DELAY LINE

Field of the invention

The present invention relates to delay lines. Background art

Delay lines are employed in microwave applications to introduce frequency independent phase shifts for numerous applications. One major application is for antenna phased arrays. In order to steer a transmit or receive beam in a certain direction, appropriate delay line lengths need to be introduced to create a phase gradient across the array. The use of different lines lengths for each antenna element is necessary to increase the number of beam directions that the array can achieve and for large systems such as phased array radar, frequent adjustment may be necessary. In addition the required lengths may be large depending on the array size. The delay lines may be implemented as microstrip on substrates, or as coaxial cables. To change the delay line length the use of switch banks is typical.

A previous approach has employed a configuration as depicted in Figure 1. In Figure 1 an antenna 2 is connected to a delay line 4 which includes a sequence of single pole single throw switches 6 to selectively connect paths of different lengths between the antenna 2 and a connection 8, which may for example be an input connection to a transmitter or receiver. Depending on which switch 6 is closed, the length of signal path provided by the delay line 4 is adjusted. Whilst this approach is simple and straightforward it requires a lot of space for large phased arrays where the delay line length requirements may be large. In addition, when applying various switch options, relatively long open circuit stub lengths are inevitably introduced, comprising in particular the length of line 10 that is not connected in the selected signal path. This leads to system errors, losses and introduces difficulties in impedance matching the system.

Summary of the invention

In order to overcome or at least reduce the above noted problems, the present invention provides a delay line, comprising a radio frequency signal path configured in a convoluted form and further comprising reconfigurable switch means connected to selected convoluted portions of the signal path so that, in dependence on which switch configuration is adopted, a different preselected length of the signal path is operatively connected between an input and an output of the delay line and an inoperative part of the signal path is disconnected from the input and the output. The switch means are so configured that where part of the delay line is connected between the input and the output of the line to give the required operative length, the remaining inoperative part is disconnected from the input and output to prevent the occurrence of open circuit stubs. The avoidance of open circuit stubs is highly desirable when signal paths are to be used for signals of microwave frequency.

As a further advantage, in a preferred embodiment, the inoperative part of the signal path is not continuous in that the switch means break the continuity of the inoperative part to prevent the formation of inductive loops.

The frequency of operation of any specific delay line in accordance with the invention will depend on the intended application. The frequency may be anywhere from radio frequency (RF) microwave frequencies of about 300 MHz to 300 GHz (the limits, for the purposes of the present specification, for microwave frequencies), through terahertz frequencies up to optical frequencies, between 10 ¹³ to 10 ¹⁸ Hz. As preferred however the frequency of operation is microwave.

The delay line may take any convenient convoluted form. Where the delay line is composed of flexible cable, the cable may be coiled, for example as a flat spiral or as an elongate tube in three dimensions. Where the delay line is composed as microstrip or other waveguide formed on a substrate of for example an integrated circuit, the delay line has a two-dimensional shape, for example sinuous, meandering, zigzag, or as preferred spiral. Individual turns of the spiral may be circular, rectangular, or any other convenient shape. In some

applications the substrate may be curved or flexible; in some circumstances the delay line may be formed stepped in a plurality of levels.

For the purposes of the present patent specification, the term "convoluted" is intended to include within its scope a variety of two or three- dimensional forms that a delay line can take, including but not limited to spiral, sinuous, meandering, zigzag and twisted. The noun "convolution" is intended to refer to an identifiable section of a convoluted line, in particular a part having a characteristic shape that is repeated, for example a loop of a spiral or a repeated shape in a meandering, zigzag or sinuous line. The reconfigurable switch means may take a variety of forms. As preferred an input line of the delay line extends across at least part of the convoluted form of the signal path, and a series of switches are connected between the input line and individual convoluted portions of the signal path to give the desired length increments. Where the delay line is formed as a flat spiral, the input line may extend in a radial direction towards the centre. Alternatively the switch means may be formed as a composite multi-element unit extending across the convoluted form of the signal path.

Preferably, each of a plurality of the selected convoluted portions is provided with a respective break and the switch means provide, for each of the plurality of convoluted portions, a first position in which the respective break is maintained and the respective convoluted portion is connected to the input or the output, and a second position in which the respective break is closed and the respective convoluted portion is disconnected from the input and the output, and the switch means have a plurality of switch configurations, each configuration comprising, for one of the plurality of convoluted portions, the switch means in the first position, and for the remainder of the plurality of convoluted portions, the switch means in the second position.

In a preferred embodiment, a further convoluted portion of the signal path has a break therein and the switch means provide, for the further convoluted portion, one position in which the break is maintained and the further convoluted portion is disconnected from the input and the output, and a further position in which the break is closed and the further convoluted portion is connected to the

- A - input or the output, and in each of the plurality of switch configurations, the switch means are in the one position in respect of the further convoluted portion to prevent the formation of an inductive loop in the signal path.

As preferred the switch means comprises a series of single pole double throw switches connected in a first position to the input line and connected in a second position to close the respective break in the delay line.

