H. Nguyen, J. Whittington, J. C Devlin, V. Vu and, E. Custovic.

An Accurate phase calibration Technique for digital beamforming

in the multi-transceiver TIGER-3 HF radar system

PACal workshop Thursday 27th September 2012

Department of Electronic Engineering, La Trobe University, Australia

• A new generation digital SuperDARNradar

• 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

TIGER-3

• 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

TIGER-3 Features

* 2nd & 3rd arrays normally used in receive mode, but can transmit for calibration or other special functions

Frequency Band: 8 - 18MHz

Antenna Arrays: Tx/Rx Array: 16 Horizontally polarised TTFD

2nd Array: 3 Horizontally polarised TTFD*

3rd Array: 1 Horizontally polarised TTFD*

Field of View: >90° Azimuth

Beam Widths: Horizontal: 4° at 10MHz, 3° at 14MHz, 2° at 18MHz

Vertical: 50°

Lobe Levels: < - 12dB for both back and side lobes @ 12MHz

Transmitters: Main Array 16 x 2.4kW (one transceiver per antenna)

Total Peak Power: 38.4kW

Mean Power: 1kW (in stereo mode)

Effective Radiated Power: ~10kW in main beam direction @ 12MHz

Tx Signals: 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

Receiver Sensitivity 60nV (20 receivers)

Instrument Range: > 5000Km

TIGER-3 Specifications

TIGER-3 System Overview

TIGER-3 Overview

• Significantly improved hardware & antenna performance provides a much larger FoV

• Covers ~20 million km2, more than 3 times Bruny & Unwin

TIGER-3 Field of View

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.

δT1 δT2 δT3δT16

From transceivers

Field of

view

Boresight Beam

direction

Antenna

array

Common Timing and Phasing Control

• AIM: Radiated power electronically steered toward a desired direction, providing:

- Rapid beam scanning.

- High accuracy.• The beam pattern and beam

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.- Fine phase resolution.

Digital Beamforming

1 2 3 N

Φ

d.cos(Φ)

d

Array

boresight

d

16-element uniform array, half wavelength spacing

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

- Referenced signals:

Phase Measurement Concept

- Unknown phase signal:

- Mixed signals:

- Using lowpass filters to eliminate high frequency components:

Phase measurement implementation on Virtex-5 FPGA using System Generator

Phase Measurement Implementation

• 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 phasemeasurment accuracy.

–Once quantised up to 14 fractionalbits, stopband Att. < 80 dB

– 16 fractional bits with 36 taps is atrade-off between filter performanceand resources.

Lowpass filter design

• Xilinx CORDIC IP core performs inversetangent by sequentially rotating inputvector in micro-rotation steps:

• Quantisation error:

– 16-bit quantisation provides ±6 10-4

degrees precision.

CORDIC based Arctan design

Design verification

Phase error:

– Floating-point model: 1.89 10-3

– Fixed-point model: 3.53 10-3

• Sequentially measures the phasedifference between the RF signalgenerated by each Tx/Rx and areference signal at the samefrequency:

• Each transceiver, in turn, adjustsits DDS phase offset in order toalign to the reference phase.

• Phase calibration algorithm forTIGER-3 system with 20 Tx/Rx (atright):

Digital Phase Measurement & Cal

Simulink simulation

• Two DDSs mimic the role of

dummy RF signals fed back

from two Tx/Rx.

– DDS1 initial phase: 54o

– DDS2 initial phase: 234o

• Activated by a PS_En enable

pulse, the phases of the two

DDSs are adjusted and

aligned after 10 clock cycles.

TIGER-3 Hardware Implementation

Hardware Results

These results show two Tx signals at bore-

site, firstly un-calibrated and then calibrated:

• Two transceivers operated at 10.5 MHz

• Phase offset:

– Originally: 1.2625o

– After calibration: 0.0405o

Hardware Results

• Applying this technique to multiple transceivers

• Four transceivers at bore-site, un-calibrated:

• And after calibration:

• 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.

Conclusion