Patent Description:
There exist many ways to design on-chip monolithic transformers. The main electrical parameters of interest to a circuit designer are the transformer turns ratio n and the coefficient of magnetic coupling k. If the magnetic coupling between windings is perfect (i.e., no leakage of the magnetic flux), k is unity, while uncoupled coils have a k-factor of zero. A practical transformer will have a k-factor between these two extremes. Typically for on-chip monolithic transformers, a k-factor between <NUM> and <NUM> can be achieved.

Architectures for on-chip monolithic transformers include parallel architecture (Shibata type), interleaved architecture (Frlan type), and stacked architecture (Finlay type). <FIG> illustrates the Shibata type, in which the primary and secondary windings <NUM>, <NUM> are provided as nested coils. This topology is easy to design but the total lengths of the primary and secondary windings are not equal. Hence the transformer turns ratio can differ from unity despite having the same number of turns of metal on each winding. <FIG> illustrates the Frlan type, which partially eliminates this asymmetry by providing the primary and secondary windings <NUM>, <NUM> as interleaved or bifilar coils. <FIG> illustrates the Finlay type, in which the primary and secondary windings <NUM>, <NUM> are provided as stacked coils. This requires less area than an interleaved architecture and hence allows higher coupling coefficients to be reached. However, the upper metal layer is generally much thicker than the intermediate metal layer, leading to higher electrical resistance and thus increased IL (Insertion Loss) of the upper winding. This can lead to an asymmetry in the electrical response of the transformer. Also, the upper winding is electrically shielded from the "conductive" substrate by the lower winding, and hence the parasitic capacitance to the substrate (and the associated dissipation) differs for each winding. In addition, there is a large parallel plate component to the capacitance between windings due to the overlapping metal layers, which limits the frequency response in view of mm-Wave applications.

However, existing techniques and architectures for designing transformers and auto-transformers having intermediate or high magnetic coupling coefficient are not suitable for applications requiring a weak (e.g., less than <NUM>) and negative coupling coefficient. Low magnetic coupling coefficients cannot be achieved by simply increasing the separation of the primary and secondary windings in existing architectures as this leads to a large area and increased losses.

<CIT> describes a phase shifter module, multiplexer/demultiplexer and communication apparatus. <CIT> describes an integrated circuit based transformer.

Aspects of the disclosure are set out in the accompanying claims. Combinations of features from the dependent claims may be combined with features of the independent claims as appropriate and not merely as explicitly set out in the claims.

According to an aspect of the disclosure, there is provided a transformer comprising:.

An advantage of the transformer of the present disclosure is that it may provide a transformer (including an autotransformer) having both a low absolute value of magnetic coupling coefficient, for example less than <NUM>, and a relatively small area or footprint. The transformer may also have a negative magnetic coupling coefficient.

Successive lobes of the first plurality of lobes may be formed by portions of the first conducting element arranged to wind in opposite senses.

As a result, a current flowing in the first conducting element may produce a magnetic field in the plurality of enclosed areas which may be oriented in opposite directions in adjacent enclosed areas. This may also help to reduce the sensitivity of the transformer to coupling of external radiation. Similarly, successive lobes of the second plurality of lobes may be formed by portions of the second conducting element arranged to wind in opposite senses.

Each lobe of said first and/or second plurality of lobes may comprise only a partial turn of the respective conducting element.

In this way, each enclosed area of said plurality of enclosed areas may be defined by a partial turn of the first conducting element on one side, and a partial turn of the second conducting element on the other side. For example, each lobe may comprise a half turn of the respective conducting element. Adjacent enclosed areas of said plurality of enclosed areas may be separated by crossing points of said first and second conducting elements.

Said first lobed portion and said second lobed portion may together form a symmetric pattern.

By providing an even number of lobes, cancellation of magnetic fields at adjacent enclosed areas and immunity to coupling of environmental radiation may be improved.

