We use a simple directly modulated closed loop to develop a 2.1-mW, 2.4-GHz frequency-shift keying (FSK) transmitter that operates on 400-mV DC supply. Connecting a capacitor bank switch via a transformer in the voltage control oscillator (VCO) to the frequency-divider circuit expands the frequency tuning range without reducing VCO performance. A prototype was fabricated using the 65-nm standard CMOS process with a chip size of 1.65 × 1.85mm2. A modulation output signal spectrum of %42 dBc at 1.5MHz with %6dBm at the PA buffer output terminal; moreover, a VCO phase noise of %101 dBc/Hz at 1MHz is achieved. The FSK transmitter can readily use voltages supplied by harvested energy because of the low power consumption of the sensor network.
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