WO2016029946A1

WO2016029946A1 - A voltage controlled oscillator without varactor

Info

Publication number: WO2016029946A1
Application number: PCT/EP2014/068238
Authority: WO
Inventors: Mingquan Bao
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2014-08-28
Filing date: 2014-08-28
Publication date: 2016-03-03

Abstract

A voltage controlled oscillator (200) for generating a signal with a frequency range. The voltage controlled oscillator (200) comprises an oscillator core (210) comprising an amplifier (212) and a resonator (214) and a tunable capacitance cell (220) having an input/output port. The input/output port is connected to the resonator (214) of the oscillator core (210). The tunable capacitance cell (220) comprises a cross-coupled transistor pair (Q11; Q12), the base or gate of the first transistor (Q11) and the base or gate of the second transistor (Q12) are coupled to a control signal to tuning an input capacitance and conductance of the tunable capacitance cell (220).

Description

A VOLTAGE CONTROLLED OSCILLATOR WITHOUT VARACTOR

TECHNICAL FIELD

Embodiments herein relate to a Voltage Controlled Oscillator (VCO). In particular, they relate to a varactorless VCO for generating a signal with a frequency range.

BACKGROUND

Wireless or fibre communication systems usually comprise transceivers which comprise receivers and transmitters. A low phase noise voltage controlled oscillator is usually required to provide a local oscillator (LO) signal with a certain frequency range for a transceiver in a wireless or a fibre communication system. As operation frequencies of transceivers increase continually, e.g. a wireless micro- or millimeter-wave transceiver operates at 71 -76 GHz and 81 -86 GHz E-band or at 1 10-170 GHz D-band, it is a challenge to fulfil all requirements on performance of a voltage controlled oscillator, e.g. lower phase noise, low supply voltage, less variation on output power etc. The phase noise of a voltage controlled oscillator is affected by a quality factor, Q, of a resonator which usually consists of an inductor and a varactor. Unfortunately, the varactor's quality factor decreases with increasing operation frequency significantly. At millimeter-wave frequency, the quality factor of a varactor is a dominate limitation on the quality factor of the resonator. To avoid this limitation, several varactorless voltage controlled oscillators have been developed.

In Chen, W. et al, 10 GHz Quadrature-Phase Voltage Controlled Oscillator and Prescaler, Proc. of the 29th European Solid-State Circuits Conference, 2003, pp. 361-364, a quadrature phase oscillator comprising a quadrature voltage controlled oscillator pair is disclosed. Varying a frequency of the quadrature phase oscillator is achieved by controlling a coupling coefficient between two oscillators in the quadrature voltage controlled oscillator pair.

In Yang, C. Y. et al, A Voltage-Controlled Varactorless LC-tank Oscillator with a Transformer Feedback Technique, Microwave and Optical Technology Letters, November 2007, Vol. 49, No. 1 1 , and in Cusmai, G. et al, A Magnetically Tuned Quadrature Oscillator, IEEE Journal of Solid-State Circuits, December 2007, Vol. 42, No. 12, transformers comprising two coupled inductors are used as a part of the LC-tank resonator to tune frequencies of the oscillators via varying a mutual inductance between the two inductors in the transformers. In Kwok, K. C. et al, A 23-to-29 GHz Transconductor-Tuned VCO MMIC in 0.13 um CMOS, IEEE Journal of Solid-State Circuits, December 2007, Vol. 42, No. 12, four transistors form a differential transconductor which is tunable for both positive and negative trans-conductance values G_m by using current sources at source/emitter of the four transistors. The admittance of the differential transconductor depends upon the value of G_m. Therefore, a resonant frequency of a LC-tank comprising the differential transconductor becomes tunable with the value of G_m.

In Veenstra, H. et al, Varactorless, tunable LC-VCO for microwave frequencies in a 0.25μηι SiGe technology, Proc. IEEE BCTM, pp. 54-57, 2007 (4), a bias-dependent collector- base junction capacitance of two transistors is used to tune a frequency of the tunable LC- VCO.

