A FAST AND ACCURATE SCHEME FOR CALIBRATION OF ACTIVE PHASED-ARRAY ANTENNAS G.A. Hampson, A.B. Smolders Netherlands Foundation for Research in Astronomy (NFRA) P.O. Box 2, 7990 AA Dwingeloo, The Netherlands Web:www.nfra.nl,
[email protected],
[email protected]
Submitted to the IEEE APS Symposium, Orlando USA, 1999 1. Introduction The international radio-astronomy community is currently making detailed plans for the development of a new radio telescope: the Square Kilometre Array (SKA) [1]. This instrument will be a hundred times more sensitive than telescopes currently in use. One approach for this new telescope is to use a phased array consisting of more than 106 receiving elements with a mixed RF/digital adaptive beamformer. At this moment, a demonstrator receive-only active phased-array system is being tested at NFRA, the One Square Metre Array (OSMA). This is a scale model with 144 receiving elements with a mixed RF/Digital beamforming architecture. One of the main issues in getting a high-performance phased-array system with accurate beam control and low side-lobes is an accurate and fast on-line calibration facility. The proposed “Multi-Element Phase-toggle” (MEP) method is an extension of the phase-toggle method used in [2] and allows groups of elements to be calibrated simultaneously by using FFT signalprocessing techniques. Measured results from the OSMA system performance and of the remaining errors after MEP calibration will be presented. 2. OSMA System Layout OSMA is a phased-array receive-only antenna with a mixed RF and digital adaptive beamforming architecture operating in the frequency range of 1.75 GHz to 3.5 GHz. The linearly polarised antenna consists of an 8 by 8 element active centre region surrounded by two rows of passive elements (total of 80 elements), as shown in figure 1(a). The array is built up of broadband bow-tie antenna elements with an integrated balun [3] and the distance between adjacent array elements is 75mm in both directions. The array is backed by a ground plane which is rounded at the array edges to reduce diffraction effects. In addition, the array has four calibration elements that will be used for on-line calibration of the active elements. The system is tested in the antenna measurement facility at NFRA.
Figure 1:(a) OSMA front view (passive elements not installed.) (b) Beamforming architecture. The top-level beamforming architecture of OSMA is illustrated in figure 1(b). The active elements are connected to 16 RF beamformer units (RFBF I), whereas the passive elements are terminated with matched loads. The outputs of the RFBF I units can be connected to both a 16-
channel adaptive digital beamforming (ADBF) unit or to a second stage 16-channel RF beamformer unit (RFBF II). The RFBF I and II units are identical, however the LNA is no longer necessary in the RFBF II units. OSMA will be used in two different modes; a RF beamforming mode, or a mixed RF/digital adaptive beamforming mode. The two modes of operation can occur simultaneously, either generating two RF beams or alternatively one RF and two digital beams. More details about the adaptive digital processing can be found in [4]. The functional layout of the RFBF units is illustrated in figure 2 where the active components are denoted by LNA (Low Noise Amplifier), TDU (Time Delay Unit) and VAT (Variable ATtenuator). The TDU units can be either used as real time-delays which results in a maximum scan angle up to 40 degrees, or they can be used as phase-shifters. In the latter case, the maximum scan angle is unlimited, at the expense of a maximum instantaneous RF bandwidth of 200 MHz. A digital control circuit remotely controls the settings of the VAT and TDU units.
Figure 2. Functional layout of RF Beamformer I (RFBF I) 3. Calibration Procedure The purpose of the calibration procedure is to correct for all amplitude and phase errors that occur in the antenna elements and the RFBF I and II units. A schematic diagram of the calibration principle of OSMA is shown in figure 3. Two type of signals can be injected into each array element; an external signal generated by a far-field source, or a signal injected through mutual coupling with the calibration elements.
Figure 3: RF calibration scheme (where only element k is shown). The first step in the calibration procedure is to relate the received signals (at the output of each active element) from a test signal of one of the calibration elements to the received signals due to one or more incident plane waves from the far field source. This part of the calibration procedure is called Off-line calibration, since it needs to be done only once in a high-quality anechoic room. The second step is to measure the amplitude and phase at the output of each element due to a signal from the calibration elements. This part of the calibration procedure is called On-line calibration, since it can be done while the system is operational in an out-door environment. By combining the results from both calibration steps, the measured gain and amplitude of each element can be related to an incident plane wave.
