A phase array antenna system and a method for its calibration

Abstract

A phased array antenna system 2 comprises: an array of multiple elements 20 each having an antenna element (202, Figure 2) and element communication circuitry (203, Figure 2) for adjusting one or more

Description

The present invention relates to a phase array antenna system and a method for its calibration. It has particular, but not exclusive, application to active electronically scanned array antenna. An electronically scanned active array antenna of conventional design comprises an array of elements each comprising an element antenna and an element level control device, controlling amplitude, phase and / or or delay of a signal to / from the element antenna, together with a passive beamforming network through which the signals from each element are combined when the antenna is receiving a signal and through which a signal is divided for receipt by each control device when the antenna is transmitting. Following manufacture of an electronically scanned antenna, a calibration procedure is carried out in which the array elements are adjusted to ensure the antenna is able to accurately steer beams and form desired beam patterns. This is required because current manufacturing processes do not enable each array element to be formed identically and because the path provided by the passive beamforming network between each element and a network’s output may not have identical characteristics. A typical calibration procedure involves use of external test equipment such as, for example, a Near Field Scanner that provides an accurate external reference signal. In the example of a Near Field Scanner, the test equipment is placed in front of each element in turn and the element energised. The control device associated with that element can then be adjusted to compensate for any imperfections. The element level control devices can be adjusted during operation to account for changes in performance that may be a result of changes in temperature. These adjustments are typically made based on expected deviation as a result of sensing a change of temperature of the array. Through these adjustments and because the characteristics of the passive beamforming network typically remain stable over the operational lifetime of the antenna, an antenna can be kept calibrated for a long period of time without need for repeating the initial calibration procedure. This approach is simple but has the disadvantages described below: a) If any part of the array fails and is replaced, the entire array may need to be re-calibrated, which may involve sending it back to the factory where it was manufactured. b) Because it is generally impractical to monitor the performance of the array in operation without the use of external test systems, failures or degradations of the element’s electronics may degrade array performance without notice. An alternative known variant antenna structure comprises two passive beamforming networks which allows a test signal to be transmitted through one network to the elements and the modified test signal from the elements to be simultaneously received through the other network. This dispenses with the need for external test equipment but significantly increases the overall size of the antenna array. As a consequence, antennas with this structure are usually impractical for use on platforms that demand considerable size and weight constraints, as is common on airborne platforms. According to a first aspect of the invention there is provided a phased array antenna system comprising an array of elements, each element arranged for transmitting and / or receiving a radio frequency (RF) signal; each element comprising an antenna and element communication circuitry, the element communication circuitry comprising means for adjustment of one or more of amplitude, phase and delay of a signal received and / or transmitted by the element; the phased array antenna further comprising a passive bi-directional network for combining received RF signals or splitting a transmitted RF signal from a common port to each of the elements; characterised in that phased array antenna system comprises a local RF reference signal source having an input connected to the passive bi-direction network; the local RF reference signal source configured to synchronise a local RF reference signal with a master RF calibration signal transmitted through the passive bi-directional network from a master RF calibration signal source; and a comparator configured to compare, during a calibration procedure, the synchronised local RF reference signal with the master RF calibration signal following transmission of one of them through the element communication circuitry. Through provision of a local reference signal source configured to provide a local reference signal that is synchronised with a master calibration signal, it is possible to calibrate an antenna having only one passive bi-directional network in both transmit and receive configurations without the need for external test equipment. When calibrating the antenna in a receive configuration, the passive bi-directional network can be used to carry the master RF calibration signal to the or each local signal source. Following this, the synchronised local reference signal can then be transmitted through the element communication circuitry and back through the manifold for comparison with the central master reference signal. When calibrating the antenna in a transmit configuration, the synchronised local reference signal can be compared with the master RF calibration signal following it transmission through the element communication circuitry The communication circuitry of each element may comprise a first port connected to the passive bi-direction network and a second port connected to the antenna; and wherein the local RF signal source is configured to transmit the synchronised local RF reference signal to the element communication circuitry via the second port. The system may further comprise a system side comparator configured to compare the