An alternative structure for UHF RFID rectifier circuitry is the Dickson charge pump (Fig. 2).⁵ It is employed mostly in nonvolatile memory to achieve the high programming voltage required in electronically erasable programmable read-only memory (EEPROM) circuits. Since most RFID chips also contain nonvolatile memory, a designer can reuse the circuit topology to implement the high-voltage generation circuitry, saving development time in the process.

The simplified equation of Dickson structure is shown in Eq. 1:

where:

V_p,RF = the amplitude of the incoming RF signal and

V_f,D = the forward voltage drop across the diode.

A Dickson charge pump can be built using almost any semiconductor device, although Schottky diodes⁶ and low-threshold-voltage (V_TH MOSFETs⁷ yield the best results. The circuit requires very little input voltage, and the designer can choose the desired output voltage and input impedance by manipulating the number of stages (N). However, power conversion efficiency is low due to the large number of rectifying devices, leakage, and parasitic elements.

The RFID modulator transmits transponder data back to the RFID interrogator or reader. Backscatter modulation is used exclusively in UHF RFID systems. In the EPC Gen2 protocol,⁸ two modulation schemes are specified: ASK and PSK. In ASK, two impedance states (in which only pure resistance is changed—either open, short, or two nonzero resistances) are switched between two antenna pins. A user can select either state to represent digital logic 1 or 0.⁹ In PSK, there are also two impedance states being manipulated, but only imaginary components are switched. To toggle between two imaginary reactance values, designers often use large MOSFET or varactor devices with voltage-dependent capacitances.

In a UHF RFID chip implementation, the impedance seen by the antenna can be represented as a resistance in parallel with a reactance (Fig. 3) .¹⁰ Assuming an antenna with minimum scattering, the amplitude of the backscattered power has the expression:

where:

P_EIRP = the effective isotropic radiated power;

R_A = the antenna resistance;

R = the chip resistance; and

A_e = the effective radar-cross-section (RCS) area.

In the case of ASK modulation, the impedance seen by the antenna is real (X >> R) and is modulated by the data signal between two values, R₁ and R₂. It is sufficient to choose:

R₁ X R₂ = R²_A to have equal impedance mismatch in both states. In such a condition in both states, the same power is transferred from the antenna to the load. Assuming R₂> R₁, in order to modulate the resistance seen by the antenna, it is possible to use a switch, driven by the data signal, to connect a resistance RMOD in parallel with the input resistance (R₂) of the transponder in such a way that R₁ = R₂||R_MOD. When resistance R_MOD is not connected, the antenna sees a resistance R₂ and all the power P_IN2 transferred from the antenna to the load can be used to supply the transponder. When resistance R_MOD is connected, the antenna sees a resistance R₁, and only a fraction of the power P_IN1 transferred from the antenna to the load can be used to supply the transponder, while the remaining part is dissipated on the resistance R_MOD. When P_IN2 and P_IN1 are equal, R₁ = R₂. From a design standpoint, it can be concluded that under ASK scheme, it is not possible to achieve a constant power supply to the tag IC. The corresponding equations are:

where:

P_AV = the average power and

A_e = the effective RCS area.