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This thesis focuses on reducing the latency and energy consumption associated with the start-up of crystal oscillators in (duty-cycled) low-power wireless systems. A comparison of the state-of-the-art in crystal oscillator start-up shows that energy injection (EI) systems start up faster than negative resistance boosted (NRB) circuits, but do not improve in terms of energy consumption. The main challenge in Energy Injection is the generation of a precise injection signal, which is required to ensure that the injection signal is in-phase with the crystal resonance over the entire injection duration. This thesis proposes a self-timed injection technique that derives the injection waveform timing from the crystal oscillation itself, eliminating the need for an auxiliary injection oscillator. A prototype demonstrates a start-up time of only 6 µs and a start-up energy of 3.7 nJ for a 50 MHz crystal. This research also addresses one of the main energy consumption issues in energy injection circuits by introducing step-wise charging, which reduces energy consumption by 2-4 times. The start-up time and energy are further reduced by combining this with further refinements to the self-timed injection technique. A second prototype starts up in only 2.8 µs, consuming only 1.9 nJ in the process when using the same crystal and using identical measurement conditions as for the first prototype. This work also gives an overview of the design space and limitations in energy injection circuits, aiding in tailoring them to the application and giving insight toward a fair comparison of different techniques. The comparison of various circuits and methods highlights the importance of equal or similar measurement conditions for a fair evaluation. Overall, this thesis aims to achieve fast start-up times, and minimize energy consumption in crystal oscillators. It introduces self-timed injection, as well the application of step-wise charging to crystal oscillators. Compared to other energy injection methods, the proposed techniques offer competitive start-up times while significantly reducing energy consumption.

This thesis is focused on reducing the electromagnetic interference (EMI) due to the residual power after the output filter at the pulse-width modulation (PWM) frequency, the so called ripple. To reduce the ripple current through the speaker leads, it is possible to use a higher order filter, use a multi-phase or multi-level output stage or to inject a cancellation signal after the output filter. First the disturbance is sensed and processed and afterwards a cancellation signal is synthesized and injected into the circuit. A development board utilizing the AX5689 digital amplifier controller IC was used as a design vehicle to realize a prototype bridge-tied-load (BTL) Class-D amplifier with ripple reduction. The built-in analog-to-digital converters (ADCs) were used to measure the output signals of both bridge halves. A selective digital band-pass loop filter was designed in the digital-signal-processing (DSP) core of the AX5689 to only provide high loop-gain around the PWM frequency without affecting the audio band. A ripple reduction of 28 dB for the common-mode (CM) and 18 dB for the differential-mode (DM) was achieved across all signal frequencies and output powers up to 10 W. The Class-A drivers used to inject the anti-ripple have a negative impact on the total system efficiency, which drops from 84% without ripple reduction to 79% with ripple reduction enabled. The technique has virtually no impact on THD+N performance and could be used as an add-on to any Class-D amplifier with a fixed (PWM) frequency. In summary, this thesis shows a feedback technique to reduce the ripple current after the output filter. Thereby the amount of EMI that can be radiated off the speaker leads decreases. The effect of unit delays in the feedback loop has been mitigated by applying a special filter design method to obtain a more stable system. A prototype has been made around an existing amplifier to show the effectiveness of the proposed feedback ripple reduction solution.