Foreign CDMA-based RFID air interface research work, so far still remain in the active tag, only send but not receive the research stage, the direct cause is usually attributed to passive tags failed to achieve the chip low-power design, visible chip low-power design is very necessary. Therefore, only by first figuring out the power supply mechanism of passive tags, and then analyzing the application environment of UHF RFID air interface, it is possible to find a complete solution. The purpose of this paper is to introduce the UHF RFID passive tag chip special power supply mechanism.
1. UHF RFID passive tag power supply characteristics
1.1 Power supply by wireless power transmission
Wireless power transmission is the use of wireless electromagnetic radiation method to transmit electrical energy from one place to another. The working process is the electric energy by radio frequency oscillation converted into radio frequency energy, radio frequency energy by transmitting antenna converted into radio electromagnetic field energy, radio electromagnetic field energy through the space propagation to the receiving antenna, and then by the receiving antenna converted back to the radio frequency energy, the checkwave into the direct current energy.
In 1896 Italian Marconi (Guglielmo Marchese Marconi) invented the radio, realized the radio signal transmission across the space. 1899, the American Tesla (Nikola Tesla) put forward the idea of wireless power transmission, and Colorado established a 60m high, the bottom of the sensing, the top of the antenna with the capacity. Using a frequency of 150kHz, the 300kW input power is transmitted over a distance of up to 42km, and 10kW of wireless received power is obtained at the receiving end.
The UHF RFID passive tag power supply follows this idea, with the reader supplying power to the tag via RF. However, the UHF RFID passive tag power supply is vastly different from the Tesla test: the frequency is nearly 10,000 times higher and the antenna size is a thousand times shorter. Since wireless transmission loss is proportional to the frequency squared and proportional to the square of the distance, it is clear that the growth in transmission loss is enormous. The simplest wireless propagation mode is free-space propagation, the propagation loss is inversely proportional to the square of the propagation wavelength, and proportional to the square of the distance, the free-space propagation loss is LS=20lg(4πd/λ). If the distance d unit for m, the frequency f unit for MHz, then LS = -27.56 + 20lgd + 20lgf.
UHF RFID system based on wireless power transfer mechanism, passive tags do not have their own power supply, with the help of receiving the RF energy emitted by the reader, through the doubling of the rectifier, i.e., the Dixon pump (Dickson charge pump) to establish a DC power supply. Power supply.
UHF RFID air interface applicable communication distance is mainly determined by the reader transmit power and space basic propagation loss. UHF band RFID reader transmit power is usually limited to 33dBm. by the basic propagation loss formula, ignoring any other possible loss, can be calculated through the wireless power transfer to reach the tag RF power. UHF RFID air interface communication distance and the relationship between the basic propagation loss and reach the tag RF power. The relationship between the basic propagation loss and the RF power arriving at the tag
Note: Assuming that the transmit power of the reader is 33dBm.
UHF RFID wireless power transmission is characterized by high transmission loss, and the tag can be supplied with a low power because the RFID complies with the national rules for short-range communication, and the transmit power of the reader is limited. As the communication distance increases, passive tags receive RF energy by frequency square rate decreases, the power supply capacity is rapidly weakened.
Bluetooth RFID Reader
1.2 Power supply through on-chip capacitor charging and discharging
1.2.1 Capacitor Charging and Discharging Characteristics
Passive tags utilize wireless power transfer to obtain energy, which is converted to DC voltage, and charges the on-chip capacitor to store energy, and then supplies power to the load through discharging. Therefore, the power supply process of passive tags is the capacitor charging and discharging process. Capacitor charging and discharging process shown in Figure 2, the establishment of the process is a pure charging process, the power supply process is a discharge and supplemental charging process, supplemental charging must be in the discharge voltage reaches the chip before the start of the minimum supply voltage.
1.2.2 Capacitor charging and discharging parameters
- Charging parameters
Charging time: τC=RC×C
Charging voltage:
Charging current:
where RC is the charging resistance and C is the energy storage capacitance.
- Discharge parameters
Discharge time: τD=RD×C
Discharge voltage:
Discharge current:
where RD is the discharge resistance and C is the energy storage capacitance.
