This work presents an architectural solution of CMOS interrogator with large dynamic range and noise immunity, which is aimed at UHF mobile RFID readers. Generally, in RFID systems based on backscattering communications, its RX and TX simultaneously operate, and it results in serious performance degradation in RX due to the self-jammer signal from the TX. Conventional RFID readers adopt off-chip solutions to attenuate the leakage signal; however, they cannot prevent the TX leakage signal reflected from antenna. It severely increases the system burden in dynamic range requirement in RX, and causes baseband DC offset and PN-induced noise problem. It is also inappropriate for the mobile RFID applications due to the low system integrity. Besides, direct conversion transmitters with linear power amplifiers have disadvantages in low system efficiency. To achieve low-power and low-cost RF front-end for mobile RFID readers, we must cope with these various design challenges in RX and TX.
As an approach for an isolator-less mobile RFID front-end, a simple and effective architecture of self-correlated RX is proposed. It generates self-LO signal using clamped RX input signal, and therefore the cause of PN-induced noise can be fundamentally eliminated. Besides, at the first stage of RX, a highly-linear passive mixer is located for obtaining large dynamic range. Through these two methodologies, an optimum RX is designed, which resolves the problems in dynamic range and PN-induced noise of RX. To the best of author’s knowledge, it is the firstly suggested solution to minimize two main problems in RX simultaneously.
For low-power and highly-linear ASK modulator, polar architecture based on switching power amplifier with envelope feedback linearization technique is adopted. To provide robust performance against PVT variations and parameter mismatches, we propose a new passive-type RF envelope detector. Compared to conventional envelope detectors using trans-conductance characteristics, it has advantages in precise signal detection owing to the switching operation of transistors. Along with the high gain differential-difference integrator, an ideal type-I feedback loop is achieved, and therefore, we can acquire output RF envelope identical to the reference baseband signal.
To verify its performances, a prototype RF front-end for UHF mobile RFID reader is fabricated with a standard CMOS technology. It contains self-correlated RX, RF envelope detector, differential-difference integrator, and polar TX, and its total area excluding the bonding pads is $1.28mm^2$.
The self-correlated RX exhibits SNR performance more than 20dB when TX signal up to 20dBm is applied. Especially for the TX power range from 2dBm to 10dBm, the SNR performance is enhanced by almost 10dB, which corresponds to the sensitivity level about -75dBm. The RF envelope detector has linear dynamic range of 27dB over the wide frequency range. Under voltage and temperature variations, its conversion gain is maintained within ±0.5dB error. At 1GHz RF frequency band, the detection bandwidth for dynamic envelope signal is measured to be maximally 0.5MHz. The polar TX delivers maximum CW output power of 23dBm while obtaining drain efficiency of 33%. With the feedback loop closed, its linear range reaches up to 27dB, and the ACPR requirement is satisfied with margin more than 10dB. The total current consumption for the implemented RX and TX is 10mA and 198mA from a 3.3V supply, respectively.