Array of coherent sources system based on novel dual-loop PLL infrastructure and its applications

Abstract

This thesis presents a novel phased array technology known as coherent sources based on dual-loop phase locked loop (PLL) infrastructure. This technology provides phase controllable signals based on multi-PLL synchronization through a low cost pure analog continuous phase shifting method, which has never been proposed before. Although this phase shifting method is based on the Indirect Controlled Phased Source (ICPS), an essential and intelligent development is proposed to achieve continuous and accurate phase control for phased array realization as well as various applications. It not only remains combining the oscillator and the phase shifter to form a series of oscillating source with controllable phase in a single element as ICPS does, but also provides a real-time continuous phase calibration for multiple elements with synchronized signals. Since the output phase of the voltage controlled oscillator (VCO) is forced to follow the phase of the reference oscillator in a PLL structure due to its synchronization and tracking characteristic, multiple output signals can share the same reference signal to be synchronized and phase controlled with a real-time feedback network for the high accuracy phase calibration among them. With this feature, all the phases of multiple synchronized signals at high frequency can be accurately controlled by different low frequency phase shifters in continuous way separately but synchronized. This proposed method has advantages of no insertion loss and lower cost for the phase shifting. In addition, due to the micro controller unit (MCU) controlled dual-loop infrastructure, phase error caused by variation of process, supply voltage and temperature (PVT) as well as component aging and temperature drift issue can be real-time calibrated, which makes the proposed technology suitable for phased array realization. Moreover, modular structure of each element makes this method a good candidate for flexible plug-and-play active antenna array design. First, the proposed array structure of coherent sources based on novel dual-loop PLL infrastructure is described on system level. The key innovation is the dual-loop structure. The first loop is a conventional PLL synthesizer, which consists of a reference divider, a phase detector with charge pump, a loop filter, a VCO and a main divider. It realizes both frequency and phase locking for a single element. The second loop provides an accurate phase calibration to the array of coherent sources with the help of external phase detector array and MCU. Second, the proposed array circuits of coherent sources based on novel dual-loop PLL infrastructure is analyzed on component level. It contains several individual elements, and each of them consists of three main parts: modified ICPS, 360° high precision phase detector and MCU. The modified ICPS is made up of several parts, which basically include reference oscillator, low frequency phase shifter, PLL chip with phase detector, frequency dividers and charge pump included, VCO and variable gain amplifier (VGA). The novel S-band phase detector with a high precision is then proposed and demonstrated. It consists of two analog phase detectors as well as digital circuits to improve a single analog phase detector's performance by eliminating the large error detection regions and merging four linear detection regions. For a demonstration with commercially available components, it provides a full 360° phase difference detection range with a maximum detection error less than 4° for frequency at 2.4 GHz. All these several parts are designed and measured individually first, and then be integrated to generate accurately phase controllable synchronized signals with the maximum amplitude variation less than 1% and the phase variation within ±5°. In addition, the stability of the integrated phased source circuits has been tested with a measured maximum phase variation within ±3° for 2 hours' operation time achieved. Third, for the integrality of the whole phased antenna array system, a novel printed vertically oriented unidirectional antenna is proposed. The proposed antenna is composed of a bowtie-shaped electric dipole, a half-circular loop antenna works as a magnetic dipole, and a microstrip-to-stripline transition balun. All of them are printed in the same plane perpendicular to the ground. The antenna operates at 2.6 GHz with an impedance bandwidth of 17.1% for SWR ≤ 2, stable unidirectional radiation pattern with low cross polarization, low back radiation, nearly identical E- and H- plane patterns and stable gain of around 6.5 dBi over the operating frequencies. Then with a single vertical planar structure as well as large beamwidth, the proposed antennas are employed in an array design. From the simulation of the array in both E- and H- plane beamforming realization, it finds that the H-plane eamforming array with the proposed antenna elements achieves better performance in terms of directivity and maximum allowable steering angle. Finally, we demonstrate the proposed system in two different applications. One is a four-element transmitter prototype based on the proposed array of coherent sources. It realizes a good beam-steering resolution as 2° for a total steering range of 34° and also achieves a good agreement among measured, simulated and theoretical beam tilt performance. It then demonstrates its capability of element failure compensation by applying phasor rotation method. Practical LTE modulation EVM test is executed with results all fulfill 3GPP's requirements to prove the proposed technique has a potential to be employed in practical applications. The second one is a high-directivity phased array using combination of beam-steering technique and novel null tracking technique for secure signal transmission. The proposed phased array provides super high directivity and constant data beamwidth for a wide scanning angle without alternating transmitting power. It achieves that by generating jamming signal with a null towards the direction of data signal's beam as well as a stable twin-beam tracking with the null. A 3X4 antenna array prototype is designed and fabricated. It demonstrates high signal-to-noise ratio (S/N) directivity of over 25 dB, as well as a constant data beamwidth (S/N >12 dB) of 15° and a large jamming window (S/N <0 dB) to interfere the spy receivers for a wide scanning angle between ±27° for GSM1800 application. Furthermore, it also proves that our proposed coherent sources method is capable to achieve a wide operation bandwidth.

Date of Award	2 Oct 2013
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Quan XUE (Supervisor)