RADIO FREQUENCY OSCILLATOR

Abstract

The invention relates to a radio frequency oscillator, the radio frequency oscillator comprising a resonator circuit being resonant at an excitation of the resonator circuit in a differential mode and at an excitation of the resonator circuit in a common mode, wherein the resonator circuit has a differential mode resonance frequency at the excitation in the differential mode, and wherein the resonator circuit has a common mode resonance frequency at the excitation in the common mode, a first excitation circuit being configured to excite the resonator circuit in the differential mode to obtain a differential mode oscillator signal oscillating at the differential mode resonance frequency, and a second excitation circuit being configured to excite the resonator circuit in the common mode to obtain a common mode oscillator signal oscillating at the common mode resonance frequency.

Claims

1. A method for configuring a radio frequency oscillator, the method comprising: exciting the radio frequency oscillator; and producing two oscillator signals, wherein the two two oscillator signals are configured to be synchronized; and wherein the radio frequency oscillator comprises: a first inductor, and a second inductor; wherein two ends of the first inductor are coupled to a first capacitor, wherein two ends of the second inductor are coupled to a second capacitor, wherein the first capacitor comprises a pair of single-ended capacitors, wherein the second capacitor comprises a pair of differential capacitors, and wherein the pair of single-ended capacitors are connected in series.

2. The method for configuring a radio frequency oscillator of claim 1, wherein the pair of differential capacitors are connected in series, wherein the pair of single-ended capacitors are configured to provide a reference to ground potential, and wherein the pair of differential capacitors are configured to be not seen by excitation signals of the common mode.

3. The method for configuring a radio frequency oscillator of claim 2, wherein the pair of single-ended capacitors comprise a first capacitor and a second capacitor, wherein a first end of the first capacitor is coupled to a first end of the first inductor, and a second end of the first capacitor is coupled to a ground potential, and wherein a first end of the second capacitor is coupled to a second end of the first inductor, and a second end of the second capacitor is coupled to the ground potential; wherein the pair of single-ended capacitors further comprise a first transistor and a second transistor; wherein the first transistor is configured to selectively connect and disconnect the first capacitor between the first end of the first winding and the ground potential, and wherein the second transistor is configured to selectively connect and disconnect the second capacitor between a second end of the first winding and the ground potential.

4. The method for configuring a radio frequency oscillator of claim 3, wherein a common mode resonant frequency of the radio frequency oscillator is independent from the second capacitor, and a differential mode resonant frequency of the radio frequency oscillator is dependent on the second capacitor and the first capacitor.

5. The method for configuring a radio frequency oscillator of claim 1, wherein the inductance associated with the first inductor in common mode is greater than the inductance associated with the first inductor in differential mode.

6. The method for configuring a radio frequency oscillator of claim 1, wherein the pair of single-ended capacitors comprise a first capacitor and a second capacitor, wherein a first end of the first capacitor is coupled to a first end of the first inductor, and a second end of the first capacitor is coupled to a ground potential, and wherein a first end of the second capacitor is coupled to a second end of the first inductor, and a second end of the second capacitor is coupled to the ground potential.

7. The method for configuring a radio frequency oscillator of claim 6, wherein the pair of single-ended capacitors further comprise a first transistor and a second transistor; wherein the first transistor is configured to selectively connect and disconnect the first capacitor between the first end of the first winding and the ground potential, and wherein the second transistor is configured to selectively connect and disconnect the second capacitor between a second end of the first winding and the ground potential.

8. The method for configuring a radio frequency oscillator of claim 7, wherein the gate of the first transistor and the gate of the second transistor are coupled to a same control signal from an inverter.

9. The method for configuring a radio frequency oscillator of claim 8, wherein the pair of differential capacitors comprise a first differential capacitor and a second differential capacitor, and wherein the first differential capacitor and the second differential capacitor are a pair of balanced capacitors.

10. The method for configuring a radio frequency oscillator of claim 9, further comprising: a switch coupled between the first differential capacitor and the second differential capacitor, wherein the switch is configured to selectively connect and disconnect the first differential capacitor and the second differential capacitor to and from one another.

11. The method for configuring a radio frequency oscillator of claim 10, wherein the pair of differential capacitors comprises a plurality of switched capacitors configurable to be arranged in parallel.

