WO2009113927A1

WO2009113927A1 - An improved voltage controlled oscillator

Info

Publication number: WO2009113927A1
Application number: PCT/SE2008/050281
Authority: WO
Inventors: Mingquan Bao
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2008-03-13
Filing date: 2008-03-13
Publication date: 2009-09-17

Abstract

A voltage controlled oscillator, VCO (200, 300, 400, 500, 600, 700, 800, 900) comprising a first bipolar transistor (110, 210, 410, 615, 705, 830, 925) and a first resonator (120; 405; 620, 640, 650; 732, 740, 745, 755; 805, 815, 825, 835). The VCO also comprises an upward impedance transformer (210; 415,420; 610; 730; 810, 815; 905, 915) connected between a terminal of the transistor and the resonator. The VCO can also be a balanced VCO (300; 500; 700,800, 900), and comprise a second bipolar transistor (110'; 410'; 710, 840,940) and a second resonator (120'; 405'; 720; 805, 825, 835, 815: 910, 905, 920, 930), with a second upward impedance transformer 210'; 405'; 720; 820,825; 930,935) connected between a terminal of the second transistor and a terminal of the second resonator, with the second transistor being connected to the first transistor via control terminals of the transistors.

Description

TITLE

An improved Voltage Controlled Oscillator.

TECHNICAL FIELD The present invention discloses a voltage controlled oscillator, a VCO, which comprises a transistor and a resonator.

BACKGROUND

Voltage Controlled Oscillators, VCOs, are used frequently within many areas of technology, such as, for example, telecommunications. An important factor in the performance of a VCO is the VCO's so called phase noise, which is a parameter to measure the spectral purity of a signal source. The phase noise affects the performance of a unit such as a transceiver in which the VCO is used, for example regarding such parameters as dynamic range, sensitivity, frequency selectivity, etc.

The type of VCO at which the invention is aimed is a VCO which comprises a transistor and a resonator. The phase noise L of such a VCO as a function of a frequency offset Δω from the intended output frequency ω of the VCO can be estimated using the following expression:

In expression (1 ) above, Δω is the frequency offset from the intended output frequency from the VCO, P_SIG is the signal power, Q is the quality value of the resonator which is comprised in the VCO and T is the temperature.

As can be seen from expression (1), the phase noise can be reduced by increasing the signal power. The signal power, P_Sj_g, for a VCO can be written as: ¹ P s = ^■ v^: ig (2)

R

where V_CE is the collector-emitter "voltage swing in the case of a bipolar transistor and R is the resistance of the resonator at oscillation frequency.

As can be seen in (2), the signal power can be increased by reducing R. However, reducing R will also affect the so called Q value of the resonator, which is detrimental to the phase noise performance of the VCO.

With further reference to equation (2) above it can be seen that the signal power can also be increased by increasing the voltage swing VCE Unfortunately, V_CE may not exceed the so called breakdown voltage of the transistor, and the breakdown voltage decreases as the scale of the transistor is reduced for high frequency applications. This means that there are difficulties in increasing P_SIG in the case of high frequency VCOs.

SUMMARY

Thus, as has been outlined above, there is a need for a VCO which generates less phase noise than previously known VCOs, while avoiding drawbacks of some known methods of reducing the phase noise of a VCO. Such a VCO should be possible to use in high frequency applications without adversely affecting the Q value of the resonator in the VCO.

A VCO which meets these needs is offered by the present invention, in that it discloses a voltage controlled oscillator which comprises a first bipolar transistor and a first resonator, as well as an upward impedance transformer.

In a VCO of the invention, the upward impedance transformer is connected between a terminal of the transistor and the resonator. As will be shown in more detail below, this will aid in increasing the signal power through the VCO, which in turn will reduce the phase noise of the VCO. In one embodiment of the invention, the VCO is a so called balanced VCO, i.e. a VCO which also comprises a second bipolar transistor and a second resonator, with a second upward impedance transformer which is connected between a terminal of the second transistor and an input terminal of the second resonator, so that the balanced VCO in effect comprises two "individual" VCOs, which are connected via control terminals of the two transistors

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following, with reference to the appended drawings, in which

Fig 1 shows a prior art VCO, and Figs 2-9 show various embodiments of a VCO of the invention.

