An Accurate phase calibration Technique for digital beamforming in the multi-transceiver TIGER-3 HF radar system

Transcription

1 An Accurate phase calibration Technique for digital beamforming in the multi-transceiver TIGER-3 HF radar system H. Nguyen, J. Whittington, J. C Devlin, V. Vu and, E. Custovic. Department of Electronic Engineering, La Trobe University, Australia PACal workshop Thursday 27 th September 2012

2 TIGER-3 A new generation digital SuperDARN radar Completely redesigned from the ground up Operates in a similar manner to existing radars, but capable of much more RF sampling with virtually all transmitter and receiver functions performed in the digital domain (in hardware) Through minimisation of analog circuitry and implementation of digital phase calibration and control virtually eliminate phase variation (particularly in receivers) Use of Field Programmable Gate Array (FPGA) enables possibility of reconfigurable hardware

3 TIGER-3 Features One Transceiver per antenna Four times the transmit power: up to 2 kw Fewer losses in output filter and phasing matrix & switches (~12dB improvement) Greater receiver sensitivity: 60nV (c.f. ~100nV) improvement (~3dB) through DSP averaging & downsampling for pulse radar Greater range at least 5000km (110 range gates) Twin-Terminated Folded Dipole (TTFD) antennas Modified antenna layout, improves AoA determination Greater azimuth FoV, up to ± 45 Flexible beam placement - can be set at any position in FoV Capable of extended operational modes

4 TIGER-3 Specifications Frequency Band: Antenna Arrays: Field of View: Beam Widths: Lobe Levels: Transmitters: Total Peak Power: Mean Power: 8-18MHz Tx/Rx Array: 16 Horizontally polarised TTFD 2nd Array: 3 Horizontally polarised TTFD* 3rd Array: 1 Horizontally polarised TTFD* >90 Azimuth Horizontal: 4 at 10MHz, 3 at 14MHz, 2 at 18MHz Vertical: 50 < - 12dB for both back and side 12MHz Main Array 16 x 2.4kW (one transceiver per antenna) 38.4kW 1kW (in stereo mode) Effective Radiated Power: ~10kW in main beam 12MHz Tx Signals: Receiver Sensitivity Instrument Range: Pulse pattern duration: ~ 100ms Pulse width: 300us Bandwidth: 10kHz at -20dB Duty Cycle: 1.33% carrier frequency, per channel Second independent channel available in stereo mode 60nV (20 receivers) > 5000Km * 2 nd & 3 rd arrays normally used in receive mode, but can transmit for calibration or other special functions

5 TIGER-3 System Overview

6 TIGER-3 Overview

7 TIGER-3 Field of View Significantly improved hardware & antenna performance provides a much larger FoV Covers ~20 million km 2, more than 3 times Bruny & Unwin

8 Common Timing and Phasing Control Why? Each transceiver generates its own RF signals, Tx pulses & Rx sampling Thus, accurate synchronisation and coordination of all 20 transceivers is vital All transceivers must generate exactly the same phase referenced frequency Phase delays vary with frequency, voltage, temperature, Correct beamforming requires accurate phase delays between antennas For the same beam a different phase delay is required for each frequency RF output signals must be accurately aligned, so that digital beamforming can be performed with further additional of appropriate phase offsets. Boresight Field of view δt 1 δt 2 δt 3 δt 16 From transceivers Beam direction Antenna array

9 Digital Beamforming AIM: Radiated power electronically steered toward a desired direction, providing: - Rapid beam scanning. Array boresight - High accuracy. d.cos(φ) The beam pattern and beam Φ steering capability determined by: - The number of geometrical arrangements. - Relative amplitudes. - Relative phases. Progressive phases generated using DDSs in FPGAs: - Programmable frequency. - Fine frequency resolution. 1 d 2 d 3 N - Fine phase resolution. 16-element uniform array, half wavelength spacing

10 Phase Calibration Requirement Digital signal generation (DDS) identical in each transceiver However, necessary RF analogue circuitry introduces differing phase delays e.g. below are RF outputs from four transmitters at the bore-site Thus, differing (analogue circuit) phase delays will impact beam-forming accuracy To correct (calibrate) we must first be able to measure phase differences

11 Phase Measurement Concept - Referenced signals: - Unknown phase signal: - Mixed signals: - Using lowpass filters to eliminate high frequency components:

12 Phase Measurement Implementation Phase measurement implementation on Virtex-5 FPGA using System Generator

13 Lowpass filter design FIR filter designed with FDAtool: Passband Freq: 1 MHz Stopband Freq: 15 MHz Passband Att: 84.6 db Stopband Att: 0.6 db Quantisation error: Fixed-point arithmetic limits phase measurment accuracy. Once quantised up to 14 fractional bits, stopband Att. < 80 db 16 fractional bits with 36 taps is a trade-off between filter performance and resources.

14 CORDIC based Arctan design Xilinx CORDIC IP core performs inverse tangent by sequentially rotating input vector in micro-rotation steps: Quantisation error: 16-bit quantisation provides ± degrees precision.

15 Design verification Phase error: Floating-point model: Fixed-point model:

16 Digital Phase Measurement & Cal Sequentially measures the phase difference between the RF signal generated by each Tx/Rx and a reference signal at the same frequency: Each transceiver, in turn, adjusts its DDS phase offset in order to align to the reference phase. Phase calibration algorithm for TIGER-3 system with 20 Tx/Rx (at right):

17 Simulink simulation Two DDSs mimic the role of dummy RF signals fed back from two Tx/Rx. DDS1 initial phase: 54 o DDS2 initial phase: 234 o Activated by a PS_En enable pulse, the phases of the two DDSs are adjusted and aligned after 10 clock cycles.

18 TIGER-3 Hardware Implementation

19 Hardware Results These results show two Tx signals at boresite, firstly un-calibrated and then calibrated: Two transceivers operated at 10.5 MHz Phase offset: Originally: o After calibration: o

20 Hardware Results Applying this technique to multiple transceivers Four transceivers at bore-site, un-calibrated: And after calibration:

21 Conclusion The multi transceiver HF TIGER-3 radar uses identical DDS circuits to create accurate phase offsets for beamforming. Necessary RF analogue circuitry introduces phase errors A hardware technique for rapid measurement and accurate measurement of phase differences has been developed as part of the TIGER-3 system This technique enables on-the-fly phase calibration, and correction of variable phase delays introduced by the RF analogue circuitry. Thus improving beamforming accuracy.