master RF reference signal with local RF reference signal following its transmission through the element communication circuitry and passive bi-directional network. With this arrangement the local RF reference signal can to mimic an RF signal received through the antenna element. The output of the system side comparator provides can be used to derive an error value resulting from the modification by the element communication circuitry and the passive bi-directional network. Each element may comprise a first comparator having a first input connected to the local signal source, and a second input connected between element communication circuitry and the passive bi-directional network; the first comparator configured to compare the synchronised local RF reference signal received through a first input from the local signal source with a modified synchronised local RF calibration signal through the second input being the local RF calibration signal as modified by the element communication circuitry, the first comparator configured to output a first comparator signal indicative of said comparison. The output of the first comparator provides an indication of the modification of the signal by the passive bi-directional network alone. Consequently, the output of the first comparator together with the output of the system side comparator it is possible to distinguish characteristics of each of the passive bi-directional network and the element communication circuitry when operating in a receive configuration. The first comparator signal may be an analogue signal. Where so each element may comprise a first analogue to digital converter (ADC) configured to digitise the first comparator signal. Each element may comprise a second comparator having a first input connected to the local signal source and a second input connected between element communication circuitry and the antenna; the second comparator configured to compare the synchronised local RF reference signal received through a first input from the local signal source with the master RF calibration signal through the second input following transmission through the element communication circuitry; the second comparator configured to output a second comparator signal indicative of said comparison. Through this arrangement, the output of the second comparator provides an indication of the characteristics of the passive bi-directional network in combination with the element communication circuitry operating in a transmit mode. The second comparator signal may be an analogue signal. Where so, each element may comprise a second analogue to digital converter (ADC) configured to digitise the second comparator signal. The system may comprise multiple local RF signal sources, for example element of the array of elements may comprise a separate local RF signal source. Each of the multiple local RF signal sources may have a separate connection to the passive bi-direction network and configured to synchronise with the master RF calibration signal transmitted through the passive bi-directional network from the master RF calibration signal source. The system may comprise multiple semiconductor chips, each chip carrying electronic circuitry implementing element communication circuitry and the local RF reference signal source for a different element of the array of elements. The or each local RF reference signal source may comprise a phase lock loop (PLL). In another aspect of the invention there is provided a semiconductor chip carrying the electronic circuitry implementing the local RF reference signal source and at least a portion of the element communication circuitry of at least one of the elements of the system of any one of the appended claims 1-10. The invention may be framed as a method and therefore according to another aspect of the invention there is provided a method of calibrating a phased array antenna; the phased array antenna comprising an array of elements, each element arranged for transmitting and / or receiving a radio frequency (RF) signal; a passive bi-directional network for combining received RF signals or splitting a transmitted RF signal from a common port to each of the elements; and a local RF calibration signal source configurable to produce a local RF calibration signal wherein each element comprises: an antenna; and element communication circuitry; the element communication circuitry comprising means for adjustment of one or more of amplitude, phase and delay of a signal received and / or transmitted by the element; the method comprising: transmitting a master RF calibration signal from the common port to each element through the passive bi-directional network; synchronising the local calibration signal to the master RF calibration signal received through the passive bi-direction network; and using the synchronised local RF calibration signal to calibrate the phased array antenna. The invention will now be described by way of example with reference to the following figures in which: Figure 1 is a schematic of a phase array antenna system comprising an electronic active array antenna; Figure 2 is an enlarged schematic of a single element of the antenna array; Figure 3 is a schematic of the system showing, enlarged, a single element of the antenna array in a first stage of the calibration to synchronise the local calibration signal sources; Figure 4 is a schematic akin to Fig 3 illustrating operation during calibration in a receive configuration; Figure 5 is a schematic akin to Fig 3 illustrating operation during calibration in a transmit configuration; and Figure 6 is a flow diagram of the calibration process. Fig 1 illustrates a system 1, e.g. radar system, comprising an active array antenna 2, a receiver 3, transmitter 4, system controller 5, a calibration store 6, a master calibration signal source 7 and a system side comparator 8. The active array antenna 2 comprises an array of elements 20. Each element 20 of the array is configured for transmitting and / or receiving a radio frequency (RF) signal. The array may comprise tens or hundreds of elements