In Chen, Y. et al, Wideband varactorless LC-VCO using a tunable negative-inductance cell, IEEE Transactions on Circuits And Systems, October 2010, Vol. 57, No. 10, a tunable negative inductive cell is used to a LC-VCO, which comprises two bipolar transistors forming a cross-coupled pair with their emitters degenerated by an inductor. Further, in Chen, Y.et al, A varactorless VCO with 15% continuous frequency tuning range and 0.2 dB output power variation, European Microwave Integrated Circuits Conference (EuMIC), 2010 pp. 361 - 364, either a tunable negative capacitive cell or a tunable negative inductance is applied to voltage controlled oscillators, which comprises two bipolar transistors forming a cross- coupled pair with their emitters degenerated by either a capacitor or a inductor

correspondingly. The impedance of the negative inductive cell or the capacitive cell can be tuned by changing DC (Direct Current) currents of current sources at emitters of two cross- coupled transistors. The existing varactorless techniques or topologies applied for VCOs described above have different kinds of problems. For instance, the quadrature phase oscillator comprising a quadrature voltage controlled oscillator pair is applicable only for a quadrature VCO. Using transformers will introduce losses of power. The transconductor-tuned VCO suffers from a trade-off between frequency tuning range and the tank quality factor. Using the bias- dependent collector-base junction capacitance of two transistors has the same problem as using varactor, because it utilizes the junction capacitor. Further, the tunable negative capacitance/inductance cell also has problems, e.g. the inductor/capacitor at the emitter degrades the gain of the cell; the voltage headroom needed for the current sources requires a high supply voltage; the parasitic capacitance at the output port of the current sources has effect on the tuning range of the cell etc. SUMMARY

Therefor it is an object of embodiments herein to provide a varactorless voltage controlled oscillator with improved performance.

According to one aspect of embodiments herein, the object is achieved by a voltage controlled oscillator for generating a signal with a frequency range. The voltage controlled oscillator comprises an oscillator core comprising an amplifier and a resonator. The voltage controlled oscillator further comprises a tunable capacitance cell having an input/output port, where the input/output port is connected to the resonator of the oscillator core. The tunable capacitance cell comprises a cross-coupled transistor pair, which is connected as the following:

a base or gate of a first transistor is coupled to a collector or drain of a second transistor via a first capacitor;

a base or gate of the second transistor is coupled to a collector or drain of the first transistor via a second capacitor;

an emitter or source of the first transistor and an emitter or source of the second transistor are connected to a ground; and

the collector or drain of the first transistor and the collector or drain of the second transistor form the input/output port; and further

the base or gate of the first transistor and the base or gate of the second transistor are coupled to a control signal to tuning an input capacitance and conductance of the tunable capacitance cell. Since the voltage controlled oscillator according to embodiments herein uses a cross- coupled transistor pair with capacitors at bases or gates of the transistors to form a tunable capacitance cell, using a varactor with low quality factor Q is avoided. The tunable capacitance cell has a high quality factor because the cross-coupled transistor pair operates actively and therefor has a negative conductance within a tuning range. This results in a good phase noise performance, especially, at high frequencies for the voltage controlled oscillator comprising the tunable capacitance cell. The input capacitance of the tunable capacitance cell is tuned by changing the control signal at the transistor's base or gate, no current sources are needed at emitters or drains of the two transistors, and the tunable capacitance cell is connected to the resonator of the oscillator core and thus is in parallel with the oscillator core, hence no transistor is stacked or cascoded with the other transistors. As a result the voltage controlled oscillator according to embodiments herein can operate at low supply voltage. In addition, when the control signal, e.g. a tuning voltage, becomes large, the capacitance and the negative conductance of the tunable capacitance cell increase too. The increasing negative conductance increases the output power from the tunable capacitance cell, which compensates the output power dropping of the whole voltage controlled oscillator due to decreasing of the impedance of the resonator, Z = jL/C, where L and C are resonator's inductance and capacitance including the capacitance of the tunable capacitance cell. Consequently, the variation of the output power of the whole voltage controlled oscillator is reduced.

