Current-Reused 2.4-GHz Direct-Modulation Transmitter With On-Chip Automatic Tuning

This paper presents the design, analysis, and experimental verification of a self-calibrating current-reused 2.4-GHz direct-modulation transmitter for short-range wireless applications. The key contributions are the design/analysis of a stacked power amplifier (PA)/voltage-controlled oscillator (VCO) architecture, the nonlinear frequency-dependent analysis of a Gilbert-cell-based root-mean-square detector, and an on-chip $LC$ -tank calibration circuit that needs no analog-to-digital convertor (ADC)/digital signal processor. The stacked architecture reduces the number of required regulators, utilizes supply headroom effectively, and allows for an “ADC-less” calibration loop that can dynamically tune the PA center frequency by sensing the transmitted signal. The very nature of direct-modulation architecture obviates additional high-purity signal generators, reducing complexity and allowing online calibration. The system was implemented in TSMC 0.18 $\mu{\rm m}$ CMOS, occupies 0.7 ${\rm mm}^{2}~({\rm TX})+0.1~{\rm mm}^{2}$ (self-tuning), and was measured in a QFN48 package on an FR4 PCB. Automatically correcting PA/VCO tank misalignment in this case yielded ${>}{\rm 4}~{\rm dB}$ increase in output power. With the automatic tuning active, the transmitter delivers a measured output power ${>}{\rm 0}~{\rm dBm}$ to a 100- $\Omega$ differential load, and the system consumes 22.9 mA from a 1.8-V core-circuit supply.