WO2007148282A2

WO2007148282A2 - Varicap replacing circuit for voltage controlled oscillators

Info

Publication number: WO2007148282A2
Application number: PCT/IB2007/052343
Authority: WO
Inventors: Eduard F. Stikvoort; Mihai A. T. Sanduleanu
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-06-21
Filing date: 2007-06-19
Publication date: 2007-12-27
Also published as: WO2007148282A3

Abstract

The invention relates to an electronic device for controlling a controlled oscillator. The electronic device provides a control input for supplying a control signal for tuning the oscillating frequency of the oscillator. Further the electronic device provides a transconductor having a transfer function in the frequency range with a substantially first- order high frequency roll off characteristic. The transconductor is adapted to be coupled to a tank circuit (L0, C0) and to act as a variable capacitance, such that the control signal controls the value of the variable capacitance of the transconductor for tuning the oscillating frequency of the controlled oscillator.

Description

VARICAP replacing circuit for voltage controlled oscillators

Field of the invention

The invention relates to an electronic device for radio frequency applications, more specifically to an electronic device for a voltage controlled oscillator.

Background of the invention Voltage controlled oscillators are well known in the art. They are typically used in a closed loop arrangement, e.g. phase locked loops or the like, in order to produce a predetermined frequency. Usually, the frequency of the oscillator is compared to a reference signal supplied to the loop. The result of the comparison is translated into a voltage, and supplied to the control input of the voltage controlled oscillator in a closed loop, such that the oscillation frequency is continuously adjusted. The components including the oscillators, or parts of them, are typically implemented as integrated circuits including e.g. integrated coils or integrated capacitors. There are several different ways to control the frequency of the oscillators in response to a voltage.

One example for controlling the oscillator frequency is a variable capacitor implemented by a junction diode usually designated as VARICAP. According to this arrangement, the oscillator includes an integrated coil and a VARICAP for tuning the oscillator frequency. The VARICAP usually consists of a junction diode having a depletion layer as insulating layer of the VARICAP. The DC voltage across the diode (VARICAP) controls the thickness of the depletion layer. The thickness of the depletion layer in turn is proportional to the value of the capacitor. This way, a voltage controlled capacitance is established.

A main disadvantage of the VARICAP resides in its series resistance. The series resistance of a VARICAP is intrinsic to these diodes resulting from the not- depleted part of the diode. For integrated VARICAPs, the voltage that is used to control the extension of the depletion layer is relatively low. Accordingly, the doping level should be low giving rise to a relatively high series resistance. When the oscillating frequency increases the impedance of the capacitor decreases, such that for the same series resistor, the quality factor of the capacitor decreases. A low quality factor of the capacitor impairs the properties of the oscillator and the applicability of the oscillator for many applications, in particular high frequency applications. As in today's applications frequencies up to 60 GHz and more are used, these aspects become more and more relevant.

US 5,929,716 relates to a high-performance voltage controlled oscillator. The voltage controlled oscillator according to this prior art includes a differential amplifier having a differential pair of transistors, an LC resonance circuit having a coil and a capacitor, a phase shift circuit for receiving a differential output of the differential amplifier, and a current control circuit for variably controlling an operation current of the phase shift circuit. The frequency control of the oscillator is performed by introducing a phase shift in the feedback path.

Summary of the invention It is an object of the invention to provide a radio frequency voltage controlled oscillator having reduced power consumption and an improved overall performance at high frequencies.

The object is solved by the subject matter of claim 1. Accordingly, an electronic device for controlling a controlled oscillator is provided, that includes a control input for supplying a control signal for tuning the oscillating frequency of the oscillator. Further, the device according to the invention includes transconductor means (e.g. a transconductor) that provides a phase shift. The phase shift is substantially the phase shift that would be introduced by a capacitance. Accordingly, the transfer function of the transconductor in the frequency range can be substantially a first-order high frequency roll off characteristic. Accordingly, the transconductor means behave similar to a capacitance, i.e. an input current to the transconductor means lags the input voltage. The transconductor means are adapted to be coupled to a tank circuit and to act as a variable capacitance, wherein the control signal controls the value of the variable capacitance of the transconductor for tuning the oscillating frequency of the controlled oscillator. According to the present invention, the frequency of a voltage controlled oscillator is controlled by tuning the transconductor. Accordingly, the tuning function of variable capacitors, as VARICAPs or the like, is replaced by an electronic circuit that is adapted to generate the current of a variable capacitor in response to an AC voltage applied to the input. The transconductor circuit according to the invention is typically connected in parallel to a coil or a tank circuit of an oscillator. This might be any kind of tank circuit as long as the oscillating frequency of the tank circuit can be controlled by changing the value of a capacitance being coupled to the tank circuit. A transconductor having a first order roll off in the frequency range, can, if properly coupled, provide a phase relation of the input voltage and the input current that is similar to those produced by a capacitor. The phase relation of the input voltage and input current of the transconductor depends further on the transconductance of the transconductor. If the (internal) transconductance of the transconductor is varied (e.g. by changing the bias currents), the phase relation of the input signals of the transconductor will change, similar to a variation of the capacitance of a capacitor used instead of the transconductor. Accordingly, the transconductor can act like a variable capacitor, or at least like an approximation of variable capacitor. By using a transconductor as a variable capacitor, the invention provides an electrically controlled capacitor circuit that overcomes the damping effects being introduced by the losses of a VARICAP in an RF-VCO. The device according to the invention is suitable to replace the VARICAP by conventionally integrated circuit elements, as transistors, capacitors and resistors. A primary benefit of the present invention is that the RF-VCO can be designed without a VARICAP. Accordingly, the