The switch means may, for microwave frequencies, comprise an array of switching transistors. Alternatively it may comprise a series of MEMS switch devices. The switch means may comprise a single MEMS device with a multi position, multi contact switch that is arranged in each of its positions to give a required delay line length with the remainder of the delay line being decoupled from the required length.

The technology for implementing the delay line of the invention will depend on the frequency of operation. For optical frequencies wave guides may be found as silica tracks (or other materials) on a substrate, or optical fibre;

Switches for optical frequencies may be of any suitable type, to give the required speed of operation.

Depending on the degree of resolution, any number of switches may be employed. For a coarse resolution, a large number of convolutions of the delay line may be disposed between consecutive switches; for a fine resolution, a lower limit of resolution is to have consecutive switches on adjacent convolutions.

In use the input of the delay line may be connected to an antenna. The output of the delay line may be coupled to a receiver, transmitter or transceiver. It has been found that the invention is particularly useful with reflect antenna arrays, in which case the delay line output is coupled to a short circuit connection.

Brief description of the drawings

Preferred embodiments of the present invention will now be described by way of example with reference to the accompanying drawings wherein;-

Figure 1 is a schematic view of a known switched delay line;

Figure 2 is a schematic view of a first preferred embodiment of the present invention comprising a switched spiral delay line; and

Figure 3 is a schematic view of a second preferred embodiment of the present invention comprising a switched meandering delay line. Description of the preferred embodiment

The preferred embodiments comprise a delay line specifically adapted for microwave applications with the delay line being formed of an electrical conductive microstrip in an integrated circuit.

Referring to figure 2, the first preferred embodiment of the invention comprises an input connection 10 connected to an antenna 12 and an output connection 14 coupled to a short circuit 15 for a reflect antenna array (or to a transmitter or receiver device). A delay line 16 is arranged between input and output 10, 14 in a spiral form. As shown conceptually the delay line is of rectangular form, but a circular form or other form may be employed. The input connection 10 comprises an input line 18 which extends radially of the spiral form and is superposed on the turns 20 of delay line 16 and crosses each turn by means of an air bridge 22. Air bridges within the integrated circuit provide the requisite insulation.

A switch array comprising switches 261 , 262, 263, 264 is connected between input line 18 and the turns 20 of delay lines 16. As shown conceptually, the delay line has four turns 201-204, and each turn has a respective switch 261-264 connected between the respective turn and input line

18. In practice further turns (not shown) may be provided between each turn

201-204. Each switch is a single pole double throw switch apart from switch 264 which is a single pole single throw switch, and permit selective connection of the respective turn to the input. For each switch 261-163, a movable switch element M is connected to contact I of the input line in a first position. In a second position of the switch, element M is connected to a contact T to close a break B in the delay line. For single pole switch 264, the element M is movable to connect to input line contact I and at the same time close break B in one position, and in the other position the switch is open.

Each switch may comprise, for a microwave circuit, a transistor (FET), or electromechanical MEMS device.

In order to introduce different line lengths, the various single pole double throw (SPDT) switches 261-263 are selectively actuated. Starting from the top, when the first switch 261 is connected to contact I of input line 18, the other switches 262, 263 are moved to contacts T, to close the respective breaks B in the delay line: except the bottom switch 264 which is open. This introduces the shortest line length. Movement of switches 262, 263 to close breaks B ensures that open circuit stubs are not coupled to the delay line; switch 264 remains open to prevent inductive loops being formed in the inoperative part of the delay line. As we progress down the line, when a particular switch is connected to the input line, all preceding switches above it are closed; all successive switches below it are closed except for the bottom switch which is open. This introduces successively longer line lengths. Thus, if switch 262 is connected to contact I of input line 18, upper switch 261 is connected to contact T to close break B, so that the two outer turns of the spiral form the operative part of the delay line. However switch 263 is connected to contact T to close break B, to disconnect the inoperative part of the delay line from input line 18. Switch 264 is open. Finally, the bottom switch is closed and all preceding switches are closed to introduce the longest line length. Various numbers of line lengths are therefore introduced and this will correspond to the number of switches.

In accordance with the invention, use of a delay line in a spiral form provides a very compact configuration wherein long open circuit stub lengths are avoided. Due to the fact that during various switching stages, the circuit introduces only minute open circuit stubs, the performance of the delay line is largely unaffected. This is in contrast to current available techniques.

A second embodiment of the present invention will now be described with reference to Figure 3. Similar principles apply in the operation of a delay line according to this second embodiment as apply in the first embodiment described above with reference to Figure 2.

Referring to Figure 3, a delay line is provided in the form of a meandering transmission line 32 wherein switches 341 , 342, 343, 344 are provided to select

respective portions of the meandering form of delay line 32 to connect between the input 10 and the output 14 of the delay line 16, in a similar way to that provided by the switches 261-264 in Figure 2. In practice, the arrangement according to this second preferred embodiment, having a meandering form of delay line 32, is likely to be slightly less compact, and the potential length of unused stub of input line 18 is likely to be slightly longer than with the spiral form of delay line 20 in the first preferred embodiment described above. However, in comparison with prior art arrangements, this second embodiment represents a significant improvement in those respects.