4. Multi-Element Phase Toggle The Multi-Element Phase-toggle (MEP) method is proposed here in order to calibrate groups of elements simultaneously. Additionally, errors are reduced significantly when compared to conventional calibration measurements. It can be used for both steps of the calibration procedure. The method proposed by Lee et al. [2] is extended for multiple elements. The MEP technique exploits the Fourier properties of the beamformer. The step frequency of each TDU is an arbitrary function of the element index, k. However to make use of all phase settings of the TDU, fk must be odd for all k, if the number of TDU settings is even, e.g. fk=4k+1. In the OSMA system, the RFBF units have 16 phase-states with a range of 3600 at 2 GHz. To ensure each TDU unit cycles through all possible settings, there must be atleast 16 different measurements, denoted by n. It is possible to calibrate up to eight elements simultaneously at this frequency since eight odd numbers exist in the range of 0 to 15. The following example will calibrate a 4 by 1 sub-array. Firstly, the characteristics of the RFBF unit are measured outside the system for the behaviour of the TDU and VAT settings. The nonlinearity of the VAT and TDU is corrected using the previously measured RFBF data. The maximum achievable uniform gain in the four elements is selected. Secondly, for the 16 measurements, the phase setting of each elements TDU is stepped at the step frequency fk. The received signal Sn (for n=[0..15]) from each measurement is now given by: 2 jnf k
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The complex amplitudes from the four array elements, sk, are found by applying a Fast-Fourier Transformation (FFT) to the 16 measured signals. The complex amplitude of element k is then found in bin fk of the FFT spectrum. Figure 4(a) shows an example of a measured FFT spectrum (amplitude and phase) of a single RFBF unit with four elements calibrated simultaneously. The gain peaks indicate the step frequency values of 1, 5, 9 and 13. The MEP method has two advantages, firstly all interfering signals (without phase-toggle) are located in bin 0 of the FFT spectrum. Secondly, an average amplitude and phase offset is determined using all TDU states.
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The MEP technique was also tested using a calibrated phase shifter resulting in phase errors of less than half a degree. Figure 4(b) firstly illustrates the array (8 by 4 elements) phase offsets measured using the MEP technique. The phase has some curvature as the array is too large for the anechoic chamber and consequently the source is in near-field operation. Smaller fluctuations are due to offsets within the RFBF unit. These phase offsets can be combined into 100 80 60 Element Element Element Element
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Figure 4: (a) FFT spectrum (4 elements calibrated), (b) Errors calculated using MEP method.
the RFBF calibration data and the MEP technique can be applied again. The results in the lower graph of figure 4(b) indicate that the phase errors are within half an LSB. With 16 settings at 2 GHz, the ideal phase quantisation is approximately 22.50 (or a LSB.) 5. Measured Beam Patterns In this section the RF beamforming performance of OSMA before and after calibration will be presented for an 8 by 4 array configuration. Figure 5(a) illustrates measured patterns with and without the MEP calibration data, for the beamformer steered to broadside using an uniform 0
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Figure 5: (a) Measured patterns with and without calibration (E-plane, 2 Ghz.) (b) 2D Measured pattern at 2 Ghz, (E-Plane is along vertical axis.) taper. The patterns are measured in the E-plane along the smaller dimension of the array. The MEP calibration data improves the symmetry of the side lobes as well as compensating the offset of the main beam. The remaining degradation of the OSMA patterns after calibration are due to the limited number of phase settings in the RFBF units, the limited size of the anechoic room and variations in the element patterns. To see the effect of calibration in two dimensions, figure 5(b) shows an image of the measured pattern with calibration. 6. Conclusions The MEP technique has been shown to be a fast and accurate method of calibrating active phased arrays. The algorithm for calculating the phase and gain offsets makes use of the Fourier transform relationships, which has many advantages over other techniques. Changing system configuration, interconnections, results in phase and gain offsets which can be quickly corrected using the MEP technique. Results have been presented using the OSMA demonstrator phased array, where the measured results after calibration are within half a LSB. References [1] A. Ardenne and F.M.A. Smits: “Technical aspects for the square kilometer array interferometer” ESA Workshop on Large Antennas in Radio Astronomy, The Netherlands, 1996, pp. 117-128 [2] K.M. Lee, R.S.Chu and S.C. Liu “A built-in performance-monitoring/fault isolation and correction (PM/FIC) System for active phased-array antennas” IEEE Transactions on Antennas & Propagation, AP-41, 1993, p. 1530-1540. [3] A.B. Smolders and M.J. Arts “Wide-band antenna elements with integrated balun” Proceedings of the IEEE Antennas and Propagation Society International Symposium, Atlanta, 1998, Vol.3, p. 1394-1397. [4] M. Goris, G.A. Hampson, A. Joseph and F.M.A. Smits “An adaptive beamforming system for radio frequency interference rejection” Accepted for publication in IEE Proceedings on Radar, Sonar and Navigation.