VARICAP can be omitted from the design library of an IC process. The electronic device according to the invention can be implemented in any technology, like CMOS, Bipolar etc.

According to an aspect of the invention, the transconductor means include a resistor or a capacitor for introducing additional phase delay to the transconductor means. The additional, external components are helpful to adapt the conductance of the transconductor, if the range for varying the capacitor by internal biasing or the like is insufficient. In particular, if the control signal is only suitable to adjust the electrical properties of the transconductor means within a specific range, the additional components can provide an additional phase shift, such that the transconductor behaves like an ideal capacitor. A phase shift of 90 degrees between the voltage across the input nodes of the transconductor and the input current, according to an ideal capacitor, can be achieved. Capacitors are typically arranged in the feedback path of the transconductor means, in order to add phase delay.

According to another aspect of the invention, the transconductor means include further a capacitance for compensating an amount of the phase delay introduced by the transconductor means. This aspect of the invention is useful in circuit designs, where the transconductor provides to much phase delay. Those properties of the transconductor may occur, when a specific technology or specific design rules impose inappropriate electrical properties of the transconductor, such as significant parasitic capacitances. The phase delay can be compensated by introducing a capacitor, in particular in the forward path of the transconductor.

According to an aspect of the invention, the transconductor means are implemented by bipolar transistors. Accordingly, the transconductor includes at least a common emitter circuit consisting of a differential pair of transistors. The control signal may then control the DC current through the common emitter circuit, e.g. by use of a controlled current source. According to this configuration, the conductance of the transistors in the common emitter configuration is biased by the DC current. As the conductance is the dominating factor that defines the transfer function and thereby the capacitive behavior of the transconductor, this is a preferred manner of tuning the oscillating frequency of a tank circuit being coupled to the transconductor. The controlled current source can be implemented as current mirror or the like.

According to still another aspect of the invention, the common emitter circuit including the two transistors is controlled by changing the base-to-emitter voltage of the transistors of the common emitter circuit. This configuration is more suitable for low power supply applications, as there is no need for a controlled current source between the common emitters of the transistors and the negative supply voltage. It the base-to- emitter voltages of the transistors are controlled, the current source can be coupled between the positive supply voltage and the common emitters.

According to an aspect of the present invention the electronic device includes further an inductor and some capacitances, which are arranged in accordance with a Colpitts oscillator configuration. Accordingly, the electronic device results in a voltage controlled Colpitts oscillator. These oscillators are typically used for high frequency applications. Brief description of the drawings

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter and with respect to the following figures:

Fig.l shows a differential oscillator including a variable capacitor according to an embodiment of the prior art,

Fig.2 shows a simplified schematic of a transconductor used for the invention, Fig.3 shows a simplified schematic according to a first embodiment of the invention,

Fig.4 shows a simplified schematic according to a second embodiment of the invention,

Fig.5 shows a simplified schematic according to a third embodiment of the invention, including the circuit that undamps the tank circuit, and

Fig.6 shows a simplified schematic according to a fourth embodiment of the invention, including the circuit that undamps the tank circuit.

Details of the preferred embodiments Fig.l shows a differential oscillator according to the prior art. The prior art solution shown in Fig.l includes a differential pair of transistors Tl and Tl' being coupled to an oscillator tank circuit by their respective bases. The current through transistors Tl, Tl' is biased by a fixed value of the current Io. The tank circuit consists of inductor Lo and capacitor Co serving as basic resonant elements. Fig.l shows further a variable capacitor C_var being dedicated to tune the frequency of the oscillator in response to a control voltage, which is not shown. The variable capacitor C_var is typically implemented by two VARICAP diodes (not shown). Fig.l shows further an input stage comprising two transistors T2 and T2' and two resistors R2 and R2'. The basic principle of the oscillator shown in Fig.l is well known in the art. The transistors Tl, Tl', and the two transistors T2 and T2' arranged as emitter followers, provide a feedback loop for the tank circuit while compensating the losses, such that a continuous oscillation occurs. The frequency of the oscillation is tuned by varying the value of the variable capacitor, i.e. the capacitance of VARICAPs represented by C_var.