Thus, embodiments herein provide a voltage controlled oscillator with improved performance, such as low phase noise, less variation on output power and low supply voltage etc. while without using varactors.

BRI EF DESCRIPTION OF THE DRAWINGS

Examples of embodiments herein are described in more detail with reference to attached drawings in which:

Figure 1 is a general block view of a voltage controlled oscillator according to embodiments herein.

Figure 2 is a schematic diagram illustrating a voltage controlled oscillator according to

embodiments herein.

Figure 3 is a schematic diagram illustrating a tunable capacitance cell with transistor's

parasitic capacitance according to embodiments herein.

Figure 4 is a schematic diagram illustrating an equivalent circuit for the tunable capacitance cell in Fig.3.

Figure 5 is a diagram illustrating input capacitance and conductance of the tunable

capacitance cell according to embodiments herein.

Figure 6 is a diagram showing the frequency of the output signal versus tuning voltage for the voltage controlled oscillator according to embodiments herein.

Figure 7 is a diagram showing output signal's phase noise versus tuning voltage for the voltage controlled oscillator according to embodiments herein.

Figure 8 is a diagram showing output signal power versus tuning voltage for the voltage controlled oscillator according to embodiments herein. Figure 9 is a diagram showing DC currents versus tuning voltage for an oscillator circuit and the tunable capacitance cell according to embodiments herein.

Figure 10 is a block diagram illustrating a wireless or a fibre communication system in which embodiments herein may be implemented.

DETAILED DESCRIPTION

A general view of a voltage controlled oscillator 100 for generating an output signal with a frequency range according to embodiments herein is shown in Figure 1. The voltage controlled oscillator 100 comprises an oscillator core 110 which comprises an amplifier 112 and a resonator 114. The voltage controlled oscillator 100 further comprises a tunable capacitance cell 120. As shown in Figure 1 , the tunable capacitance cell 120 is connected to the resonator 1 14 of the oscillator core 1 10 and thus is connected in parallel with the resonator 1 14. According to one embodiment, the voltage controlled oscillator 100 may be

implemented by circuits shown in Figure 2, where the voltage controlled oscillator 100 is now denoted as a voltage controlled oscillator 200, the oscillator corel 10 may be implemented by a circuit referred as an oscillator core 210. The amplifier 1 12 may be implemented by a circuit, referred to as amplifier 212 and the resonator 1 14 may be implemented by a circuit referred to as resonator 214.

As shown in Figure 2, the voltage controlled oscillator 200 comprises the oscillator core 210 which comprises an amplifier 212 and a resonator 214. The amplifier 212 comprises a cross coupled transistor pair 0.21/0.22, the resonator 214 comprises inductors L and capacitors C, and therefore is an inductor-capacitor (LC) resonator. Further, the voltage controlled oscillator 200 comprises a tunable capacitance cell 220 having an input/output port, the input/output port is connected to the resonator 214 of the oscillator core 210. The tunable capacitance cell 220 comprises a cross-coupled transistor pair Qn/Q₁₂, which has two transistors with capacitors at bases or gates of the two transistors.

Although transistors On, Q12, Q21, Q22 are shown as bipolar transistors having a base, a collector and an emitter, other type of transistor, such as field-effect transistor having a gate, a drain and a source may be used.

