TECHNICAL FIELD

The present invention discloses an improved voltage controlled oscillator.

BACKGROUND

In many high frequency applications, such as, for example, microwave communications equipment or radar systems, so called voltage controlled oscillators, VCOs, with very low phase noise, are often required. Usually, a VCO will comprise a so called varactor. However, at high frequencies, a varactor will usually cause the VCO's phase noise to increase.

SUMMARY

As has emerged from the description above, there is a desire for obtaining a VCO without a varactor, i.e. a "varactor-less" VCO. Such a VCO is offered by the present invention in that it discloses a voltage controlled oscillator, a VCO, which comprises a resonator connected to an amplifier, so that signals from the resonator can pass through the amplifier and be returned to the resonator.

The VCO of the invention also comprises a tuning circuit for tuneably altering the amplitude of a signal which is input to the tuning circuit, and the tuning circuit is connected to a first point in the VCO for accessing a signal and to a second point in the VCO for re-injecting said signal altered in amplitude.

According to the invention, the accessed and re-injected signal is given a phase shift which is greater or smaller than the phase shift of a signal which is returned to the resonator after having passed through the amplifier. In one embodiment of the VCO of the invention, the tuning circuit comprises a variable gain amplifier with an inherent phase shift. In one embodiment of the VCO of the invention, the tuning circuit comprises a variable gain amplifier and a phase shifter.

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 basic diagram of the invention, and

Fig 2 shows a second principle of the invention, and

Fig 3 shows a first embodiment of the invention, and

Figs 4-7 show various embodiments of the invention.

DETAILED DESCRIPTION

Fig 1 shows a diagram of a basic embodiment 100 of a VCO, a Voltage Controlled Oscillator, of the invention. As shown in fig 1 , the inventive VCO 100 comprises a resonator 105 and an amplifier 1 0, as well as a tuning circuit 1 15, in this case a tuneable amplifier, for altering the amplitude of a signal which is input to the tuning circuit 115. The tuning circuit is tuneable with respect to the amount of amplitude change which is causes to an input signal, i.e. it is tuneable with respect to its amplification. Naturally, in other embodiments, the tuning circuit can also be tuneable with respect to the phase shift it gives to an input signal.

As is also shown in fig 1 , the tuning circuit is connected to a first point A in the resonator for accessing a signal, and to a second point B in the resonator for re-injecting the accessed signal, the signal having been altered in amplitude and phase. The first and second points A and B, i.e. the input/output points for the altering circuit, are, as can be seen in fig 1 , points between the resonator 105 and the amplifier 110, and are on different sides of the amplifier, said sides of the amplifier being with respect to the amplifier's input and output connections. The re-injection of the signal at point B is shown by means of a connection at that point. The phase of the injected signal, l _ln j, and the phase of the signal from the amplifier 100, l _0S c. are shown symbolically in a small phase diagram in fig 1. Also shown is the phase of the combined (resultant) signal, Itotai- corresponding diagram is shown in fig 2, which shows the amplitude and phase of the injected signal, lj _n j, the VCO signal, l _osc , and the amplitude and phase of the resulting signal, I total -

The oscillation frequency of the VCO 100 of the invention is made tuneable in the following manner: As will be realized from the diagram of fig 2, by varying the amplitude or phase of the injected signal, the phase of the resulting signal, Itotai, will also be altered. In a preferred embodiment, the amplitude is made variable by means of making the amplification of the tuning circuit variable, for example by using a so called Variable Gain Amplifier as the altering circuit 1 15. As will also be realized, a phase shift with respect to l _osc needs to be introduced into the signal l _in j which is accessed at point A before it is injected at point B, since the two phase vectors l _osc and l _in j of fig 2 will not give rise to a phase shift in the signal l _to t if they coincide totally with each other in phase. In most applications, such a phase shift will be obtained "for free", since the amplifier 1 10 and the tuning circuit 1 15 will inherently introduce phase shifts which differ from each other. However, if it is noticed that the components of the VCO 100 are such that they do not introduce inherently give rise to the necessary phase shift, a phase shifter, suitably a tuneable one, can be added to the tuning circuit 115.

Fig 3 shows a first detailed embodiment of a VCO 300 of the invention: the VCO 300 comprises a resonator 305, which can be designed in a variety of ways, but in the embodiment 305 shown in fig 3 the resonator 305 comprises first and second inductors, 302, 301 , connected in series to each other via first and second capacitors 303, 304. The VCO 300 also comprises an amplifier 310, which can also be designed in a variety of ways, but which in the embodiment shown in fig 3 comprises a transistor 306, in this case a Field Effect Transistor, a FET. The amplifier 310 is connected in parallel to the resonator 305, in the embodiment shown in fig 3 via connections "outside" of the two capacitors 303, 304.

As has also been described above, according to the invention, the VCO 300 also comprises a tuning circuit 315 for altering the amplitude and phase of a signal which is input to the altering circuit 315. In the embodiment shown in fig 3, the tuning component 315 is a Variable Gain Amplifier, a VGA, which comprises a transistor 316, in this case a FET, and a resistor 317. In other embodiments, the VGA can, for example, comprise a variable resistance.

