Low power local oscillator

This application claims priority to French Patent Application No. 1752076, filed on Mar. 14, 2017, which application is hereby incorporated herein by reference.

Embodiments of the invention relate generally to integrated circuits, and in particular embodiments, to low power consumption oscillators, e.g., in transceiver systems.

Transceiver systems generally comprise local oscillators for generating local oscillator signals at useful frequencies, e.g. of the order of a gigahertz, notably allowing frequency transpositions to be performed.

FIG. 1 represents an example of a common local oscillator DISo.

The local oscillator DISo comprises an oscillator module OSCo, in which an inductive element Lo and a capacitive element Co mounted in parallel form a conventional LC resonator known per se, generating an oscillating signal V 1 at the terminals of the inductive element Lo.

The elements Lo and Co of such an oscillator module OSCo are usually selected to satisfy a compromise notably between phase noise, energy consumption and surface area occupied.

A sustaining amplifier SUST, supplied by a current source I, allows the oscillations of the signal V 1 to be maintained by compensating resistive losses.

Such a setup offers good performance in terms of phase noise, energy consumption and surface area occupied by the circuit.

A frequency divider fDiv divides the frequency of the oscillating signal present on the inductive element Lo in order that the LC resonator can be configured for generating an oscillating signal V 1 at a higher frequency. Indeed, this makes it possible to reduce the surface area occupied by the inductive element Lo and to reduce the coupling between the oscillator OSCo and the output signal of the divider fDiv or an amplified version of this signal, since the fundamental frequencies of the signals delivered by OSCo and fDiv are different.

The local oscillator signal LO is delivered at the output of the frequency divider fDiv, generally in the form of a square-wave voltage.

To produce such a signal the frequency divider fDiv must receive at its input a signal the amplitude of which is significantly above the thresholds of the transistors that it comprises.

However, high amplitude signals accentuate the “varactor” effect on the transistors of the input stage of fDiv, i.e. the variation in the capacitive component of their input impedance, which results in adding phase noise at the oscillator OSCo. More broadly this effect also occurs on all the other components connected to the LC resonator, such as the sustaining amplifier SUST and the amplitude controller MCTR. Thus, such local oscillators DISo are usually provided with a buffer amplifier ISO used both to isolate the LC resonator from the frequency divider fDiv inputs, and to generate an amplified signal V 2 of sufficient amplitude for the input requirements of the frequency divider fDiv, from the oscillating signal V 1 .

Indeed, the capacitive value at the inputs of the buffer amplifier ISO is sufficiently small and sufficiently insensitive to the “varactor” effect not to interfere with the LC resonator.

On the other hand, this type of buffer amplifier ISO has the drawback of having a variable gain with many factors, such as temperature, the frequency of the signal at its input, or the vagaries of manufacture. Furthermore, this type of buffer amplifier ISO tends to degrade the duty cycle of the signal at its input (i.e., the ratio between the time at the high state and the period of the signal).

Accordingly, a usual solution consists in over-dimensioning the buffer amplifier ISO so that it is less sensitive to these factors. Over-dimensioning the buffer amplifier ISO, however, may create undesirable excessive energy consumption.

Accordingly, embodiments of the invention limit the energy consumption of this type of local oscillator without reducing its performance.

Thus, according to one aspect, a local oscillator device is provided comprising an oscillator module with a first inductive element and a capacitive element mounted in parallel, and a frequency divider coupled to the oscillator module for delivering a local oscillator signal.

According to a general feature of this aspect, the local oscillator device comprises an autotransformer including the first inductive element and two second inductive elements respectively coupled to the terminals of the first inductive element and to two output terminals of the autotransformer, the output terminals being further coupled to input terminals of the frequency divider.

It is recalled here that an autotransformer is a transformer in which the primary and secondary circuits have a common part.

Thus, the two second inductive elements of the autotransformer supply an increased amplitude oscillation sufficient to meet the aforementioned requirements at the input of the frequency divider.

This overcomes the need for the buffer amplifier while maintaining a low oscillation level in the oscillator module, a suitable phase noise and an unchanged occupied surface area.

Furthermore, the autotransformer makes it possible to obtain a new degree of freedom in the architecture of the integrated circuit by placing the output terminals at a location other than the terminals of the first inductive element.

For example, the terminals of the first inductive element are located on a first side of the periphery of the autotransformer, and the output terminals are each located on another side of the periphery of the autotransformer.

Advantageously, the output terminals are located on a second side of the periphery of the autotransformer opposite the first side with respect to a central point of the autotransformer.

The central point of the autotransformer may be, for example, a geometric center, such as a center of symmetry of the autotransformer.

This allows placing the output terminals of the transformer at a more convenient place for connection to the input of the frequency divider, and thus saves space. Furthermore, this makes it possible to avoid a degradation in the quality factor of the oscillator module usually introduced by numerous and lengthy conductive tracks coupled to its terminals.

According to one embodiment in which the first inductive element comprises a first flat conductive track having an interlaced coil shape, and the second inductive elements each comprise a second flat conductive track having a coil shape, the second conductive tracks being interlaced with each other and located on the periphery of the first inductive element, the ratio of the width of the second conductive tracks over the width of the first conductive track is advantageously between 0.1 and 0.7.