DETAILED DESCRIPTION

Fig 1 shows a prior art VCO 100, for reference. The invention can be applied to more or less any type of VCO which is based on a transistor connected to a resonator, (or multiple "branches" which comprise transistors connected to resonators) and as will be realized by the man skilled in the field, the number of different such VCO types is so large that it is impossible to enumerate them in this text.

Thus, although the invention will be described in this text with reference to a number of different types of VCOs which comprise transistors connected to resonators, this should not be seen as excluding the invention from being applied in other types of VCOs which also comprise transistors connected to resonators. The use of certain types of VCOs in order to illustrate the invention is merely in order to facilitate the reader's understanding of the invention. Returning now to the VCO 100 of fig 1 , the VCO 100 is a so called "single ended Colpitts VCO", and comprises a transistor 110, a first terminal of which is connected to a resonator 120. A control voltage can be input at terminal 120, and an output signal is produced at point 126 in fig 1.

The resonator 120 as such comprises an inductance 122 connected in parallel with two serially coupled capacitors 124 and 126. A feedback connection is employed to connect a second terminal of the transistor 110 to a point between the two capacitors 124, 126. As shown in fig 1 , one of the capacitances, in this case the capacitance 124, is shown as a varactor, i.e. the capacitance of the component 124 can be varied by means of applying a (not shown) control voltage, thereby influencing the frequency of the output signal of the VCO. This is a principle which will be adhered to throughout this text and the drawings associated with the text, i.e. if one or more capacitances are shown as varactors, the control voltage to the varactor can be used to vary the frequency of the output signal of the VCO in which the varactor is comprised.

In the example of fig 1 , and also in some of the following examples, the transistor which is used is a so called bipolar transistor, so that the terminals of the transistor are the collector, the emitter and the base of the transistor. In the example used in fig 1 , with the nomenclature used above, the first terminal is the collector, and the second terminal is the emitter. As can be seen in fig 1 , the base of the bipolar transistor is connected to AC ground in this example.

As has been shown previously in this text, the signal power P_SIG, which should be increased in order to reduce the phase noise of the VCO, is proportional to V_CE and inversely proportional to the resistance of the resonator at the oscillation frequency. Since, as explained above, V_CE is limited by the breakdown voltage, increasing V_CE in order to increase P_SIG is usually not practical. Instead, the present invention proposes to reduce the impedance of the resonator by reducing the impedance at a terminal which connects the transistor to the resonator. In the case of the Colpitts VCO of fig 1 , this is achieved by inserting an upward impedance transformer between the transistor's collector and the resonator of the VCO.

One embodiment 200 of a VCO of the invention is shown in fig 2. The VCO 200 comprises the components of the VCO 100 of fig 1 , which have retained their reference numbers from fig 1 , and the VCO 200 also comprises an upward impedance transformer 210, inserted between a terminal of the transistor and the resonator, so that the transistor connects to the resonator via the upwards impedance transformer 210.

The basic function of the upwards impedance transformer 210 is shown by the insert in fig 2: the transformer 210 has a first port 212 and a second port 214, and as seen from the first port 212, the impedance Zi of the transformer 210 increases in the direction of the second port, so that it is transformed to the impedance value Z₂. Conversely, the impedance Z₂ of the transformer, as seen from the second port 214 decreases into Zi at the first port 212.

It can be mentioned that a large Z₂ at the port 214 which is connected to the resonator is desired in order to avoid an overly heavy load of the resonator.

Fig 3 shows another embodiment 300 of the VCO of the invention. The embodiment 300 also uses the Colpitts topology, but is a so called "balanced Colpitts VCO", as opposed to the "single ended Colpitts VCO" of figs 1 and 2. Thus, as can be seen in fig 3, the VCO 300 basically comprises two of the single ended VCOs 200 of fig 2, with the "second" VCO being referenced as 305 in fig 3. The components of the first VCO of fig 2 have retained their reference numbers from fig 2. As compared to the VCO 200 of fig 2, the VCO 300 comprises a second resonator 120', a second transistor 110' and a second transformer 210'. The second transformer 210 is connected to a terminal of the second transistor 110', in this case the collector, and the second transformer 210 is also connected to the second resonator 120', so that the second transformer 210 is placed "between" the second resonator 120'and the second transistor 110'.