arranged in a one or two dimensional array. The active array antenna 2 also comprises a passive reciprocal manifold network 21, hereafter referred simply as the manifold 21. Each element 20 of the active array antenna 2 is connected to the receiver 3 and transmitter 4 via the manifold 21. The manifold 21 is arranged to carry RF signals from each of elements 20 to a common port 21 A, and divide any RF signal entering the network 21 from the common port 21A such that it is received by each element 20. With reference to Fig 2, which shows an enlarged view of one element 20, each element 20 includes a primary port 201 through which the element 20 is separately connected to the manifold 21, an element antenna 202, and signal modifier circuitry 203. The signal modifier circuitry 203 comprises a first port 203A through which it is connected directly to the primary port 201, and a second port 203B through which the signal modifier circuitry 203 is connected directly to the element antenna 202. The signal modifier circuitry 203 of each element 20 is controlled independently from the others by external control signals 500 from the system controller 5 to control one or more of amplitude, phase and delay of a RF signal travelling to or from the element’s 20 element antenna 202. In this example, each signal modifier circuitry 203 includes a high power amplifier 203 C for amplifying RF signals to be transmitted by the antenna by the antenna element 202, and a low noise amplifier 203D for amplifying RF signals received through the antenna element 202. By providing external control signals 500 independently to each element 20, the system controller 5 is able to achieve specific system functions, e.g. to form a specific beam shape in a specific direction. Each element 20 further includes: an analogue phase lock loop (PLL) circuit 204, providing the function of a synchronisable local RF calibration signal source; a first switch 205; a first comparator 206 having an output 206C connected to the input of a first analogue to digital converter (ADC) 207; and a second comparator 208 having an output 208C connected to an input of a second ADC 209. The PLL circuit 204 has an input 204A and an output 204B. The input 204A is coupled to the primary port 201 to receive RF signals directly from the manifold 21, namely without first passing through the signal modifier circuitry 203. The first switch 205, operative in response to control signals 500 from controller 5, allows the input 204A to be selectively connected or isolated from the manifold 21. The output 204B of the PLL 204 is connected to each of first inputs 206A 208A of the respective first and second comparators 206, 207. Additionally, the output 204B of the PLL circuit 204 is selectively coupled, through a second switch 210, to the signal modifier circuitry 203 via the second port 203B. Though this connection a local RF calibration signal outputted from the PLL 204 is receivable at the second port 203B of the signal modifier circuitry 203 to mimic an RF signal received by the antenna element 202. The first comparator 206 and second comparator 208 each have a second input 206B, 208B for receiving a RF signal. The second input 206B of the first comparator 206 is configured to receive RF signals traveling out of port 203A towards the manifold 21. The second input 208B of the second comparator 208 is configured to receive RF signals that have passed out of port 203B towards the antenna element 202. Advantageously, the signal modifier circuitry 203 and PLL circuit 204, may be implemented on the same semiconductor chip. Further advantageously, the first and second comparators 206, 208 and first and second ADCs 207, 209 may also be implemented on same said semiconductor chip. The circuit arrangement relies on the antenna elements 202 being highly reproduceable such that there their electrical characteristics are substantially identical. This is easily achievable with conventional phase array antenna manufacturing techniques. In normal operation in a transmit mode, a RF signal from the transmitter 4 received at common port 21 A, is carried by the manifold 21 to each element 20 of the antenna 2 through each element’s primary port 201. Each signal modifier circuitry 203 modifies (e.g. one or more of phase, amplitude and delay) the RF signal independently from the other elements 20 for the purposes of beam forming and / or steering based on the control signal 500 from system controller 5. The modified RF signal is amplified by the high power amplifier 203C transmitted out through second port 203B and into free space through antenna element 202. In normal operation in a receive mode, a RF signal received through each antenna element 202 is received by the signal modifier circuitry 203 through second port 203B, amplified by LNA 203A and modified, e.g. one or more of phase, amplitude and delay, for the purposes of beam forming and / or steering based on the control signals from system controller 5. The modified received RF signal is outputted through first port 203A and primary port 201 to the manifold 21. The manifold 21 carries and combines the RF signals from each element 20. The combined RF signals are outputted from the manifold 21 to the receiver 3 via the common port 21. Calibration Because manufacturing processes of electronically scanned array antenna are imperfect, it is unlikely that each element 20 within the array will be formed identically. Further there may be imperfections within the manifold 21. To correct for these errors the antenna 2 requires calibrating before use. First Stage - Synchronising Local Calibration Signals to Master Calibration Signal The calibration procedure is undertaken to ensure that each element 20 of the array of the antenna 2 is aligned on transmission and reception. In a first stage of the calibration procedure the PLL circuits 204 of each element 20 are locked to a master calibration signal. With reference to Figs 3 and 6, with the switches 205 of all elements 20 closed to put the input 204A of the PLL 204 of each element in communication with the manifold 21