Fig.2 shows a simplified schematic of the tuning circuit to be used according to the present invention. The present invention suggests to use a transconductor as the tuning means in order to eliminate the VARICAPs from the circuit. The transconductor shown in Fig.2 has the following characteristic. In response to an input voltage V_1n that is supplied across input nodes IN+ and IN-, an output current I_out = -V_1n •gm on the output nodes OUT- and OUT+ is established. The magnitudes of the input current I_1n and the output current I_out are equal. If an additional phase shift (delay) is introduced in the transconductor of Fig.2, the output current I_out (and so the input current I_1n) is delayed with respect to the input voltage V_1n. Such a delay or phase shift may originate from a first-order time constant in the transfer function of the transconductor, resulting in the following frequency behavior of g_m:

g_m(ω)=g₀/(l+jωτ)

Accordingly, the relationship of the input current to the input voltage accommodates the following equation:

The above equation is equal to

I_in / V_in = Gωτ-l) g₀/(l+ωV)

The real term of this equation corresponds to a negative conductance of g_o/(l+ωV). The term of jωτ g₀ V_1n/ (1+ωV) describes the current of a capacitor of

C_v = τg₀ /(l+ωV), Accordingly, a transconductor with a first-order roll off provides a negative conductance in parallel with a capacitor which is proportional to the conductance

This is refined and implemented in the VARICAP replacing circuit and the embodiment. The above considerations can be applied to voltage controlled oscillator circuits for radio frequency applications. According to the present invention, the VARICAP diodes represented by variable capacitance C_var shown in Fig.l can be omitted, if the above teaching is correctly applied to an oscillator circuitry.

Fig.3 shows a first preferred embodiment according to the present invention. Accordingly, Fig.3 shows an input stage including two emitter followers T2 and T2' and a pair of transistors Tl and Tl' in a common emitter configuration (CE) . The transistors T2 and T2' are coupled to the oscillator tank circuit consisting of Lo and Co. The two transistors Tl and Tl' are coupled to capacitor Ci and to two resistors Rl and Rl' by their respective bases. In this configuration, the common emitter circuit (CE) including Tl and Tl' operates as a transconductor with a first order roll off, as explained above with respect to Fig.2. At high frequencies, the common emitter circuit introduces a phase delay in the input currents relative to the input voltage as explained with respect to Fig.2 (input voltage and input current are not shown in Fig.3). As a lagging current with respect to an input voltage is characteristic for a capacitance, the phase shift renders the common emitter circuit of Fig.3 more capacitive and less conductive. The emitter followers T2, R2 and T2', R2' serve to provide a higher input impedance and they establish a desired additional delay (phase shift). The common emitter circuit comprising Tl, Tl' are biased by a common current source CCl. The transistors Tl, Tl' are biased such that the total phase shift of the circuit is close to or even at 90 degree, such that no additional phase shifting network is required. The value of the capacitance that is represented by the transconductor is controlled by the conductance go of the common emitter transistors Tl and Tl'. go is changed by changing the DC biasing current supplied by the controlled current source CCl. Accordingly, instead of tuning the VARICAPs according to the prior art, the transconductor is tuned by controlled current source CCl that adjusts the conductance of the common emitter transistors Tl, Tl'. The controlled current source might be implemented in many ways, e.g. as a current mirror. The emitter followers T2, R2 and T2', R2' serve additionally to reduce the effect of the collector base capacitor of the input transistors Tl, Tl'. The rather optional resistors Rl and Rl' and the capacitor Cl serve to increase the delay of the common emitter circuit.