In some embodiments, said first conducting element further comprises a first outer portion connected at one end to a first end of said first lobed portion, and said second conducting element further comprises a second outer portion connected at one end to a first end of said second lobed portion.

Said first outer portion and said first lobed portion may be arranged to form a first loop; and said second outer portion and said second lobed portion may be arranged to form a second loop, said second loop at least partially overlapping said first loop.

In some embodiments, at least a portion of said first outer portion and/or said second outer portion is substantially straight.

This may be helpful in reducing the overall area of the transformer.

In some embodiments, a second end of said first lobed portion is connected to a second end of said second lobed portion.

In this way, the transformer may be configured as an autotransformer. Advantageously, the connection between the second ends of the first and second lobed portion may provide convenient access for a center tap of the autotransformer. Said second end of said first lobed portion may be adjacent or overlapping said second end of said second lobed portion. For example said second ends of said first and second lobed portions may be located at a crossing point of said first and second conducting elements.

Said second end of said first lobed portion and said second end of said second lobed portion may be connected to ground, for example to a ground plane.

The connection to ground may be via a switch.

In some embodiments, the transformer is an autotransformer.

The first and second conducting elements may provide the primary and secondary sides respectively of the autotransformer, the first and second conducting elements being connected in series.

In some embodiments, said first outer element is further arranged to define a full turn of the first conducting element, and/or said second outer element is further arranged to define a full turn of the second conducting element.

Including a full turn in one or both of the first and second outer elements may facilitate optimising the design of the transformer for a specific application. to a specific frequency or frequency range depending on the application. However, these tuning elements may be omitted if required to reduce the footprint or overall area of the transformer.

The transformer may be formed on a substrate.

For example, the transformer of the present disclosure may be implemented as a planar monolithic transformer. For example, the transformer may be formed on a PCB or on any dielectric or semiconductor substrate, including silicon and GaN. The transformer may be implemented in an integrated circuit, for example an RFIC.

According to a further aspect of the present disclosure, there is provided a passive phase shifter comprising a transformer as defined above.

The transformer defined above may be particularly useful in applications where Phase-to-Gain must be minimised, for example in integrated <NUM>/<NUM>° Passive Phase shifter (PPS) for analog beam-forming applications, which may require the Phase-to-Gain to vary by less than <NUM>. 5dB between the two modes, since this is only achievable for weak negative values of k, for example in the range -<NUM> to -<NUM>.

Example embodiments of the present disclosure will be described, by way of example only, with reference to the accompanying drawings in which like reference signs relate to like elements and in which:.

<FIG> illustrates a configuration of a transformer <NUM>, in the form of a planar monolithic autotransformer <NUM>, according to an example embodiment of the present disclosure. The autotransformer <NUM> comprises a first conducting element <NUM> and a second conducting element <NUM>, effectively forming the primary and secondary sides respectively of the autotransformer <NUM>. The first conducting element <NUM> includes a first outer portion <NUM> and a first lobed portion <NUM>, which together form a first loop. The first outer portion <NUM> extends from a first terminal P1 to a first end of the first lobed portion <NUM>. Similarly, the second conducting element <NUM> includes a second outer portion <NUM> and a second lobed portion <NUM>, which together form a second loop, the second outer portion <NUM> extending from a second terminal P2 to a first end of the second lobed portion <NUM>. The first lobed portion <NUM> is arranged to form a first plurality of lobes <NUM>, and the second lobed portion <NUM> is arranged to form a second plurality of lobes <NUM>.

Each of the first and second pluralities of lobes <NUM>, <NUM> corresponds to a sequence of partial turns of the respective conducting element <NUM>, <NUM>, with each successive lobe <NUM>, <NUM> winding in an opposite sense to the preceding lobe <NUM>, <NUM>. The first and second conducting elements <NUM>, <NUM> are arranged such that the first lobed portion <NUM> overlaps the second lobed portion <NUM> to define a plurality of enclosed areas <NUM>. Each enclosed area <NUM> is bounded by a lobe <NUM> of the first conducting element <NUM> on one side, and by a lobe <NUM> of the second conducting element <NUM> on the other side, with adjacent enclosed areas <NUM> being separated by crossing points of said first and second conducting elements <NUM>, <NUM>.