The two single ended VCOs of the balanced VCO 300 are connected by means of control terminals of the transistors, in this case the base of the two transistors 210, 210', being connected to each other. As shown in fig 3, the transistors 110,110' shown in fig 3 are connected to each other directly, but as an alternative they may also be connected to each other via an impedance.

The balanced design 300 of fig 3 can be used if, for example, it is desired to obtain the advantages of the invention, while cancelling a phase shift which may have been introduced by an impedance transformer.

Turning now to the upward impedance transformer used in a VCO of the invention, a large number of different designs for such a transformer can be used, as is known by the man skilled in the field. Examples of transformer designs which can be used are so called L-match or π -match transformers, etc, as well as distributed elements, such as transmission-lines, stubs, etc.

Examples of improved Colpitts VCOs using an L-match transformer are shown in figs 4 and 5. Fig 4 shows a single ended Colpitts VCO 400 of the invention, with an L-match transformer comprised of an inductance L_m 415 coupled serially between the transistor 410 of the VCO and the resonator 405 of the VCO, and a capacitance C_m 420 which couples to ground.

Fig 5 shows a balanced Colpitts VCO 500 of the invention, which basically comprises two branches, each of which comprises one of the VCOs of fig 4, one of which is thus referenced as 400 and the other as 400'. Thus, the two "single" VCO's 400, 400' of the balanced VCO 500 in fig 5 each comprises an L-match transformer, which comprises an inductance L_m connected serially between the transistor and the resonator of the branch, and a capacitance C_m which couples to ground. In addition, the components of the VCO 400 have retained the reference numerals from fig 4, and the components of the other VCO 400' have been given the reference numbers of the VCO 400, but with a prime (') attached, so that, for example, the resonator of the VCO 400' has been given the reference number 405'.

In some applications, the transistor's collector (in the case of a bipolar transistor) may be AC-grounded. In such cases, an upwards impedance transformer may be inserted between the transistor's emitter and the resonator in order to decrease V_CE, thus decreasing the phase noise of the VCO.

An inventive VCO 600 based on another single ended Colpitts topology, with an impedance transformer 610 connected in the manner suggested above is shown in fig 6. The small impedance port Z₁ is connected to the transistor's emitter and the large impedance port Z₂ is connected with the resonator. Hence, the collector of the transistor 615 of the VCO 600 is AC-grounded, and the emitter of the transistor is connected to the impedance transformer 610, which in turn connects to the resonator which comprises capacitors 640 and 620, as well as an inductor 650, with the capacitor 640 also connecting to the transistor's 610 base.

Fig 7 shows an example of a balanced VCO 700, i.e. a VCO which comprises two branches with a "single VCO" in each branch. As can be seen, the collector of each of the transistors 705, 710 in each branch of the VCO is AC-grounded, and the emitter of each transistor 705, 710 connects serially to a first inductor 734, 724, said first inductor in turn connecting to a second inductor 732, 722. Thus, the first inductors 734, 724 of each branch of the VCO 700, together with the second inductor 732, 722 of each branch of the VCO 700 constitute an upward impedance transformer in each branch, which connects the transistor of each branch to a resonator.

In order to clarify the notion of the resonator, the resonator of the branch of the transistor 705 comprises the inductor 755, and capacitors 740 and 745. In addition, a capacitor 740 connects to ground, as well as a serially connected capacitance 745 and an inductance 755, via which the branch of the transistor 705 connects to the ("mirror image") branch of the second transistor 710.

By now, it will be obvious to the man skilled in the art that it is difficult or impossible to list all of the possible implementations of the present invention, but the principles behind the invention should also be obvious to those skilled in the art by now. However, two more embodiments of VCOs of the invention will be shown in figs 8 and 9, and described below. The VCO 800 of the invention comprises a first and a second bipolar transistor 830,840, which are connected to each other via their respective bases over a varactor 835.

The collector of each of the transistors 830, 840 is connected to respective inductances 810, 820. The output signals of the VCO 800 are produced at the collectors of the transistors 830 and 840. The end of the inductances which is not connected to the collector of the transistor is connected to a terminal for supply voltage, V_cc.