The above illustrates the power supply characteristics of passive tags, neither a constant voltage source, nor a constant current source, but the energy storage capacitor charging and discharging. When the on-chip energy storage capacitor charging to reach the chip circuit operating voltage V0 above, it will be able to supply power to the label. Energy storage capacitor began to supply power at the same time, its supply voltage began to fall, down to the chip operating voltage V0 below, the energy storage capacitor to lose power supply capacity, the chip will not continue to work. Therefore, the air interface tag should have enough to supplement the charging capacity of the label.
It can be seen that the passive tag power supply method is compatible with its burst communication characteristics, passive tag power supply also needs to have continuous charging support.
1.3 Supply and Demand Balance
Float-charged power supply is another type of power supply where the float-charged power supply capacity is compatible with the discharge capacity. But they all have a common problem, that is, the UHF RFID passive tag power supply needs to be balanced supply and demand.
1.3.1 for burst communication supply and demand balance power supply
UHF RFID passive tags current standard ISO/IEC18000-6 belongs to the burst communication system, for passive tags, receiving time does not emit signals, answering time, although receiving the carrier, but is equivalent to obtaining the oscillator source, so it can be considered as a simplex mode of operation. For this application, if the receiving time is taken as the charging time of the storage capacitor and the answering time is taken as the discharging time of the storage capacitor, the equal amount of charging and discharging charges to keep the supply and demand in balance becomes a necessary condition to maintain the normal operation of the system.
From the above UHF RFID passive tag power supply mechanism can be seen, UHF RFID passive tag power supply is neither a constant current source, nor a constant voltage source. When the tag storage capacitor charging to higher than the circuit’s normal operating voltage, the start of the power supply; when the tag storage capacitor discharge to lower than the circuit’s normal operating voltage, stop the power supply.
For burst communication, such as passive tags UHF RFID air interface, the tag can be sent to answer the burst before charging enough to ensure that the answer is completed before maintaining a sufficient voltage. Thus, in addition to the tag receiving strong enough RF radiation, the chip is also required to have a large enough on-chip capacitance and a long enough charging time. Label response power consumption and response time must also be compatible. Due to the distance between the tag and the reader is different, the response time is different, the energy storage capacitance area is limited and other factors, the use of time-division supply and demand balance may be difficult.
1.3.2 for continuous communication floating charging power supply
For continuous communication, to maintain uninterrupted power supply of the energy storage capacitor, it is necessary to do with the release of charging, charging speed and discharge rate is similar to the end of the communication, that is, before the end of the power supply capacity is maintained.
Passive tags code points RFID and UHF RFID passive tags the current standard ISO/IEC18000-6 has common characteristics, the tag receiving state needs to demodulate and decode, answer state to modulate and send, therefore, more should be designed according to continuous communication tag chip power supply system. In order to make the charging speed and the discharge rate is similar, it is necessary to use most of the energy received by the tag for charging.
2. shared RF resources
2.1 passive tag RF front-end
passive tag on the RF energy from the reader, in addition to the tag as a chip power supply, more importantly, through wireless data transmission to achieve the reader to the tag command signal transmission, the tag to the reader’s answer signal transmission. Passive tags on the application of RF energy from the reader
tag to receive RF energy to be divided into three, respectively, for the chip to establish the power supply, demodulation signals (including command signals and synchronization clock) and to provide the answer carrier.
The current standard UHF RFID mode of operation has the following characteristics: downlink channel using the broadcast mode of operation, the uplink channel using multiple tags to share a single channel sort of response, therefore, as far as information transmission is concerned, belongs to the simplex mode of operation. However, because the label can not provide its own transmission carrier, the label response needs to provide carrier with the help of the reader, so in the label response, as far as the sending state is concerned, the two ends of the communication are in the duplex working state.
In different states of operation, the label into the work of different circuit units, different circuit units work the power required is not the same, all the power comes from the RF energy received by the label. Therefore, it is necessary to reasonably allocate the appropriate time to control the RF energy distribution.
2.2 Application of RF energy in different working hours
When the label into the reader RF field to start establishing power supply, regardless of what signals are sent by the reader at this time, the label will all receive RF energy provided to the doubling rectifier circuit, on-chip energy storage capacitor charging, in order to establish the chip power supply power.
