University of Twente Student Theses


Higher order feedback loop for a pulse width modulator

Stapelbroek, W. (2006) Higher order feedback loop for a pulse width modulator.

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Abstract:This report is about the research of a feedback loop for a Pulse Width Modulator. Different loop filters are investigated. The optimal type of loop filter depends on the types of noise in the system. In general a higher filter order of the loop-filter leads to a better noise suppression, but there are limits. Out of this study it emerged that a 3rd order Butterworth type filter with at 700 kHz carrier signal inserted in the last integrator is a good option for implementation. A SINAD of 112 dB in the audio band is possible in case of 50% modulation depth. This is 29 dB better than for a 2nd order Butterworth filter with a 350 kHz carrier signal. This last filter is used the Philips’ design which was the motive for this project. In the simulations the noise/distortion sources are jitter noise from the carrier signal, supply voltage distortion and intrinsic modulation errors. Distortion in the output stage is neglected. Also a start is made for the implementation of the actual loop-filter. A chain of integrators with distributed feedforward is chosen as loop-filter topology, because it is considered as the best choice in perspective of the power consumption and internal component requirements. All integrators in the loop filter are of the Miller type, due to their linearity, low power consumption and because of simplicity reasons. Although some components inside the loop filter are still idealized, a lot of parasitic effects are visible. Parasitic zeros and poles from the integrators are analyzed. The simulations results of this implementation show that a dynamic range of 122 dB in the audio band would be possible if only the internal thermal noise is considered as noise source. The component values for this implementation are calculated so that the internal thermal noise is not too large, the voltage swings of the integrators are within limits, the parasitics do no harm and that the component values are realistic.
Item Type:Essay (Master)
Philips Semiconductors
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Subject:53 electrotechnology
Programme:Electrical Engineering MSc (60353)
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