University of Twente Student Theses
Distortion analysis of Voltage-Controlled Oscillator-based Analog-to-Digital Converters
Besselink, M. (2013) Distortion analysis of Voltage-Controlled Oscillator-based Analog-to-Digital Converters.
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| Abstract: | In this work, Analog-to-Digital Converters (ADCs) based on a Voltage-Controlled Oscillator (VCO) were analyzed on distortion performance. It is widely known that the performance of these ADCs is limited by the VCO nonlinearity. Since this is the most limiting factor, this MSc project was focused on the linearity of VCOs, at which a distinction was made between static distortion and dynamic distortion. The static distortion was determined from the actual (nonlinear) VCO control characteristic, that was obtained by simulations at several fixed (static) input voltages. The dynamic distortion was determined from frequency spectra with a sinusoidal input voltage to indicate frequency dependent distortion. In order to cancel the VCO nonlinearity by digital calibration, the frequency dependent distortion also known as memory effects, should be lower than 0.1 dB to gain 40 dB in performance. Before the VCO nonlinearity was analyzed and simulated on circuit level, high-level system simulations of VCO-based ADCs were performed. They show beside the desired operation of the system, an interesting aspect of this kind of ADCs, namely the inherent first-order noise shaping. Since the phase of the VCO is a continuous signal and sampled for the A/D conversion, the residual phase at a current sampling period becomes the initial phase of the next sampling period. This introduces the first-order noise shaping. With ideal building blocks on high-level it was shown that the VCO-based ADC generates no distortion. However, by adding a small amount of nonlinearity to the voltage-to-frequency gain KVCO to the ideal ADC, the performance was significant decreased. Regarding the static distortion on circuit level, the proposed ring VCO from former MSc project was analyzed and simulated. The mathematical analysis showed that the VCO nonlinearity was caused by the nonlinear V-I characteristics of the MOS transistors. Several alternative VCO circuits were treated and compared to the proposed ring VCO and the proposed ring VCO achieved the highest performance qua static distortion in comparison with the alternatives. Nevertheless, the proposed VCO needs an input buffer, which cannot be implemented easily without influencing the overall static distortion performance. The saw-tooth oscillator, which is a completely different oscillator, is an interesting alternative, because it rendered only 1.6 dB more static distortion than the proposed ring VCO while it does not need an input buffer. The proposed ring VCO and the saw-tooth oscillator were also analyzed on frequency dependent (dynamic) distortion. Since determining the instantaneous VCO frequency by time periods rendered distortion issues, a frequency quantizer was implemented to investigate the dynamic distortion of VCOs. Single-tone simulations of the proposed ring VCO and the saw-tooth oscillator showed distortion results that correspond to the static distortion results. Two-tone simulations were performed with both circuits to indicate memory effects, where it turned out that the variations of the distortion terms cannot easily be related to these effects. Several other methods were explored to indicate memory effects, but without successful results. |
| Item Type: | Essay (Master) |
| Faculty: | EEMCS: Electrical Engineering, Mathematics and Computer Science |
| Subject: | 53 electrotechnology |
| Programme: | Electrical Engineering MSc (60353) |
| Link to this item: | https://purl.utwente.nl/essays/74212 |
| Export this item as: | BibTeX EndNote HTML Citation Reference Manager |
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