The cross-coupled transistor pair Qn/Q₁₂ is connected as the following: A base or gate of a first transistor Qn is coupled to a collector or drain of a second transistor Q₁₂ via a first capacitor Cn. A base or gate of the second transistor Q₁₂ is coupled to a collector or drain of the first transistor Qn via a second capacitor C_t2- An emitter or source of the first transistor On and an emitter or source of the second transistor Q₁₂ are connected to a ground, and the collector or drain of the first transistor Qn and the collector or drain of the second transistor Q₁₂ form the input/output port. Further, the base or gate of the first transistor Qn and the base or gate of the second transistor Q₁₂ are coupled to a control signal, e.g. a voltage signal, i.e. the base bias voltage V_tune, to tuning an input capacitance and conductance of the tunable capacitance cell 220. The control signal may also be a current signal. Further, the base or gate of the first transistor Qn and the base or gate of the second transistor Q₁₂ are coupled to the control signal via respective inductors Ι__Μ and L_b2, or via respective resistors. As the tunable capacitance cell 220 is connected to the resonator 214 of the oscillator core 210, the admittance or capacitance of the resonator 214 is changed with the bias voltage Vtune, thus the frequency of the output signal from the voltage controlled oscillator 200 is changed or tuned by the base bias voltage V_tune-

In order to analyse the performance of the tunable capacitance cell 220, a schematic diagram of the tunable capacitance cell 220 with transistor's parasitic capacitance according to embodiments herein is shown in Figure 3. As shown in Figure 3 and described above, the base of one transistor is connected with the collector of the other one via a capacitor C_t. The emitters of two transistors are grounded. The transistor's parasitic capacitances are taken into account, where C_M and C_n represent the transistor's collector-base capacitance and the base-emitter capacitance, respectively. The base bias voltage, denoted as V_t, is tunable to control the transistor's trans-conductance, g_m. Consequently, the admittance at the input/output port of the tunable capacitance cell 220, Y_in=1 /Zi_n, is controlled by V_t.

An equivalent circuit for the tunable capacitance cell 220 in Figure 3 is shown in Figure 4. Two transistors operate in a differential mode, i.e. the base voltages of the two transistors have a phase difference of 180, and also their collector voltages have a phase difference of 180. According to the Kirchhoffs circuit laws (KCL), the current at the collector of the transistor, i.e. at point "A", is

I=j ω C_t(V_c+V_t)+j ω C _μ (V_c-V_t)+g_mV_t Eq. (1 )

Where, g_m represents the trans-conductance of the transistor. The current at the base of the transistor, i.e. at point "B", is

Eq. (2) From Eq. (2), one obtains

Inserting Eq. (3) into Eq. (1 ), one obtains

J<u(c -C_t)[/6)(-C +C_t)+g_m]

Y = G + jB = = ja>(C_t + C_ll) + Eq. (4)

Where, Y is the input admittance, its real part, G, is the conductance, and the imagery part, B, is the susceptance. From Eq. (4) one obtains

and

B = ω C_t + C_M + Eq. (6) ω² (^+^+¾)² + (^)²

Equations (5) - (6) show that the input admittance, Y, can be varied by the trans- conductance, g_m, which can be tuned by changing the base bias voltage, V_t, for the common- emitter transistors.

The input capacitance, Β/ω, of the tunable capacitance cell 220 is determined by the base capacitor C and the transistor's parasitic capacitance which depends on the transistor's size, i.e. the emitter's length and width.

If the capacitance C_t is much larger than the transistor's parasitic capacitance C_M and

Cn, one obtain

and

From Eq. (7) and (8), it is more visible that the input capacitance and conductance vary with the trans-conductance, g_m.

In order to demonstrate the variation of the capacitance and conductance at the input/output port of the tunable capacitance cell 220, a simulation is performed, where the capacitance C_t is 270 fF. Two Indium phosphide (InP) Doubled Heterojunction Bipolar Transistors (DHBT) are used. Their emitter width and length are 0.25 μηη and 20 μηη, respectively. Under an optimal bias condition, the transistors are able to achieve a transition frequency ft and a maximum frequency f_max of around 350 and 600 GHz, respectively. The collector DC supply voltage is 2.5 V.

The tunable capacitance cell 220 is driven by a differential voltage signal at the input/output port. The peak-to-peak voltage swing at the input/output port is about 3 V at a frequency of 62 GHz. As the base voltage Vt is increased from 0 to 0.8 V, the input capacitance is changed from 100 fF to 288 fF, and the input conductance is varied between - 6.7 to 0.3 Ω, as shown in Figure 5. Since the cross-coupled transistor pair operates actively with differential signal, the input conductance has negative values for a large range of the base bias voltages and has quite low positive values for a small range of the base bias voltages.