[1001] , a master RF calibration signal is transmitted by the master reference signal source 7 through the manifold 21 to each element 20 simultaneously

[1002] , The master calibration signal is received at the input 204A of each PLL circuit 204. As is conventional, the PLL circuit 204 comprises a signal generator (e.g. a voltage controlled oscillator) to generate a local signal and means, typically comprising a phase comparator and feedback loop, to phase lock the local signal to an input signal, in this case the master calibration signal received at input 204A. In this way the PLL 204 is configured to provide a signal at its output 204B, hereafter referred to as the local RF calibration signal, that matches the phase, and thus also the frequency, of the master calibration signal as received at input 204A

[1003] , Once the PLLs 204 are locked to the master signal, each PLL 204 is isolated from the manifold 21 by opening switches 205

[1004] , Importantly, subsequent to isolation from the manifold 21, the PLL 204 continues to output the local calibration signal. The PLL circuits 204 are designed to ensure that the phase of the local calibration signal remains stable for the duration required to carry out of the, to be described, second and third stages of the calibration procedure; typically this requires a few microseconds. The local calibration signal of each PLL 204 will be gain and phase shifted relative to the master calibration signal as a consequence of a transfer function applied to the master calibration signal as it travels through the manifold 21. These shifts are likely to differ between elements as a result of imperfections in the manifold 21. During the first calibration stage, the switches 210 may be open and / or the modifier circuitry 203 disconnected or otherwise configured to ensure a signal is not outputted by the signal modifier circuitry 203 through first port 203A that could be received at input 204A. For the same reason, and as it conventional, the signal modifier circuitry 203 is impedance matched at the first port 203A to minimise reflection of signals received through the manifold 21. Second Stage - Calibration of Antenna in Receive Configuration With reference to Figs 4 and 6, in a second stage of the calibration procedure the antenna 2 is calibrated in a receive configuration. The switches 205 of each element 20 are open to isolate the input 204A of the PLLs 204 from the manifold 21. Each element 20 is tested one at atime. Switch 210 of the first element 20 selected for test is closed placing the output 204B of the PLL circuit 204 in communication with the second port 203B of the signal modifier circuitry 203 such that the local RF calibration signal outputted by the PLL 204 is received by the signal modification circuitry 203 through second port 203B

[2001] , The local RF calibration signal is also received at the first input 206A of the first comparator 206. The local calibration signal passes through the signal modification circuitry 203 where it is amplified and modified in a manner controlled by external control signal 500 from controller 5 as though it were a signal received through antenna 202. The modified local calibration signal passes out of the first port 203A, through the manifold 21 and out via the common port 21A to system side comparator 8. Here it is compared with the master calibration signal from the master reference signal source 7. The output of the comparator 8 is digitised to form a first measurement signal 700 received by the controller 5

[2002] , Measurement signal 700 provides an indication of the receive characteristics of the signal modifier circuitry 203 in receive mode and the characteristics of the manifold 21. The modified local calibration signal outputted at first port 203A is also received, via a coupler, at the second input 206B of the first comparator 206. The unmodified and modified local calibration signals received at the respective first and second inputs 206A 206B are compared by the first comparator 206 and the output digitised by first ADC 207. The output X of the first ADC 207 is received by the system controller 5 as second measurement signal 600A

[2003] , Measurement signal 600A provides the control system 5 with an indicator of the receive characteristics of the signal modifier circuitry 203 of the selected element 203 in isolation from the manifold 21. The control system 5 is configured to compare the measurement signals 700 and 600A to determine the characteristics of the manifold 21. Switch 210 is then opened to isolate second port 203B from the output of the PLL 204