In the embodiment 300, the tuning circuit 315 is connected to the rest of the VCO 300 at a first point in the VCO shown as "A" in fig 3, and to a second point "B" in the VCO. The signal from "A" is thus used as input to the tuning circuit 315, and the output from the tuning circuit 315 is then injected back into the VCO at "B". The nature of the tuning circuit 31 5 is, as mentioned previously, such that it alters an input signal in amplitude and phase, by means of which the oscillation frequency of the VCO 300 is altered. In the embodiment shown in fig 3, the tuning circuit 115 is made tuneable with regard to its amplification, by means of a tuning port VTUNE a the resistor 317 connected to the gate of the FET transistor 316.

Fig 4 shows an alternative embodiment 400 of a VCO of the invention. In this embodiment, the tuning circuit comprises a variable resistor 406 and an LC impedance match network which comprises first and second capacitors 404, 408, connected to ground, and first and second inductors 407, 405, on either side of the variable resistor 406. Regarding the variable resistor 406, a CMOS transistor (Complementary Metal Oxide Semi Conductor) operating in the so called "triode region" can, in one embodiment, be used as the variable resistor 406. The resistance of such a component is then controlled by the gate bias voltage. Also, in the embedment 400 shown in fig 4, the LC network has dual functions:

• increasing the impedance at ports A and B, thus, reducing the loading to the resonator by the variable resistor 406;

• avoiding that the injected signal and the resonator signal are in-phase.

The VCO 400 may be used in applications in which there are demands for a low DC power consumption.

In alternative embodiments, the invention is applied in a so called differential VCO, as opposed to the embodiments shown and described so far, which have been based on so called single-ended VCO:s. An example of such an embodiment, i.e. one with a differential VCO is shown in fig 5, in which two single oscillators, indicated as "Osc 1" and "Osc 2" in fig 5, are synchronized through coupled sources of FET transistors Q1 and Q2 in the respective oscillators via one inductor L2 in each oscillator. Each oscillator also comprises two serially connected capacitors C1 and C2, which in each oscillator connect the gate of the FET to the drain of the FET. In each oscillator, there is also an inductor L1 connected to the drain of the FET, with the two inductors L1 also being connected to each other at a point indicated as Vp in fig 5.

The transistors and Q ₂ , as well as the inductors U, L ₂ and the capacitors Ci, C ₂ are comprised in the oscillators Osc 1 and Osc 2 respectively; the tuning circuit 515 is also a VGA, a Variable Gain amplifier, which in the VCO 500 of fig 5 comprises, for each of the two oscillators Osc 1 and Osc 2, a FET transistor Q ₃ in Osc1 and Q ₄ in Osc2, and a resistor R _g, for each of the transistors Q3 and Q4, which all together form a VGA in the VCO 500. The FET transistors Q ₃ and Q are also connected to each other via their sources, a connection which is also grounded via an inductor L _e .

Due to the "double nature" of the VCO 500, the VGA 515 accesses the signals in each of the two oscillators which form the differential VCO; these points are indicated in fig 5 as A and A'. Similarly, the amplified and phase shifted signals from the VGA 515 are re-injected into the two oscillators Osc 1 and Osc 2 at two points, indicated as B and B' in fig 5. The oscillation frequency of the VCO 500 is tuned by means of tuning voltages V _tU ne at the gates of each of the two transistors Q3, Q ₄ .

In one embodiment, the VCO of the invention can comprise a resonator which is a so called transmission line resonator. Such an embodiment is shown as 600 in fig 6, with the transmission line resonator being shown as 605. The basic principle of the VCO of the invention is, as can be seen in fig 6, adhered to in such an embodiment as well: the VCO 600 comprises an amplifier 610 connected to the transmission line resonator 605, and there is also a tuning component 615, for altering the phase, and tuneably altering the amplitude of an input signal, i.e. a VGA amplifier, a variable gain amplifier.

As can be seen in fig 6, the tuning component 615 is connected to a first point A for accessing the signal in the VCO 600, and to a second point B for re-injecting this signal altered in amplitude and phase; the first A and second B points are points between the resonator 605 and the amplifier 610, on different sides of the amplifier. As can be seen, as a general principle, the first point A is on the input side of the amplifier 610, and the second point B is on the output side of the amplifier 610. As has been mentioned previously, the signal which is re-injected should also have been altered in phase, something which will usually be an inherent property of the tuning component 615. However, if the tuning component does not have this as an inherent property, a phase shifting component can be added between points A and B.

An alternative way of ensuring that the accessed and re-injected signal has a phase shift which is greater or smaller than the phase shift of a signal which is returned to the resonator after having passed through the amplifier is shown in fig 7: in the embodiment 700 shown in fig 7, the resonator 706 comprises two serially connected transmission line resonators 705, 705'. The point A at which the signal is accessed for the tuning circuit, in this case a VGA 715, is at a point outside the resonator 706, at an end of the first transmission line resonator 705 which is distal from the point where it is connected to the second transmission line resonator 705'. The point B where the signal is re-injected into the VCO is a point in between the two transmission line resonators 705, 705'. As can be seen in fig 7, this will also give the desired effect, i.e. a difference in phase shifts between the signal which is re-injected at point B and the signal which is returned to the resonator 706 after having passed through the amplifier 710.

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.