In order to demonstrate advantages of the voltage controlled oscillator 100, 200 according to embodiments herein, simulations have been done for the voltage controlled oscillator 200 shown in Figure 2, where the amplifier 212 is a Class-D amplifier, hence the tunable capacitance cell 220 is applied to a Class-D oscillator. Here a Class-B/C/D amplifier refers to the cross-coupled, common-emitter/source configured transistors in the amplifier 212 operate in Class-B/C/D modes. Without the tunable capacitance cell 220, the Class-D oscillator, i.e. the oscillator core 210, operates at an oscillation frequency of 69.82 GHz with an output power of 7.86 dBm. The phase noise at 1 MHz offset is -101 .7 dBc/Hz. In the Class-D oscillator, i.e. the oscillator core 210, two InP DHBT transistors which forming the amplifier in the oscillator core have an emitter width and length of 0.25 urn and 20 urn, respectively. The supply voltage to collectors of the two transistors is 2.5V. As the tunable capacitance cell 220 is connected in parallel with the resonator in the Class-D oscillator, i.e. the oscillator core 210, the oscillation frequency is a function of the V_t, as shown in Figure 6. The oscillation frequency decreases from 62.3 GHz to 56.2 GHz, as V_t is swept from 0 to 0.8 V. The relative tuning range is 10.3%. The highest oscillation frequency of 62.3 GHz is lower than that of the class-D oscillator, i.e., the oscillator core 210, without the tunable capacitance cell, because of loading from the input capacitance of the tunable capacitance cell 220.

Figure 7 shows the phase noise at 1 MHz offset versus V_t for the voltage controlled oscillator with the tunable capacitance cell 220. In the whole tuning range, the phase noise at 1 MHz offset varies with the V_t. The worst phase noise is -98.6 dBc/Hz, which is only 3.1 dB higher than the Class-D oscillator, i.e. the oscillator core 210, without the tunable capacitive cell 220. This result indicates that the tunable capacitance cell 220 dose not degrade significantly the phase noise performance of the Class-D oscillator. While, the best phase noise of -101 .5 dBc/Hz is comparable to that of the oscillator core 210 without the tunable capacitance cell 220.

Figure 8 shows the output power of the voltage controlled oscillator 200 with the tunable capacitance cell 220 versus V_t. The variation of the output power is about 1.2 dB. This power variation in the tuning range of the V_t is much less than that of the oscillator core 210 with a varactor. For example, the power variation of a cross-coupled Class-D VCO with varactors may reach 10 dB.

Figure 9 shows the DC current of the oscillator core 210 and the DC current of the tunable capacitance cell 220 versus V_t. It can be seen that the DC current of the oscillator core 210 varies from 103 mA to 92 mA, as V_t increases from 0 to 0.8V. The decreasing of the DC current at large V_t is due to the decrease of the impedance of the resonator 214. In contrast, the DC current of the tunable capacitance cell 220 increases from 34 mA to 126 mA, as V_t increases from 0 to 0.8 V. The decreasing of the DC current at the oscillator core 210 will cause decreasing output power. However, the decreasing output power will be compensated partly by the increasing power from the tunable capacitance cell 220. As a result, the voltage controlled oscillator 200 according to embodiments herein has less output power variation. To summarise the discussions above, the advantages of the voltage controlled oscillator 100, 200 with the tunable capacitance cell 1 10, 210 according to embodiments herein are: Good phase noise performance, especially at high frequencies; Operating at low DC supply voltage; The variation of the output power is low, when the tuning voltage changes. Although the tunable capacitance cell 1 10, 210 is applied in the Class-D oscillator according to embodiments herein as shown in Figure 2, the tunable capacitance cell 1 10, 210 may be applied in different kinds of VCO topologies, i.e. a cross-coupled oscillator, a Class-B oscillator, a Class-C oscillator, a Colpitts oscillator or a Hartley oscillator etc.