[2004] , This process is repeated for each element 20 of the array in turn to provide separate first and second measurement signals 700 600A for each element 20

[2005] , Third Stage - Calibration of Antenna in Transmit Configuration With reference to Figs 5 and 6, in a third stage of the calibration procedure the antenna 2 is calibrated in a transmit configuration. With the first and second switches 205 and 210 of each element 20 open, the master calibration signal from the master reference signal source 7 is transmitted through the manifold 21 to each element 20 simultaneously

[3001] , The master calibration signal is received by the signal modification circuitry 203 of each element 20 at its respective first port 203A. It passes through the signal modification circuitry 203 where it is modified based on the control signals 500 as though it were a signal for transmission by the antenna element 202. A portion of the modified signal output at port 203B is coupled out to be received at the second port 208B of the second comparator 208. The second comparator 208 compares the unmodified local calibration signal from the PLL circuit 204 received at its first input 208A, with the modified master calibration signal received at its second input 208B. The difference is outputted at 208C which is digitised by second ADC 209. The output Y of the second ADC 209 is received by the system controller 5 as third measurement signal 600B. As both RF signals received at the second comparator’s 208 first and second inputs 108A 208B include a modification function of the manifold 21, the third measurement signal 600B from each element 20 provides the control system 5 with an indicator of the transmit characteristics of the respective element’s signal modifier circuitry 203 in transmit configuration. The system controller 5 is configured to use the first, second and third measurement signals 700 600A 600B received from each element 20 to derive individual transmit configuration error values and receive configuration error values for each element 20. These values are stored in store 6 for use by the system controller 5 to adjust the control signals 500 sent to the signal modifier circuitry 203 of each element 20 during normal transmit and receive operations, to adjust one or more of the amplitude, phase and delay of signals being transmitted or received by the elements 20 to correct for differences in the electrical characteristics between the elements 20 and their respective transmission paths through the manifold 21. Following completion of the above calibration procedures the array antenna 2 is calibrated for use on both transmit and receive. It will be appreciated that the second and third calibration stages could be carried out in reverse order. Although less efficient and so less preferred, each element 20 could be calibrated in receive and transmit configuration in turn. In variant embodiments of the invention the active array antenna need not comprise both a transmitter and receiver. For example, where the active array antenna 1 is used for receive only each element communication circuit 6 need only comprise a receiver. Similarly where the active array antenna 1 is used for transmit only, each sub-array communication circuitry 8 and each element communication circuit 6 need only comprise a transmitter. It will be appreciated that antenna 2 may be a sub-array of a larger antenna. In the described example, each element comprises its own PLL 204. This is thought to be preferred. Nevertheless, in a variant multiple of the elements 20, forming a subset of elements of the array that share a common path through the manifold, may share a local calibration signal from a common PLL circuit 204. This may be a preferred configuration where multiple elements 20 are formed on a single semiconductor chip. A PLL circuit 204 is a preferred implementation of the local calibration signal source as it is simple and reliable. Nevertheless, the function of the local calibration signal source could be carried out by more complex analogue and / or digital circuity such as including digital signal processing.

Claims

1. A phased array antenna system comprising an array of multiple elements, each element arranged for transmitting and / or receiving a radio frequency (RF) signal; each element comprising an antenna and element communication circuitry, the element communication circuitry comprising means for adjustment of one or more of amplitude, phase and delay of a RF signal received and / or transmitted by the element;the phased array antenna further comprising a passive bi-directional network for combining received RF signals or splitting a transmitted RF signal from a common port to each of the elements;characterised in that phased array antenna system comprises:a master RF calibration signal source configured to provide a master RF calibration signal;a local RF calibration signal source having an input connected to the passive bidirectional network; the local RF calibration signal source configurable to synchronise a local RF calibration signal with the master RF calibration signal transmitted through the passive bi-directional network; anda comparator means configured to compare, during a calibration procedure, the synchronised local RF calibration signal with the master RF calibration signal following transmission of one of them through the element communication circuitry.