12. The method for configuring a radio frequency oscillator of claim 1, wherein a common mode resonant frequency of the radio frequency oscillator is independent from the second capacitor, and wherein a differential mode resonant frequency of the radio frequency oscillator is dependent on the second capacitor and the first capacitor.

13. The method for configuring a radio frequency oscillator of claim 12, wherein the excitation circuit is configured to produce 3 dB improvement in phase noise.

14. The method for configuring a radio frequency oscillator of claim 13, wherein the frequency oscillator is implemented as a integrated circuit.

Description

SHORT DESCRIPTION OF EMBODIMENTS

[0043] Embodiments of the invention will be described with respect to the following figures, in which:

[0044] FIG. 1 shows a diagram of a radio frequency oscillator according to an embodiment;

[0045] FIG. 2 shows a diagram of a resonator circuit according to an embodiment;

[0046] FIG. 3 shows a diagram of a resonator circuit according to an embodiment;

[0047] FIG. 4 shows a diagram of a transformer of a resonator circuit according to an embodiment;

[0048] FIG. 5 shows a diagram of an input impedance response of a resonator circuit according to an embodiment;

[0049] FIG. 6 shows a diagram of a single-ended capacitor according to an embodiment;

[0050] FIG. 7 shows a diagram of a differential capacitor according to an embodiment;

[0051] FIG. 8 shows a diagram of an input inductance response and a coupling factor response according to an embodiment;

[0052] FIG. 9 shows a diagram of a radio frequency oscillator according to an embodiment; and

[0053] FIG. 10 shows a diagram of differential mode oscillator signals and common mode oscillator signals according to an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

[0054] The design of radio frequency oscillators for multi-mode multi-band cellular applications can demand for a high tuning range (TR) while ensuring low phase noise (PN) for a wide range of frequency bands and channels. Low quality factors Q of resonator circuits, e.g. comprising switched-capacitor networks for wide tuning ranges, can limit the phase noise performance of radio frequency oscillators.

[0055] Embodiments of the invention are based on an increased tuning range of a radio frequency oscillator having a single resonator circuit by introducing a common mode oscillation. The common mode oscillation increases the tuning range of the radio frequency oscillator without any die area and/or phase noise penalty. The radio frequency oscillator can be regarded as a dual-mode radio frequency oscillator having a single resonator circuit. The resonator circuit can also be referred to as a tank circuit.

[0056] FIG. 1 shows a diagram of a radio frequency oscillator 100 according to an embodiment. The radio frequency oscillator 100 comprises a resonator circuit 101 being resonant at an excitation of the resonator circuit 101 in a differential mode and at an excitation of the resonator circuit 101 in a common mode, wherein the resonator circuit 101 has a differential mode resonance frequency at the excitation in the differential mode, and wherein the resonator circuit 101 has a common mode resonance frequency at the excitation in the common mode, a first excitation circuit 103 being configured to excite the resonator circuit 101 in the differential mode to obtain a differential mode oscillator signal oscillating at the differential mode resonance frequency, and a second excitation circuit 105 being configured to excite the resonator circuit 101 in the common mode to obtain a common mode oscillator signal oscillating at the common mode resonance frequency.

[0057] In an embodiment, the resonator circuit 101 comprises a transformer, a primary capacitor, and a secondary capacitor, wherein the transformer comprises a primary winding and a secondary winding, the primary winding being inductively coupled with the secondary winding, wherein the primary capacitor is connected to the primary winding, the primary capacitor and the primary winding forming a primary circuit, and wherein the secondary capacitor is connected to the secondary winding, the secondary capacitor and the secondary winding forming a secondary circuit.

[0058] FIG. 2 shows a diagram of a resonator circuit 101 according to an embodiment. The resonator circuit 101 is resonant at an excitation of the resonator circuit 101 in a differential mode and at an excitation of the resonator circuit 101 in a common mode, wherein the resonator circuit 101 has a differential mode resonance frequency at the excitation in the differential mode, and wherein the resonator circuit 101 has a common mode resonance frequency at the excitation in the common mode.