The voltage controlled oscillator 100, 200 according to embodiments herein is suitable to provide a low phase noise signal with a certain frequency range for millimeter or macro wave transceivers in a wireless or fiber communication system, or for any electronic devices which need oscillator signals. Figure 10 shows a wireless or fiber communication system 1000 according to embodiments herein. The wireless or fiber communication system 1000 comprises a transceiver 1010, wherein the voltage controlled oscillator 100, 200 may be implemented in. The transceiver 1010 may be any transceiver, e.g. a micro or millimetre- wave transceiver, or a fibre optic transceiver etc. The wireless or fiber communication system 1000 may comprise other units, e.g. a memory 1020 and a processing unit 1030 for information storage and signal processing etc.

Those skilled in the art will understand that although transistors Qn , Qi₂ in the tunable capacitance cell 210 as shown in Figure 2 are Bipolar Junction Transistors (BJT) and InP DHBT transistors are used in the simulations, the tunable capacitance cell 210 may comprise any other types of transistors, such as Field-Effect Transistor (FET), Metal-Oxide- Semiconductor FET (MOSFET), Junction FET (JFET), etc. When using the word "comprise" or "comprising" it shall be interpreted as non- limiting, i.e. meaning "consist at least of".

The embodiments herein are not limited to the above described preferred

embodiments. Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appending claims.

Claims

1 . A voltage controlled oscillator (100; 200) for generating a signal with a frequency

range, the voltage controlled oscillator (100; 200) comprising:

an oscillator core (1 10; 210) comprising an amplifier (1 12; 212) and a resonator (1 14; 214); and

a tunable capacitance cell (120; 220) having an input/output port, wherein the input/output port is connected to the resonator (1 14; 214) of the oscillator core (1 10; 210), wherein the tunable capacitance cell (120; 220) comprises:

a cross-coupled transistor pair (Qn ;Q₁₂), wherein

a base or gate of a first transistor (Qn) is coupled to a collector or drain of a second transistor (Q₁₂) via a first capacitor (Cn);

a base or gate of the second transistor (Q₁₂) is coupled to a collector or drain of the first transistor (Qn) via a second capacitor (C_t2);

an emitter or source of the first transistor (Qn) and an emitter or source of the second transistor (Q₁₂) are connected to a ground; and

the collector or drain of the first transistor (Qn) and the collector or drain of the second transistor (Q₁₂) form the input/output port; and further the base or gate of the first transistor (Qn) and the base or gate of the second transistor (Q₁₂) are coupled to a control signal to tuning an input capacitance and conductance of the tunable capacitance cell (120; 220).

2. The voltage controlled oscillator (100; 200) according to claim 1 , wherein the

oscillator core (1 10; 210) comprises one of a cross-coupled oscillator, a Colpitts oscillator, a Hartley oscillator, a Class-B oscillator, a Class-C oscillator or a Class-D oscillator.

3. The voltage controlled oscillator (100; 200) according to any of the claims 1 -2,

wherein the base or gate of the first transistor (Qn) and the base or gate of the second transistor (Q₁₂) are coupled to the control signal via respective either inductors (Ι__Μ; L_b2) or resistors.

4. The voltage controlled oscillator (100; 200) according to any of the claims 1 -3, wherein the control signal is a voltage or current signal.

5. The voltage controlled oscillator (100; 200) according to any of the claims 1 -4, wherein the resonator is an inductor-capacitor resonator.

6. A micro or millimetre-wave transceiver (1010) comprises a voltage controlled oscillator (100; 200) according to any of the claims 1 -5.

7. A fibre optic transceiver (1010) comprises a voltage controlled oscillator (100; 200) according to any of the claims 1 -5.

8. An electronic device comprises a voltage controlled oscillator (100; 200) according to any of the claims 1 -5.

9. A wireless or a fibre communication system (1000) comprises a voltage controlled oscillator (100; 200) according to any of the claims 1 -5.