2. A phased array antenna system according to claim 1 wherein each element communication circuitry comprises a first port connected to the passive bi-directional network, and a second port connected to the antenna; and wherein the local RF signal source is configured to transmit the synchronised local RF calibration signal to the element communication circuitry via the second port; the comparator means comprising a system side comparator configured to compare the master RF calibrationsignal with the local RF calibration signal following transmission of the local RF calibration signal through the element communication circuitry and passive bidirectional network.

3. A phased array antenna system according to claim 1 or 2 wherein each element comprises a first comparator having a first input connected to the local signal source and a second input connected between element communication circuitry and the passive bi-directional network; the first comparator configured to compare the synchronised local RF calibration signal received through a first input from the local signal source with a modified synchronised local RF calibration signal through the second input being the local RF calibration signal as modified by the element communication circuitry, the first comparator configured to output a first comparator signal indicative of said comparison.

4. A phased array antenna system according to claim 3 wherein the first comparator signal is an analogue signal, and in which each element comprises a first analogue to digital converter (ADC) configured to digitise the first comparator signal.

5. A phased array antenna system according to any previous claim wherein each element comprises a second comparator having a first input connected to the local signal source and a second input connected between element communication circuitry and the antenna; the second comparator configured to compare the synchronised local RF calibration signal received through a first input from the local signal source with the master RF calibration signal through the second input following transmission through the element communication circuitry; the second comparator configured to output a second comparator signal indicative of said comparison.

6. A phased array antenna system according to claim 6 wherein the second comparator signal is an analogue signal and each element comprises a second analogue to digital converter (ADC) configured to digitise the second comparator signal.

7. A phased array antenna system according to any previous claim comprising multiple local RF signal sources each having an input connected to the passive bidirection network to receive the master calibration signal; each of the multiple local RF calibration signal source configured to synchronise with the master RF calibration signal transmitted through the passive bi-directional network from the master RF calibration signal source.

8. A phased array antenna system according to claim 7 wherein each element of the array of elements comprises a separate local RF signal source.

9. A phased array antenna system according to claim 7 or 8 comprising multiple semiconductor chips, each chip carrying electronic circuitry implementing element communication circuitry and the local RF calibration signal source for a different element of the array of elements.

10. A phase array antenna system according to any previous claim wherein the or each local RF calibration signal source comprises a phase lock loop circuit (PLL).

11. A semiconductor chip carrying electronic circuitry implementing the local RF calibration signal source and, at least a portion of, the element communication circuitry of at least one of the elements of the system of any claim 1-10.

12. A method of calibrating a phased array antenna;the phased array antenna comprising an array of elements, each element arranged for transmitting and / or receiving a radio frequency (RF) signal;a passive bi-directional network for combining received RF signals or splitting a transmitted RF signal from a common port to each of the elements;and a local RF calibration signal source configurable to produce a local RF calibration signalwherein each element comprises:an antenna; andelement communication circuitry; the element communication circuitry comprising means for adjustment of one or more of amplitude, phase and delay of a signal received and / or transmitted by the element;the method comprising:transmitting a master RF calibration signal from the common port to each element through the passive bi-directional network;synchronising the local calibration signal to the master RF calibration signal received through the passive bi-direction network;using the synchronised local RF calibration signal to calibrate the phased array antenna.

13. A method according to claim 12 wherein calibrating the phased array antenna comprises comparing, during a calibration procedure, the synchronised local RF calibration signal with the master RF calibration signal following transmission of one of them through the element communication circuitry of an element.

14. A method according to claim 13 comprising using measurements from the comparison made in claim 13 to calculate corrections to be applied to the element communication module of said element.

15. A method according to claim 13 wherein calibrating the element receiving circuitry further comprises comparing, with a comparator, the stored RF calibrationsignal transmitted through the element communication circuitry with the RF calibration signal from the RF signal store.5 16. A method according to claim 13 or 14 wherein the RF signal is further transmittedfrom the element communication circuitry through the passive bi-directional network to the common port.

17. A method according to any claim 12-17 wherein synchronising the local 10 calibration signal of each local RF calibration signal source to the master RF calibration signal received through the passive bi-direction network is carried out simultaneously.

18. A method according to any claim 12-17 comprising:15comparing a second calibration signal that has been transmitted from the common port through the element communication circuitry with the local RF calibration signal.A

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