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Fueled by unprecedented growth in mobile internet, multimedia and cloud computing applications, modern communication networks must support increasingly higher data rates. To achieve this, wireless standards have evolved to become more spectrally efficient by adopting increasingly complex modulation schemes and exploiting wider channel bandwidths which present significant design challenges to the frequency synthesizer of a wireless transceiver. This thesis presents a number of circuit and system level frequency synthesis techniques to improve the output frequency range and spectral purity of multi-band CMOS frequency synthesizers. The first part of this thesis proposes a reconfigurable dual-band and concurrent oscillator capable of generating harmonically unrelated tones in two frequency bands, 4.7-6.6GHz and 8.5-10.7GHz, either concurrently or one-at-a-time for multi-band radio applications. Such an oscillator can be used for inter-band carrier aggregation resulting in an effectively wider channel. By controlling the discrimination property of a cross-coupled pair, stable coexistence of the two tones is demonstrated to be possible using a single CMOS gm-cell for the first time. The second part focuses on improving the spectral purity and frequency resolution of wideband inductor-less injection-locked PLLs. A wideband phase-domain filtering technique capable of up to 20dB phase-noise and spur suppression is proposed. Additionally, a capacitor-ring coupled oscillator reduces delay-cell mismatches while increasing the number of phases available for injection. Demonstrated in 65nm CMOS, the prototype 1.2GHz synthesizer …