[0059] The resonator circuit 101 comprises a transformer 201, a primary capacitor 207, and a secondary capacitor 209, wherein the transformer 201 comprises a primary winding 203 and a secondary winding 205, the primary winding 203 being inductively coupled with the secondary winding 205, wherein the primary capacitor 207 is connected to the primary winding 203, the primary capacitor 207 and the primary winding 203 forming a primary circuit, and wherein the secondary capacitor 209 is connected to the secondary winding 205, the secondary capacitor 209 and the secondary winding 205 forming a secondary circuit.

[0060] The resonator circuit 101 has the common mode resonance frequency at an excitation of the primary circuit in the common mode. The resonator circuit 101 has the differential mode resonance frequency at an excitation of the primary circuit in the differential mode.

[0061] In the following, further implementation forms and embodiments of the radio frequency oscillator 100 are described.

[0062] FIG. 3 shows a diagram of a resonator circuit 101 according to an embodiment. The resonator circuit 101 comprises a transformer 201, a primary capacitor 207, and a secondary capacitor 209, wherein the transformer 201 comprises a primary winding 203 and a secondary winding 205, the primary winding 203 being inductively coupled with the secondary winding 205, wherein the primary capacitor 207 is connected to the primary winding 203, the primary capacitor 207 and the primary winding 203 forming a primary circuit, and wherein the secondary capacitor 209 is connected to the secondary winding 205, the secondary capacitor 209 and the secondary winding 205 forming a secondary circuit.

[0063] The primary capacitor 207 of the primary circuit comprises a pair of single-ended capacitors 301, 303. The secondary capacitor 209 of the secondary circuit comprises a pair of differential capacitors 305, 307. k.sub.m denotes the magnetic or inductive coupling factor between the primary winding 203 and the secondary winding 205, wherein 0≤|k.sub.m|≤1.

[0064] The inductance of the primary winding 203 can be referred to as L.sub.p, the inductance of the secondary winding 205 can be referred to as L.sub.s, the capacitance of the primary capacitor 207 can be referred to as C.sub.p, and the capacitance of the secondary capacitor 209 can be referred to as C.sub.s. According to this definition, the primary winding 203, the secondary winding 205, the primary capacitor 207, and the secondary capacitor 209 are considered as individual concentrated components.

[0065] Alternatively, the inductance of the primary winding 203 can be referred to as 2 L.sub.p, the inductance of the secondary winding 205 can be referred to as 2 L.sub.s, the capacitance of the primary capacitor 207 can be referred to as 0.5 C.sub.p, and the capacitance of the secondary capacitor 209 can be referred to as 0.5 C.sub.s. According to this definition, the primary winding 203 and the secondary winding 205 are each formed by a pair of inductors connected in series, wherein the inductance of each inductor is referred to as L.sub.p or L.sub.s, respectively. Furthermore, the primary capacitor 207 and the secondary capacitor 209 are each formed by a pair of capacitors connected in series, wherein the capacitance of each capacitor is referred to as C.sub.p or C.sub.s, respectively.

[0066] FIG. 4 shows a diagram of a transformer 201 of a resonator circuit 101 according to an embodiment. The transformer 201 comprises a primary winding 203 and a secondary winding 205, the primary winding 203 being inductively coupled with the secondary winding 205. The primary winding 203 comprises one turn and the secondary winding 205 comprises two turns, realizing a ratio of turns of 1:2. The diagram illustrates induced currents within the transformer 201 at an excitation in differential mode (DM) and an excitation in common mode (CM).

[0067] Embodiments of the invention are based on different resonance frequencies of the transformer-based resonator circuit 101 at differential mode excitations and common mode excitations. The different resonance frequencies can be due to a different inductive coupling factor k.sub.m when the primary winding 203 of the transformer 201 is excited by differential mode or common mode signals.

[0068] The induced currents at the secondary winding 205 at differential mode excitation can circulate in same directions and can provide a high coupling factor k.sub.m between the primary winding 203 and the secondary winding 205 of the transformer 201. Consequently, in differential mode, the inductance of the secondary winding 205 and the capacitance of the secondary capacitor 209 can strongly affect the differential mode characteristics of the resonator circuit 101, e.g. the differential mode resonance frequency. For a coupling factor in differential mode k.sub.m,DM>0.5, the differential mode resonance frequency can be at ω.sub.0,DM=1/sqrt(L.sub.pC.sub.p+L.sub.sC.sub.s).

