Feedback methods for inductorless bandwidth extension and linearisation of post-amplifiers in optical receiver frontends

Optical communication is increasingly important in today's telecommunications. It is not only a key component in long-haul infrastructure, but is also being brought into new applications within the datacentre, at the circuit board and integrated circuit level, and in next generation mobile networks. This thesis proposes feedback tuning approaches in order to address two challenges within optical receiver analog frontend circuits: a) the dynamic response of a prior bandwidth extension technique; and b) linearity optimisation. To address dynamic response, we begin with an inductorless method of bandwidth extension using positive feedback loops. In a multi-stage post-amplifier with local positive feedback loops, we propose an approach which tunes each positive feedback gain separately, and demonstrate that this achieves better dynamic response and eye opening than the prior equal-feedback-gain approach. We additionally propose root-locus analysis as a means of characterising dynamic response, and suggest some design guidelines based on this analysis. To address linearity optimisation, we propose the use of an interleaving negative-feedback post-amplifier topology, previously proposed only for bandwidth extension. We investigate the relationship between the feedback gains and linearity and develop a design approach for linearity optimisation. We then designed and fabricated two 70 dB 6 GHz optical receiver circuits, making use of two different post-amplifiers, in order to compare different design approaches. We achieved a linearity of 0.08 dBVrms OIP3 (quasi-static) and a THD of 0.195\% at 1 GHz

THE USE OF TUNED FRONT END OPTICAL RECEIVER AND PULSE POSITION MODULATION

The aim of this work is to investigate the use of tuned front-ends with OOK and PPM schemes, in addition to establish a theory for baseband tuned front end receivers. In this thesis, a background of baseband receivers, tuned receivers, and modulation schemes used in baseband optical communication is presented. Also, the noise theory of baseband receivers is reviewed which establishes a grounding for developing the theory relating to optical baseband tuned receivers. This work presents novel analytical expressions for tuned transimpedance, tuned components, noise integrals and equivalent input and output noise densities of two tuned front-end receivers employing bi-polar junction transistors and field effect transistors as the input. It also presents novel expressions for optimising the collector current for tuned receivers. The noise modelling developed in this work overcomes some limitations of the conventional noise modelling and allows tuned receivers to be optimised and analysed. This work also provides an in-depth investigation of optical baseband tuned receivers for on-off keying (OOK), Pulse position modulation (PPM), and Di-code pulse position modulation (Di-code PPM). This investigation aims to give quantitative predictions of the receiver performance for various types of receivers with different photodetectors (PIN photodetector and avalanche photodetector), different input transistors (bi-polar junction transistor BJT and field effect transistor FET), different pre-detection filters (1st order low pass filter and 3rd order Butterworth filter), different detection methods, and different tuned configurations (inductive shunt feedback front end tuned A and serial tuned front end tuned B). This investigation considers various optical links such as line of sight (LOS) optical link, non-line of sight (NLOS) link and optical fibre link. All simulations, modelling, and calculations (including: channel modelling, receiver modelling, noise modelling, pulse shape and inter-symbol interference simulations, and error probability and receiver calculations) are performed by using a computer program (PTC Mathcad prime 4, version: M010/2017) which is used to evaluate and analyse the performance of these optical links. As an outcome of this investigation, noise power in tuned receivers is significantly reduced for all examined configurations and under different conditions compared to non-tuned receivers. The overall receiver performance is improved by over 3dB in some cases. This investigation provides an overview and demonstration of cases where tuned receiver can be optimised for baseband transmission, offering a much better performance compared to non-tuned receivers. The performance improvement that tuned receivers offers can benefit a wide range of optical applications. This investigation also addresses some recommendations and suggestions for further work in some emerging applications such as underwater optical wireless communication (UOWC), visible light communication (VLC), and implantable medical devices (IMD). Keyword: Optical communications, Baseband receivers, Noise modelling, tuned front end, pulse position modulation (PPM)