A current flowing in the first lobed portion <NUM> thereby produces a magnetic field in each of the enclosed areas <NUM> which is opposite in direction to the magnetic field in adjacent enclosed areas <NUM>.

In this embodiment, the respective second ends of the first and second lobed portions <NUM>, <NUM> are connected to each other at a centre tap <NUM> to form an autotransformer. The centre tap <NUM> is connected to ground. Therefore the magnetic coupling between the respective lobes of the first and second lobed portions <NUM>, <NUM> at each enclosed area <NUM> is negative. As a result, the magnetic coupling between the first and second lobed portions <NUM>, <NUM> has a negative magnetic coupling coefficient k.

In this embodiment, the centre tap <NUM> is located at a crossing point of the first and second lobed portions <NUM>, <NUM> to facilitate the connection. Being located at an end of the first and second lobed portions <NUM>, <NUM> and between the first and second terminals P1, P2, the design of the transformer <NUM> provides convenient access to the centre tap <NUM>.

In this embodiment, the first and second lobed portions <NUM>, <NUM> have an equal and even number of lobes <NUM>, <NUM>, forming a symmetric pattern. This results in at least partial cancellation of the magnetic fields from adjacent pairs of enclosed areas <NUM>, thereby reducing the sensitivity of the transformer to external radiation.

In the embodiment shown in <FIG>, the first and second lobed portions <NUM>, <NUM> each comprise four lobes <NUM>, <NUM>, each comprising a half turn of the respective conducting member <NUM>, <NUM>. However, different numbers and shapes of lobes may be used, depending on the coupling strength required and the area available.

The first and second outer elements <NUM>, <NUM> further include a respective tuning element <NUM>, <NUM>, in the form of a full turn of the respective first/second conducting element <NUM>, <NUM>. The tuning elements <NUM>, <NUM> provide a means for convenient tuning of the transformer values for fine tuning and optimisation during the design stage. The dimensions of the tuning elements <NUM>, <NUM> can be stretched in one or both of the vertical and horizontal directions (i.e., in the plane of the transformer <NUM>), for example to optimise the transformer for a given frequency range. The shape of the loops of the tuning elements <NUM>, <NUM> may also be varied. However, the tuning elements <NUM>, <NUM> may be omitted if a particularly small footprint is required for the transformer.

The first and second tuning elements <NUM>, <NUM> shown in <FIG> are also coupled magnetically with a low negative coupling coefficient k, so the overall magnetic coupling coefficient of the autotransformer <NUM> is still negative.

In the embodiment shown in <FIG>, the first and second outer portions <NUM>, <NUM> have straight portions, which may be helpful in reducing the overall area of the transformer <NUM>. However, other configurations may be used.

<FIG> illustrates a transformer <NUM>' according to another example embodiment of the disclosure. Elements corresponding to those of transformer <NUM> described above with reference to <FIG> are labelled with identical reference numbers.

<FIG> differs from <FIG> in that the transformer <NUM> is shown surrounded by a ground plane <NUM>. The transformer <NUM>' is formed on a substrate, for example, any dielectric or semiconductor substrate, including silicon and GaN, or a PCB. The first and second conducting elements <NUM>, <NUM> are formed by metallization on the substrate. An (n-<NUM>)th layer of metallization is used to form the parts of the conducting members <NUM>, <NUM> indicated by a black and white pattern. An nth layer of metallization is used to form the parts of the conducting elements <NUM>, <NUM> indicated by grey shading. In the present embodiment, the conducting elements <NUM>, <NUM> are formed using only the angles (<NUM>, <NUM>, <NUM> degrees) available in the GaN fabrication process. However, other angles and curvatures may be available in other processes. The ground plane <NUM> may be formed as part of the (n-<NUM>)th or nth layer of metallization, or as a lower layer. In the embodiment shown in <FIG>, the centre tap <NUM> is connected to the ground plane <NUM> by a switch <NUM>. However, a permanent connection may be used depending on the application.