[0069] On the other hand, the induced currents at the secondary winding 205 at common mode excitation can circulate in opposite directions and can cancel each other. Hence, the coupling factor k.sub.m can be low in the common mode, e.g. k.sub.m,CM<0.3. This low coupling factor k.sub.m can be interpreted as the secondary winding 205 not being seen from the primary winding 203. Thus, the common mode resonance frequency can be at ω.sub.0,CM=1/sqrt(L.sub.pC.sub.p). Since common mode excitation signals at the primary circuit may not see the secondary circuit, in particular the differential capacitors 305, 307, the primary capacitor 207 can comprise single-ended capacitors 301, 303. The common mode resonance frequency ω.sub.0,CM can be higher than the differential mode resonance frequency ω.sub.0,DM and the frequency separation can be controlled by the ratio L.sub.sC.sub.s/L.sub.pC.sub.p.

[0070] The differential mode resonance frequency can be determined as ω.sub.0,DM=1/sqrt(L.sub.pC.sub.p+L.sub.sC.sub.s). Further differential mode resonance frequencies can be determined as ω.sub.n,DM=n ω.sub.0,DM, wherein n can be any number, and wherein n can depend on an inductive coupling factor between the primary winding 203 and the secondary winding 205, and the ratio (L.sub.sC.sub.s)/(L.sub.pC.sub.p).

[0071] The inductance associated with the primary winding 203 in common mode L.sub.pc can be different from the inductance associated with the primary winding 203 in differential mode L.sub.pd. It can be considered in the design phase that L.sub.pc>L.sub.pd. The ratio of capacitances of the primary capacitor 207 and the secondary capacitor 209 can be designed based on a value of L.sub.pc, e.g. L.sub.pc=L.sub.pd+L.sub.T, wherein L.sub.T denotes an inductance of a metal track connecting the center tap of the primary winding 203 e.g. to a constant voltage source.

[0072] FIG. 5 shows a diagram of an input impedance response 501 of a resonator circuit 101 according to an embodiment. The diagram illustrates differential mode resonance frequencies and common mode resonance frequencies.

[0073] The input impedance of the resonator circuit 101 is denoted as Z.sub.in. The quality factor in common mode Q.sub.C can be slightly less than the quality factor in differential mode Q.sub.D but may be large enough such that oscillation is possible in this mode. If the quality factor of a resonator circuit is large, an oscillation start-up can easier be realized. In the diagram, Q.sub.C is smaller than Q.sub.D but is still large enough for an efficient oscillation start-up.

[0074] Assuming a maximum to minimum capacitance ratio C.sub.max/C.sub.min=2 for the primary capacitor 207 or the secondary capacitor 209, which can be variable or switchable capacitors, enabling a sufficiently high quality factor of the capacitors, both the differential mode resonance frequency and the common mode resonance frequency can vary, e.g. by 34%.

[0075] In order to avoid any gaps between the tuning ranges in differential mode and in common mode, there can be some overlap. This can mean that at least the lowest common mode resonance frequency ω.sub.CM,low equals the highest differential mode resonance frequency ω.sub.DM,high, i.e. ω.sub.CM,low=ω.sub.DM,high. Satisfying the overlap relationship can result in L.sub.sC.sub.s=L.sub.pC.sub.p.

[0076] With these relationships, the resonance frequencies can cover an octave while going from differential mode oscillations to common mode oscillations. Practically, the ratio C.sub.max/C.sub.min may be greater than 2, due to parasitic effects and challenges associated with controlling the precise overlap between the differential mode resonance frequencies and the common mode resonance frequencies.

[0077] In an embodiment, an octave tuning range for the radio frequency oscillator is achieved upon the basis of the following equations:

[00001] $\begin{matrix} \frac{C_{\max, p}}{C_{\min, p}} = 2 & \frac{C_{\max, s}}{C_{\min, s}} = 2 \\ C_{\max, p =} C_{p,} & C_{\max, s} = C_{s} \end{matrix} \begin{matrix} \end{matrix} \begin{matrix} ω_{D} : \frac{1}{\sqrt{L_{p} C_{p} + L_{s} C_{s}}} ~ \frac{\sqrt{2}}{\sqrt{L_{p} C_{p} + L_{s} C_{s}}} \\ ω_{C} : \frac{1}{\sqrt{L_{p} C_{p}}} ~ \frac{\sqrt{2}}{\sqrt{L_{p} C_{p}}} \end{matrix}) \begin{matrix} for \\ ⟹ \\ overlap \end{matrix} L_{p} C_{p =} L_{s} C_{s}$

wherein C.sub.max,p/C.sub.min,p denotes a maximum to minimum capacitance ratio associated with the primary capacitor 207, C.sub.max,s/C.sub.min,s denotes a maximum to minimum capacitance ratio associated with the secondary capacitor 209, (OD relates to the differential mode resonance frequencies, and ω.sub.C relates to the common mode resonance frequencies.