The varactor 835 via which the bases of the transistors 830, 840 are connected to each other is part of a "loop" which also includes a first 815 and a second 825 inductor, and a second varactor 805, said "loop" forming the resonator of the VCO 800,

The transformer for the first transistor 830 is formed by the inductance 810 to which the collector of the transistor 830 is connected, and similarly, the transformer for the second transistor 840 is formed by the inductance 820 to which the collector of the transistor 840 is connected,

As seen in fig 8, each transistor 830, 840 is also provided with a DC bias via impedances (in the present case, by means of inductors) 850, 845 from the base of each transistor.

Fig 9 shows a further embodiment 900 of a VCO of the invention. The VCO 900 also comprises first 925 and second 940 bipolar transistors, which are connected to each other via their respective bases via a connecting inductance 930.

A terminal of each of the transistors 925, 940, in this case the collectors of the transistors, is connected to respective inductances 915, 935. The inductances 915, 935 couple inductively to respective inductances 905 and 930 in a "loop", which apart from said inductances also comprises a first 910 and a second 920 varactor. The "loop" is the resonator of the VCO 900.

As can be seen in fig 9, the inductance 930 in "the loop" or resonator is the same inductance which connects the bases of the transistors, and, as can also be seen in fig 9, the base of the first transistor 925 is connected to a point in "the loop" between the inductor 930 and the first varactor 910, and similarly, the base of the second transistor 940 is connected to a point in the loop between the inductor 930 and the second varactor 920.

The emitters of the two transistors 925, 940, are connected to each other, and are also connected to a current source. The outputs of the VCO 900 are taken from the collectors of the transistors 925 and 940.

The invention is not limited to the examples of embodiments described above and shown in the drawings, but may be freely varied within the scope of the appended claims. In conclusion, the invention discloses an arrangement by means of which a number of advantages are gained, for example:

• Due to the impedance transformer, the resistance at the collector/emitter is dropped. Thus, for a maximum voltage swing, namely, collector-emitter break-down voltage, the transistor can deliver more power into the resonator. Consequently, the phase noise is reduced.

* A high impedance at the port connected to the resonator minimizes the deterioration of the resonator's Q value due to the transistor.

The proposed arrangement can be implemented in any semiconductor technology, e.g., Silicon, GaAs etc; it can also be implemented both in MMICs and discrete circuits.

Claims

1. A voltage controlled oscillator, VCO (200, 300, 400, 500, 600, 700, 800, 900) comprising a first bipolar transistor (110, 210, 410, 615, 705, 830, 925) and a first resonator (120; 405; 620, 640, 650; 732, 740, 745, 755; 805, 815, 825, 835), the VCO being characterised in that it also comprises an upward impedance transformer (210; 415,420; 610; 730; 810, 815; 905, 915), said transformer being connected between a terminal of the transistor and the resonator.

2. The VCO of claim 1 , being a balanced VCO (300; 500; 700,800, 900), so that the VCO also comprises a second bipolar transistor (110'; 410'; 710, 840,940) and a second resonator (120'; 405'; 720; 805, 825, 835, 815: 910, 905, 920, 930), with a second upward impedance transformer 210'; 405'; 720; 820,825; 930,935) being connected between a terminal of the second transistor and a terminal of the second resonator, with the second transistor being connected to the first transistor via control terminals of the transistors.

3. The VCO (300; 500; 700,800, 900) of claim 2, according to which the transistors are connected to each other directly.

4. The VCO (300; 500; 700,800, 900) of claim 2, according to which the transistors are connected to each other via an impedance (750,755; 935).

5. The VCO (400, 500) of any of claims 1 -4, in which the upward impedance transformer or transformers is a so called L-match transformer (415, 420).

6. The VCO of any of claims 1-4, in which the upward impedance transformer or transformers is a so called π-match transformer.

7. The VCO (600, 700) of any of claims 1-4, in which the upward impedance transformer or transformers comprise coupled inductors (730; 720; 810, 815; 905, 915; 935, 930).

8. The VCO (200; 300; 400; 500; 600) of any of claims 1-7, said VCO comprising a Colpitts VCO.

9. The VCO (200, 300, 40O₁ 500, 600, 700, 600, 700, 800, 900) of any of claims 1-8, in which the transistor's said terminal is the transistor's collector or its emitter or its base, with the control terminal of the transistor being the base of the transistor.