Importantly, the design of the transformer <NUM>, <NUM>' of the present disclosure is optimised to allow convenient access to the centre tap <NUM>, enabling an easy and efficient ground connection in the present embodiment. Additionally, the first and second terminals P1, P2 are located symmetrically to each side of the transformer <NUM>, facilitating routing of connections and thus minimizing loses.

<FIG> illustrates a <NUM>/<NUM>° Passive Phase shifter (PPS) <NUM> comprising an autotransformer <NUM> which may be implemented using the transformer <NUM>, <NUM>' described above with reference to <FIG> and <FIG>. For this application, an autotransformer <NUM> having a low, negative value of k is required to minimise losses. In addition, the input and output must be as close as possible for ease of routing and thus minimising losses. The layout must also be compliant with a low-impedance path to the ground reference.

<FIG> presents simulation results for the transformer <NUM>, optimised for <NUM>. The upper graph shows the individual inductances (top two traces) and mutual inductance (lower trace) of the first and second conducting elements <NUM>, <NUM> (i.e., the primary and secondary sides of the transformer) as a function of frequency. It can be seen that the inductances are substantially equal across a large frequency range, and that the mutual inductance is weak and negative, with an absolute value below <NUM>. The lower graph of <FIG> shows the resistances of the first and second conducting elements <NUM>, <NUM> (i.e., the primary and secondary sides of the transformer) as a function of frequency. It can be seen that the resistances are substantially equal across a large frequency range. The transformer is operable over a wide frequency band and can be optimised for any frequency during the design phase, in particular by varying the area of the tuning elements <NUM>, <NUM>.

Whereas existing transformer designs are very area consuming or have a high magnetic coupling coefficient leading to high losses, the design of the transformer <NUM> of the present disclosure provides a transformer having a weak magnetic coupling coefficient, without consuming a high area and is therefore advantageous in applications requiring a low-k transformer. For example, the integrated <NUM>/<NUM>° Passive Phase shifter (PPS) <NUM> requires a negatively-coupled autotransformer having a low absolute value of magnetic coupling coefficient k in order to achieve low absolute losses, a low difference in losses between the <NUM>° and <NUM>° modes (that is, a low Phase to Gain or P2G) and a low variation in phase shift ΔΦ between the <NUM>° and <NUM>° modes of the passive phase shifter <NUM> as a function of magnetic coupling coefficient k.

Accordingly, there has been described a transformer and an integrated passive phase shifter comprising the transformer. The transformer includes a first conducting element having a first lobed portion arranged to form a first plurality of lobes. The transformer includes a second conducting element having a second lobed portion arranged to form a second plurality of lobes. The first lobed portion overlaps said second lobed portion to define a plurality of enclosed areas.

Embodiments of the transformer include monolithic on-chip transformers. The transformer may be fully symmetrical. The transformer is particularly suitable for implementation as an autotransformer having a weak, negative magnetic coupling.

Claim 1:
A transformer (<NUM>, <NUM>') comprising:
a first conducting element (<NUM>) having a first lobed portion (<NUM>) arranged to form a first plurality of lobes (<NUM>); and
a second conducting element (<NUM>) having a second lobed portion (<NUM>) arranged to form a second plurality of lobes (<NUM>);
wherein said first lobed portion (<NUM>) overlaps said second lobed portion (<NUM>) to define a plurality of enclosed areas (<NUM>),
characterised in that the number of lobes comprised by said first plurality of lobes (<NUM>) is an even number, and said transformer (<NUM>, <NUM>') is substantially symmetric.