[0078] FIG. 6 shows a diagram of a single-ended capacitor 301, 303 according to an embodiment. The single-ended capacitor 301, 303 comprises a capacitor 601, a capacitor 603, a transistor 605, a transistor 607, a resistor 609, a resistor 611, an inverter 613, and an inverter 615. The single-ended capacitor 301, 303 is arranged within the primary circuit of the resonator circuit 101.

[0079] By applying a digital switching signal the transistor 605 and the transistor 607 can be switched between a conducting state and a non-conducting state. Consequently, the capacitance of the single-ended capacitor 301, 303 can be digitally tuned. A plurality of single-ended capacitors 301, 303 can be connected in parallel.

[0080] FIG. 7 shows a diagram of a differential capacitor 305, 307 according to an embodiment. The differential capacitor 305, 307 comprises a capacitor 701, a capacitor 703, a transistor 705, a resistor 707, a resistor 709, an inverter 711, and an inverter 713. The differential capacitor 305, 307 is arranged within the secondary circuit of the resonator circuit 101.

[0081] By applying a digital switching signal b.sub.i, the transistor 705 can be switched between a conducting state and a non-conducting state. Consequently, the capacitance of the differential capacitor 305, 307 can be digitally tuned. A plurality of differential capacitors 305, 307 can be connected in parallel.

[0082] FIG. 8 shows a diagram of an input inductance response 801 and a coupling factor response 803 according to an embodiment.

[0083] The input inductance 801 relates to the inductances of the primary winding 203 and the secondary winding 205 of the transformer 201. The inductance 2 L.sub.s is approximately 1.5 nH. The inductance 2 L.sub.pd is approximately 0.5 nH. The inductance L.sub.pc is approximately 0.4 nH. The inductances slightly increase with frequency.

[0084] The coupling factor response 803 relates to coupling factors of the transformer 201 at an excitation in differential mode and in common mode. The coupling factor in differential mode k.sub.md is approximately 0.7. The coupling factor in common mode k.sub.mc is approximately 0.25. The coupling factors slightly increase with frequency.

[0085] FIG. 9 shows a diagram of a radio frequency oscillator 100 according to an embodiment. The radio frequency oscillator 100 comprises a resonator circuit 101 being resonant at an excitation of the resonator circuit 101 in a differential mode and at an excitation of the resonator circuit 101 in a common mode, wherein the resonator circuit 101 has a differential mode resonance frequency at the excitation in the differential mode, and wherein the resonator circuit 101 has a common mode resonance frequency at the excitation in the common mode, a first excitation circuit 103 being configured to excite the resonator circuit 101 in the differential mode to obtain a differential mode oscillator signal oscillating at the differential mode resonance frequency, and a second excitation circuit 105 being configured to excite the resonator circuit 101 in the common mode to obtain a common mode oscillator signal oscillating at the common mode resonance frequency.

[0086] The resonator circuit 101 forms an implementation of the resonator circuit 101 as described in conjunction with FIG. 3. The first excitation circuit 103 comprises a transistor 901, a transistor 903, a tail transistor 905, and a tail capacitor 907. The second excitation circuit 105 comprises a first auxiliary oscillator 909, the first auxiliary oscillator 909 comprising a transistor 911, a capacitor 913, a transistor 915, and a capacitor 917. The second excitation circuit 105 further comprises a second auxiliary oscillator 919, the second auxiliary oscillator 919 comprising a transistor 921, a capacitor 923, a transistor 925, and a capacitor 927.

[0087] The resonator circuit 101 and the first excitation circuit 103 are arranged to form a transformer-coupled oscillator to obtain the differential mode oscillator signal. The resonator circuit 101 and the second excitation circuit 105 are arranged to form a Colpitts oscillator to obtain the common mode oscillator signal.

