Fig. 2: Schematics of harmonic RFID system for near-field coherent sensing. | Nature Electronics

Fig. 2: Schematics of harmonic RFID system for near-field coherent sensing.

From: Monitoring vital signs over multiplexed radio by near-field coherent sensing

Fig. 2

a, Passive harmonic RFID tag. The harmonic tag receives the downlink RF signal (at frequency f) from the reader. It goes through the tag Antenna 1 (ANT 1) and splits into two parts: one provides d.c. power for tag circuits by energy harvesting and the other is fed into the passive harmonic generator (at frequency 2f) to be re-emitted from Antenna 2 (ANT 2). b, Schematic of the harmonic RFID reader. The digital clock is fed into two frequency synthesizers at f and 2f for coherent demodulation at 2f. The digital module performs the code-division multiple-access (CDMA) protocol25. The downlink commands from the reader to tags are modulated by the digital-to-analogue converter (DAC) and then upconverted by the mixer (blue) to the carrier at f. The harmonic tag backscatters to the reader at 2f, which is downconverted to the base band by the coherent local oscillator at 2f and sampled by the quadrature analogue-to-digital converter (ADC). Tx, transmitter; Rx, receiver; LPF, low-pass filter; HPF, high-pass filter; FPGA, field programmable gate array.