When the reader sends a command signal, the reader’s transmit signal is subject to command data encoding and amplitude modulation of the extended spectrum sequence of signals. The signal received by the label there is a carrier component and represents the command data and extended spectral sequence of the side-band component, the total energy of the received signal, the carrier energy, the size of the side-band component is related to the modulation. At this time, the modulation component is used to transmit the synchronization information of the command and the extended spectrum sequence, and the total energy is used to charge the on-chip energy storage capacitor, which simultaneously begins to supply power to the on-chip synchronization extraction circuit and the command signal demodulation circuit unit. Therefore, during the time period when the reader sends commands, the RF energy received by the tag is used for continued charging of the tag, synchronization signal extraction, command signal demodulation and identification. The tag energy storage capacitor is in the floating charge power supply state.
When the tag responds to the reader, the reader’s transmit signal is amplitude modulated by the spread spectrum chip rate sub-rate clock. The signal received by the tag has a carrier component and a sideband component representing the spread spectrum chip rate division rate clock. At this time, the modulation component is used to transmit the chip rate sub-rate clock information of the extended spectrum sequence, and the total energy is used to charge the on-chip energy storage capacitor and modulate the answer data and send an answer to the reader, and the on-chip energy storage capacitor begins to supply power to the on-chip chip synchronization extraction circuitry and the answer signal modulation circuitry unit at the same time. Therefore, during the time when the reader receives an answer, the RF energy received by the tag is used for the tag to continue charging, chip synchronization signal extraction and modulation by the answer data, and to send an answer. The tag energy storage capacitor is in the floating charge power supply state.
In short, in addition to the tag into the reader RF field, the beginning of the establishment of power supply time, the tag is all the received RF energy to provide voltage doubling rectifier circuit, on-chip energy storage capacitor charging, in order to establish the chip power supply power. Subsequently, the label from the received RF signal to extract the synchronization, the implementation of the command demodulation, or answer data modulation to send, which are to use the received RF energy.
ALN-9954 G Higgs 9 RFID Tags
2.3 RF Energy Requirements for Different Applications
2.3.1 RF Energy Requirements for Wireless Power Transfer
Wireless power transfer establishes the power supply for the label, so it requires both the voltage that is sufficient to drive the chip circuits and sufficient power and continuous power supply capability.
Wireless power transmission power supply is in the absence of power in the label by receiving the reader RF field energy, doubling rectifier to establish power, therefore, its receiving sensitivity by the front-end detector diode tube voltage drop limit, for CMOS chips, doubling rectifier receiving sensitivity between -11 ~ -0.7dBm, the bottleneck of passive tags.
2.3.2 Receive signal detection of RF energy requirements
doubling voltage rectifier to establish the chip power supply power supply at the same time, the label should be divided into a portion of the received RF energy to provide signal detection circuitry, including the command signal detection and synchronization clock detection. As the label has been established under the conditions of power supply to implement signal detection, demodulation sensitivity is not subject to the front-end detector diode tube voltage drop limitations, so the reception sensitivity is much higher than the wireless power transfer reception sensitivity, and belongs to the signal amplitude detection, there is no power intensity requirements.
2.3.3 RF energy requirements of the label answer
When the label answer is sent, in addition to the need to detect the synchronization clock, but also need to receive the carrier (containing the clock modulation envelope) for pseudo-PSK modulation and reverse transmission. At this time, the requirements of a certain power level, the value depends on the reader on the label distance and reader reception sensitivity. As the reader operating environment allows the use of more complex design, the receiver can achieve low-noise front-end design, plus code division of RFID using extended spectrum tuning, as well as extended spectrum gain and PSK system gain, reader sensitivity may be designed to be sufficiently high, so that the tag return signal requirements down to low enough.
In summary, the label will receive RF power is mainly allocated for wireless power transfer multiplying rectifier energy, followed by the allocation of the appropriate amount of label signal detection level and the appropriate amount of return modulation energy, to achieve a reasonable allocation of energy to ensure that the continuous charging of the energy storage capacitor is possible, reasonable design.