Fig.4 is a simplified schematic of another preferred embodiment according to the present invention. The emitter followers T2, R2 and T2', R2' operate in the same way as explained above. According to the embodiment shown in Fig.4, the capacitors C2, C2' are coupled to the transistors Tl, Tl' such that a phase advance is introduced to the circuit. Accordingly, a small value of the DC blocking capacitors C2, C2' can compensate a portion of the delay of the transconductor circuit. This can be helpful, if the delay due to the transconductor is too large. Particularly, if the delay introduced by the transconductor becomes too large, the negative real part of the output conductance turns into a positive real part. If the conductance becomes positive, the controlled capacitor will be lossy and starts damping the tank circuit. A small value of the DC blocking capacitors C2, C2' provides a phase advance. Further, by properly determining the values of the capacitors C2, C2' and the resistors R4, R4', the phase shift can be adjusted so that an almost ideal (lossless) capacitor at the operating range of the voltage controlled oscillator is established. The current source CCl of Fig.3 is omitted in Fig.4 and replaced by another circuitry. The current source CC2 supplies a current to resistors R3, R4, and R4' and transistor T3. Accordingly, the DC current through the CE transistors Tl, Tl' is controlled by the DC base-to-emitter voltage by supplying more or less current via CC2. This is advantageous for low power applications.

Fig.5 shows a simplified schematic of a preferred embodiment according to an aspect of the invention. Accordingly, the transconductor is used for a Colpitts oscillator configuration. Instead of the oscillator tank circuit shown in Figures 1 to 4, Fig.5 shows a differential Colpitts oscillator being coupled to the transconductor according to the invention. The Colpitts oscillator includes the three capacitors C3, C3', C4 the inductor L3 and the two transistors T3, T3'. The circuit is biased by controlled current sources CC4, CC4'. The configuration is known in the art. Further, in accordance with the invention, there is an input stage including transistors T2, T2', and a common emitter circuit Tl, Tl', representing the transconductor according to the invention. The biasing of the circuit it performed by the controlled current sources CC5, CC5', and CC3. As explained with respect to Fig.3, CC3 is used to determine the conductance go of the CE circuit (Tl, Tl'), such that an appropriate phase shit of the circuit is obtained. It might be useful to insert additional capacitors or resistors in the circuit coupled to the transconductor to adjust the capacitive behavior of the circuit as explained with respect to Figures 3 and 4.

Fig.6 is a simplified schematic of a preferred embodiment according to an aspect of the invention relating also to the Colpitts oscillator. The schematic of Fig.6 is basically the same as the one shown in Fig.5, wherein the controlled current source CC3 of Fig.5 is replaced by a circuitry including resistors R4, R4', and R3, transistor T3, and controlled current source CC2. The functionality of these components has already been explained with respect to Fig.4. As mentioned with respect to Fig.4, the circuit shown in Fig.6 also provides improved properties with respect to applications with reduced supply voltags, as the controlled current source CC2 controls the base-to-emitter voltage of the common emitter transistors Tl, Tl', by supplying a controlled current to resistor R3, and transistor T3. The DC-blocking capacitors C2, C2' provide a phase advance, if the delay of the transconductor is too large.

The VARICAP replacing circuit is applicable and useful for any radio frequency VCO for the losses of the VARICAPs at high frequencies. Although the drawings refer to bipolar transistors, the principle of a phase shifting transconductor fits well for applications in CMOS. The circuit enables the omission of the VARICAP from design libraries.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere effect that certain measures are resided in mutually different dependent claims does not indicate that a combination of these measures cannot be used as advantage.

Furthermore, any reference signs in the claims shall not be considered as limiting the scope of the claims.

Claims

CLAIMS:

1. Electronic device for controlling a controlled oscillator, comprising: a control input for supplying a control signal for tuning the oscillating frequency of the oscillator, a transconductor having a transfer function in the frequency range with a substantially first-order high frequency roll off characteristic, the transconductor being adapted to be coupled to a tank circuit (LO, CO) and to act as a variable capacitance, wherein the control signal controls the value of the variable capacitance of the transconductor for tuning the oscillating frequency of the controlled oscillator.

2. Electronic device according to claim 1, being adapted such that the control signal being applied to the control input controls the conductance (go) of the transconductor means for tuning the oscillating frequency of the controlled oscillator.

3. Electronic device according to claim 1 or 2, wherein the transconductor means comprise further a resistor (Rl, Rl') and a capacitance (Cl) for introducing additional phase delay to the transconductor means.

4. Electronic device according to claim 1 or 2, wherein the transconductor means comprise further a capacitance (C2, C2') for compensating an amount of the phase delay introduced by the transconductor means.

5. Electronic device according to claims 3 or 4, wherein the transconductor comprises a common emitter circuit (CE) and the control signal controls the DC current through the common emitter circuit (CE).

6. Electronic device according to one of claims 1, 2 or 4, wherein the common emitter circuit (CE) comprises two transistors (Tl, Tl') and the variable capacitance of the transconductor is controlled by changing the base-to-emitter voltage of the transistors (Tl, Tl') of the common emitter circuit.

7. Electronic device according to any one of the previous claims, the device being arranged in a Colpitts oscillator configuration.

8. Data processing system having an electronic device according to claim 1.