Indeed, the second inductive elements may comprise narrower tracks than those of the first inductive element without restricting the quality factor of the oscillator module, notably since the power delivered on the output terminals is low compared to the power necessary for the oscillator module.

Accordingly, the surface area initially occupied by a first inductive element alone is substantially unchanged here.

According to one embodiment in which the frequency divider is configured for dividing the frequency of a signal having an amplitude above a threshold amplitude, the oscillator module is configured for generating an oscillating signal with an amplitude below the threshold amplitude on the terminals of the first inductive element, and the autotransformer is configured for generating an amplified signal of the oscillating signal on the output terminals with an amplitude above the threshold amplitude.

The local oscillator device is advantageously implemented in an integrated manner within an integrated circuit.

A transceiver system, including an antenna intended to transmit and receive radio frequency signals, a transmitting chain, and a receiving chain coupled to the antenna, advantageously comprises a local oscillator device as defined above coupled to the transmitting and receiving chains.

An electronic apparatus, such as a mobile phone or a touch tablet, advantageously comprises such a transceiver system.

Other advantages and features of the invention will appear on examination of the detailed description of embodiments, in no way restrictive, and the accompanying drawings in which:

FIG. 1 , described previously, represents a conventional local oscillator; and

FIGS. 2 to 5 represent examples of embodiments of the invention.

FIG. 2 represents a local oscillator device DIS, implemented, for example, in an integrated manner within an integrated circuit.

The local oscillator device DIS comprises an oscillator module OSC including a first inductive element L and a capacitive element C mounted in parallel, of the LC resonator type.

The capacitive element C may, for example, comprise a bank of elementary capacitive elements in parallel, coupled or not coupled to the first inductive element L via controllable switches. This notably makes it possible to adjust the capacitive value of the capacitive element C.

The oscillator module OSC generates in the usual way an oscillating signal V 3 on the terminals La and Lb of the first inductive element L. The amplitude of the oscillating signal V 3 may, for example, be 0.5 volts.

The oscillator module OSC further comprises a sustaining amplifier SUST supplied by a current source I, and connected in parallel to the LC resonator. The sustaining amplifier SUST allows the oscillations of the signal V 3 to be maintained by compensating resistive losses, and controlling the oscillation amplitude thanks to adjusting the current I via an amplitude controller MCTR.

The local oscillator device DIS comprises a frequency divider fDiv, configured in this example for communicating two local oscillator output signals LO 1 , LO 3 in phase quadrature, and two local oscillator signals LO 2 , LO 4 respectively in phase opposition with the signals in phase quadrature LO 1 , LO 3 .

The signals LO 1 -LO 4 at the output of the frequency divider fDiv are at a frequency proportional to the frequency of the signal V 4 present between the two inputs IN 1 , IN 2 of the frequency divider fDiv.

For example, the frequency divider may divide the frequency of the signal V 4 on its input by 2 or by 4.

The frequency divider fDiv, for example, implemented in CMOS technology, requires for its operation an input signal with an amplitude above a threshold amplitude.

This threshold amplitude stems notably from the threshold voltage necessary to engage a switching of a CMOS transistor, and must be, for example, greater than 1 volt. An autotransformer TR makes it possible to generate on its output terminals OUT 1 , OUT 2 , a signal V 4 with an amplitude level above the threshold amplitude at the output of the frequency divider fDiv, from an oscillating signal V 3 generated by the oscillator device OSC having an amplitude level below the threshold amplitude.

The autotransformer TR includes the first inductive element L as the primary circuit and a secondary circuit including two second inductive elements L 1 , L 2 . The second inductive elements L 1 , L 2 are respectively coupled to the terminals La, Lb of the first inductive element L and to two output terminals OUT 1 , OUT 2 of the autotransformer TR.

The output terminals OUT 1 , OUT 2 of the autotransformer TR are directly coupled to the input terminals IN 1 , IN 2 of the frequency divider fDiv, transmitting the signal V 4 thereto.

FIG. 3 represents an embodiment of an autotransformer TR advantageously integrated according to the invention into the device DIS described in relation to FIG. 2 .

The autotransformer TR comprises the first inductive element L implemented by a first conductive track PC, e.g., made of metal and located in the first interconnection levels of an integrated circuit.

The autotransformer TR comprises two second inductive elements L 1 , L 2 , also implemented by second respective conductive tracks PC 1 , PC 2 , e.g., made of metal and located in the first interconnection levels of the integrated circuit.

In the description in relation to FIG. 3 , the first inductive element L may be designated by the term primary circuit, and the second inductive elements L 1 , L 2 by the term secondary circuit.

The conductive track PC of the primary circuit L is implemented in a plane P, and has an interlaced coil shape.

More precisely the conductive track PC, starting from the terminal La, follows a route having in its first half an octagonally-shaped coil that winds round in a first direction and then having in its second half an octagonally-shaped coil winding round in the same direction, in an interlaced manner with the winding of the coil of the first half of the route, up to the terminal Lb.

