11/L12and the inductors L31/L32within both of the first LC oscillator and the second LC tank, and the inductors L21/L22) can be implemented by a continuous metal layer without segmentation as shown inFIG. 7B.

As the dual core oscillators60and70are based on the architecture shown inFIG. 1A, the dual core oscillators60and70also have all the advantages of the LC oscillator10. Furthermore, in comparison with utilizing a single LC oscillator (e.g. any of the LC oscillators10,20,30,40and50), the dual core oscillators60and70may effectively double output signal swing/power, and thereby equivalently reduce overall phase noise (e.g. improve 3 dB) under a condition where the efficiency (e.g. FOM) is unchanged.

Briefly summarized, the embodiments of the present invention provides an LC oscillator with an embedded second harmonic filter, which combines a fundamental resonant tank and a second harmonic filter into one LC network. The LC oscillator can effectively increase the impedance regarding second harmonic signals without sacrificing the voltage gain, and impact of process variation upon overall performance can be minimized, since capacitors and inductors mismatch can be minimized by properly layout as shown in the embodiments. In comparison with the related art, the embodiments of the present invention will not greatly increase overall cost. Thus, the present invention can improve overall performance of the LC oscillator without introducing any side effect or in a way that is less likely to introduce side effects.