A 36mW/9mW power-scalable DCO in 55nm CMOS for GSM/WCDMA frequency synthesizers

The RF front-ends of modern smart phones are becoming more complicated as newer standards are introduced (e.g. LTE). Reconfigurability can be used to reduce their size, provided that power consumption is not adversely affected. For 2G/3G transceivers, local oscillator (LO) generation requires significant area and power. Reconfigurable voltage-controlled oscillators (VCOs) are generally used to maximize the achievable tuning range to reduce their number in the presence of many supported bands. However, no VCO capable to support both WCDMA and GSM has been reported that is also competitive with the power consumption achieved using two separate oscillators. In fact, the very demanding GSM phase-noise specs require a current up to four times higher (depending on the duplexer selectivity) than that used in the WCDMA case. In the design of an LC-tank harmonic oscillator, phase noise normalized to power consumption (i.e. the figure-of-merit, FOM), reaches an optimum at the maximum oscillation amplitude compatible with the supply voltage V
_dd
. This condition impairs the power reconfigurability of a VCO, since there is only one value of bias current yielding the highest FOM, once tank and V
_dd
are chosen. On the other hand, making the tank reconfigurable invariably results in a degradation of its Q, i.e. a reduced FOM.