The conductive track PC comprises n coils (i.e., n convolutions), n being a whole number, with n=2 in the representation of FIG. 3 .

The width of the metal track PC is, for example, between 10 μm and 20 μm.

The secondary circuit comprises two second conductive tracks PC 1 and PC 2 interlaced with each other and located on the periphery of the first inductive element L.

The first of these second conductive tracks PC 1 , is also implemented in the plane P and has an octagonally-shaped coil that winds round in the same first direction, starting from the terminal OUT 1 up to the terminal La. The conductive track PC 1 forms the first of the second inductive elements L 1 , coupled to the first inductive element L on its terminal La.

The second of these second conductive tracks PC 2 , is also implemented in the plane P and also has an octagonally-shaped coil that winds round in the first direction, starting from the terminal Lb up to the terminal OUT 2 , in an interlaced manner with the winding of the first second conductive track PC 1 . The conductive track PC 2 forms the second of the second inductive elements L 2 , coupled to the first inductive element L on its terminal Lb.

The conductive tracks PC 1 , PC 2 of the secondary circuit each comprise, for example, n/2 coils, n being the number of coils of the conductive track PC of the primary circuit.

Moreover, the conductive tracks PC 1 , PC 2 of the secondary circuit may be narrower than the conductive track PC of the primary circuit, e.g., according to a width ratio between 0.1 and 0.7.

The autotransformer TR has an overall octagonal shape in the example represented in FIG. 3 , but could have other forms, e.g. square or circular. A point O located in the middle of the autotransformer TR designates a central point. The central point O is substantially a point of symmetry of the autotransformer shape, at the positions of the terminals and nearby crossings.

The octagonal outline Lim defines an area of protection for the autotransformer TR inside which any signal other than that of the oscillator is excluded.

Thus, it is seen in FIG. 3 that the implementation of the second inductive elements L 1 , L 2 of the autotransformer TR do not substantially occupy more surface area than the surface area occupied by the inductive element L alone.

Thus, via electromagnetic coupling of the autotransformer TR, the voltage V 4 between the output terminals OUT 1 , OUT 2 may be expressed as: V 4 =(½)*V 3 +V 3 +(½)*V 3 =2*V 3 , in this example with n coils in the primary circuit and n/2 coils in each inductive element L 1 , L 2 of the secondary circuit.

The inductive elements L, L 1 , L 2 of the autotransformer TR are dimensioned so that the signal V 3 is optimized for the oscillator module OSC, e.g. with regard to phase noise and surface area occupation, and so that the signal V 4 is of an amplitude at least equal to the threshold amplitude of the frequency divider fDiv.

Furthermore, in the example represented in FIG. 3 , the terminals La, Lb of the first inductive element are located on a first side of the octagon, on the periphery of the autotransformer TR.

The output terminals OUT 1 , OUT 2 , are located on the side opposite this first side. The output terminals OUT 1 , OUT 2 could be located on another side of the autotransformer TR, and not necessarily on the same side, notably according to the requirements regarding the number of coils and the architecture of the integrated circuit.

In such a configuration, it is possible to place the frequency divider fDiv on the side of the outputs OUT 1 , OUT 2 of the autotransformer, which makes it possible to relieve the load congestion on the terminals La, Lb of the first inductive element, and to reduce the size of the electrical paths linking the oscillator module OSC notably to the inputs of the frequency divider fDiv.

FIG. 4 represents a simplified example of a transceiver system SYS e.g. configured for communicating at frequencies of the order of a gigahertz, advantageously comprising a local oscillator device DIS as previously described in relation to FIGS. 2 and 3 .

The transceiver system SYS includes a receiving stage RX and a transmitting stage TX.

The receiving stage RX conventionally comprises a low-noise amplifier LNA, transmitting the signal received by the antenna ANT to a symmetrical-asymmetrical transformer BLN (“balun” according to the usual term).

At the output of the balun transformer BLN, the signal is in differential mode, and each differential component of the signal is mixed by a mixer MIX 1 with a local oscillator signal LORX supplied at the output of the frequency divider fDiv of the device DIS, which makes it possible to perform frequency transpositions, e.g., in baseband.

The transposed signals are then processed by processor MTS, e.g., partly implemented by software within a processor (a baseband processor, for example).

The transmitting stage TX comprises a power amplifier PA configured for generating a transmission signal amplified on the antenna ANT, the carrier frequency of which is the frequency of a local oscillator signal LOTX supplied at the output of the frequency divider fDiv of the device DIS.

The signal received at the input of the power amplifier PA results from a frequency transposition, in the radio frequency domain, of a signal emanating from the processor MTS, e.g., of the baseband processor, with the aid of the local oscillator signal LOTX.

FIG. 5 represents an example of an electronic apparatus APP provided with an embodiment of a radio frequency communication system SYS as previously detailed, here a mobile phone. It will be clear to the person skilled in the art that the embodiments of a radio frequency communication system SYS or of a local oscillator device DIS according to the invention may be included in any other type of apparatus or other known system not described here.

Moreover, the invention is not limited to the embodiments that have been described above but embraces all the variants, e.g., the architecture of the autotransformer, the various components of the oscillator device or the radio frequency system that have been given by way of example.