Due to Bluetooth’s prevalence in cellular-phone platforms, there has been pressure to reduce the cost of the Bluetooth transceiver. The key is to achieve smaller die size and reduce any required external printed-circuit-board (PCB) components. Following this trend, a 4 x local-oscillator (LO) -based VCO has been proposed by a team of engineers at MediaTek, Inc: Sam Chun-Geik Tan; Fei Song; Renliang Zheng; Jiqing Cui; Guoqin Yao; Litian Tang; Yuejin Yang; Dandan Guo; Alexander Tanzil; Junmin Cao; Ming Kong; KianTiong Wong; Soong Lin Chew; Chee-Lee Heng; Osama Shana’a; and Guang-Kaai Dehng.
Their design strives to reduce the LO pulling effect and achieve superior receive (Rx) out-of-band blocking performance without requiring an external RF bandpass filter. As chip area is reduced, the traditional 2 x LO-based voltage-controlled oscillator (VCO) becomes more susceptible to interference from the power-amplifier (PA) second-harmonic products. Strong PA-to-VCO coupling can induce a frequency-pulling effect, which in turn degrades the transmitter modulation accuracy. It also increases output spectral regrowth in direct-conversion transmitters (DCTs).
Here, the 4-x-LO-based VCO is implemented to reduce LO pulling. It also minimizes transmit out-of-band spurious emissions. The transmitter provides +10 and +7 dBm output power in basic-data-rate (BDR) and enhanced-data-rate (EDR3) modes, respectively. It provides 1.5-kHz frequency stability and less than 6% root-mean-square (RMS) differential error vector magnitude (DEVM). Receiver sensitivity is -95.5, -96.5, and -89.0 dBm, respectively, for BDR, EDR2, and EDR3 modes. By integrating a balun shared between the transmitter and receiver, this transceiver eliminates the need for a separate transmit/receive switch. For continuous Tx transmission at +10 dBm, output power is 48 mA. The reference sensitivity level is 35 mA for continuous Rx reception. See “An Ultra-Low-Cost High-Performance Bluetooth SoC in 0.11-μm CMOS,” IEEE Journal Of Solid-State Circuits, Nov. 2012, p. 2665.