[0088] The first auxiliary oscillator 909 and the second auxiliary oscillator 919 are connected by a transistor 929. The first auxiliary oscillator 909 is synchronizable with regard to the second auxiliary oscillator 919 in frequency and phase via the transistor 929. The first auxiliary oscillator 909 can be injection-locked on the second auxiliary oscillator 919.

[0089] Exciting the resonator circuit 101, in particular the transformer 201 of FIG. 2, in the differential mode to obtain the differential mode oscillator signal oscillating at the differential mode resonance frequency can be achieved e.g. by using a cross-coupled gm-pair structure or a transformer-coupled structure within the first excitation circuit 103. A separate second excitation circuit 105 may be used in order to excite the resonator circuit 101 in the common mode. Thus, a dual mode oscillation in terms of a differential mode oscillation and a common mode oscillation can be provided.

[0090] The described structure is depicted in FIG. 9. The first excitation circuit 103 of the radio frequency oscillator 100 is structured according to a transformer-coupled structure for obtaining the differential mode oscillation in conjunction with the resonator circuit 101. The second excitation circuit 105 of the radio frequency oscillator 100 is structured as a Colpitts oscillator in conjunction with the resonator circuit 101. The Colpitts oscillator is single-ended. The Colpitts oscillator comprises two auxiliary oscillators 909, 919, in particular two cores, with the auxiliary oscillators 909, 919 being injection-locked through the transistor 929 or M7 switch. Both auxiliary oscillators 909, 919 can start oscillating at the same frequency but slightly out of phase. Later, the auxiliary oscillators 909, 919 can lock and there may be no phase shift between them. Therefore, the resonator circuit 101, in particular the transformer, is excited in the common mode to obtain the common mode oscillator signal oscillating at the common mode resonance frequency. The injection-locking of the two auxiliary oscillators 909, 919 can lead to a 3 dB improvement in phase noise performance. The radio frequency oscillator 100 can exhibit a phase noise characteristic having two distinctive regions.

[0091] When exciting in differential mode, the voltages V.sub.B2 and V.sub.B3 can be zero, i.e. V.sub.B2=V.sub.B3=0, and the transistor 929 can be switched off for providing an efficient excitation in differential mode. When exciting in common mode, the voltage V.sub.B1 can be zero, i.e. V.sub.B1=0, and the transistor 929 can be switched on.

[0092] The differential mode resonance frequency ω.sub.D and the common mode resonance frequency ω.sub.C can be determined according to the following equations:

[00002] $ω_{D} = \frac{1}{\sqrt{L_{p} (C_{p +} C_{e q}) + L_{s} C_{s}}} ω_{C} = \frac{1}{\sqrt{L_{p} (C_{p} + C_{eq})}} C_{e q} = \frac{C_{1} C_{2}}{C_{1} + C_{2}}$

wherein C.sub.eq denotes an equivalent capacitance determined upon the basis of the capacitances of the capacitors 913, 917, 923, 927.

[0093] In order to relax the start-up conditions e.g. of the Colpitts oscillator, C.sub.eq can be designed to be larger than C.sub.p through the tuning range. This may reduce the tuning range of both the differential mode and common mode oscillations. The reduced tuning range can be more visible in common mode since in differential mode the primary and secondary capacitors can both affect the resonance frequency. By choosing L.sub.sC.sub.s>L.sub.pC.sub.p, this effect can be reduced even more in differential mode, without changing the common mode conditions.

[0094] FIG. 10 shows a diagram of differential mode oscillator signals 1001 and common mode oscillator signals 1003 according to an embodiment. The differential mode oscillator signals 1001 and the common mode oscillator signals 1003 are depicted for probe positions GA, GB, DA, and DB as shown in FIG. 9. The differential mode oscillator signals 1001 and the common mode oscillator signals 1003 are sinusoidal signals over time.

[0095] Embodiments of the invention use different characteristics of a 1:2 transformer 201 in differential mode excitation and in common mode excitation, and use an additional common mode oscillator signal to a differential mode oscillator signal, using the same resonator circuit 101 and hence imply no die area penalty. Furthermore, since differential mode and common mode start-up circuits can share the same output, there may be no need for multiplexing and hence no noise floor degradation.

RADIO FREQUENCY OSCILLATOR

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Abstract

Claims

Description