Low power design often requires direct conversion
architectures, such as low-IF or zero-IF. Any of these
two possibilities needs a low power, low phase noise
voltage control oscillator (VCO) in the frequency
synthesizer. This work is focused on low power
considerations applied to the practical modern
conception of this device. Fulfilling the standard
specifications (output power, phase noise, frequency
range) should be completed with this deeper step. A
conscious design leads moreover to an improvement in
the results obtained by the classical considerations. The
increase of the quality factor of the passive elements is
one of the key points, followed by an accurate design
of the architecture scheme. Furthermore, lower current
consumption provides higher oscillation frequencies
and facilitates higher frequency ranges, which follow
the trends of modern wireless and wideband
communication standards. In order to validate the
aforementioned assumptions, a CMOS VCO has been
implemented in UMC 0.18μm 1P6M technology, with
power consumption down to 3.4mW.
A 3.3V, fully integrated 3.2-GHz voltage-controlled oscillator (VCO) is designed in a 0.18μm CMOS technology for the IEE 802.11a/HiperLAN WLAN standard for the UNII band from 5.15 to 5.35 GHz. The VCO is tunable between 2.85 GHz and 3.31 GHz. NMOS architecture with self-biasing current of the tank source is chosen. A startup circuit has been employed to avoid zero initial current. Current variation is lower than 1% for voltage supply variations of 10%. The use of a self-biasing current source in the tank provides a greater safety in the transconductance value and allows running along more extreme point operation The designed VCO displays a phase noise and output power of -98dBc/Hz (at 100 KHz offset frequency) and 0dBm respectively. This phase noise has been obtained with inductors of 2.2nH and quality factor of 12 at 3.2 GHz, and P-N junction varactors whose quality factor is estimated to exceed 40 at 3.2 GHz. These passive components have been fabricated, measured and modeled previously. The core of the VCO consumes 33mW DC power.
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