Can be seen, passive tags received by the RF energy has a variety of application requirements, so there is a need for RF power allocation design; different working hours RF energy application requirements are not the same, so there is a need for different working hours according to the demand for RF power allocation design; different applications on the size of the RF energy requirements are not the same, including wireless power transmission requirements of the largest power, so the RF power allocation should focus on the wireless Power transmission requirements.
125khz RFID Transponder
3. Conclusion
UHF RFID passive tag with the help of wireless power transmission to establish the tag power supply, therefore, the power supply efficiency is extremely low, the power supply ability is very weak, the tag chip must use low-power design. With the help of on-chip energy storage capacitor charging and discharging implementation of the chip circuit power supply, therefore, in order to ensure that the tag continues to work, must continue to charge the energy storage capacitor. RF energy received by the label has three different applications: voltage doubling rectifier power supply, command signal reception and demodulation, answer signal modulation and transmission, of which, voltage doubling rectifier reception sensitivity by the rectifier diode tube voltage drop constraints, become the bottleneck of the air interface. For this reason, signal reception and demodulation and answer signal modulation and transmission is the basic function of the RFID system must ensure that the doubled-voltage rectifier tag power supply capability, the stronger the product is more competitive. Therefore, the tag system design in the reasonable distribution of the received RF energy guidelines are: to ensure that the received signal demodulation and response signal transmission under the premise of increasing the multiplier rectifier RF energy supply as far as possible.
4. Frequently Asked Questions
4.1 How does a UHF RFID passive tag get power without a battery?
A UHF RFID passive tag relies on wireless power transmission from the reader. The reader emits RF energy, which is captured by the tag’s antenna, converted to DC voltage via a Dickson charge pump (voltage multiplier), and stored in an on-chip capacitor. This stored energy powers the tag’s chip during communication.
4.2 Why is low-power design critical for UHF RFID passive tags?
A: Since passive tags harvest limited energy from RF signals (typically just microwatts), the chip must operate at ultra-low power to function reliably. High power consumption would prevent the tag from charging sufficiently, leading to communication failure, especially at longer distances.
4.3 What limits the operating range of UHF RFID passive tags?
The range is constrained by:
Free-space path loss (RF energy decays with distance² and frequency²).
Regulatory power limits (e.g., readers are capped at 33 dBm in UHF bands).
Rectifier sensitivity (diodes need ~-11 to -0.7 dBm to start charging).
Beyond a few meters, the tag cannot harvest enough energy to respond.
4.4 How does the tag balance power between charging, receiving data, and transmitting responses?
The tag dynamically allocates harvested RF energy:
Initial phase: All energy goes to charging the capacitor.
Command reception: Energy splits between charging, clock recovery, and demodulation.
Response transmission: Energy powers backscatter modulation (reflecting the reader’s signal with data).
Efficient power-sharing algorithms ensure no function starves.
4.5 Why is the capacitor’s charge/discharge cycle important?
The capacitor acts as a temporary battery:
Charging: Must reach the chip’s minimum operating voltage (e.g., 1.2V) before use.
Discharging: Must sustain power until the next charging cycle.
If discharge is faster than recharge, the tag resets mid-communication.
4.6 Why can’t passive tags support continuous communication like active tags?
Passive tags lack a battery, so they operate in burst mode:
Charging pauses: The capacitor must recharge between transmissions.
Energy asymmetry: Transmitting backscatter signals consumes more power than receiving.
Active tags (with batteries) avoid this by providing steady power.
4.7 How does modulation (e.g., PSK) affect power efficiency?
Modulation impacts energy use in two ways:
Backscatter efficiency: PSK minimizes energy loss during data reflection.
Spectral spreading: Techniques like DSSS improve noise immunity but require more complex (power-hungry) decoding.
Optimized modulation balances reliability and power drain.
4.8 What’s the biggest bottleneck in passive RFID performance?
The rectifier’s sensitivity (voltage threshold to start charging) is the primary limit. Even with high reader power, diode losses (~0.3V per stage in charge pumps) reduce usable energy. Research focuses on low-voltage rectifiers (e.g., zero-bias diodes) to extend range.
4.9 Could graphene improve UHF RFID passive tags?
Yes! Graphene offers:
Higher conductivity: More efficient energy harvesting.
Flexible substrates: Enables tags on paper, fabric, etc.
Lower manufacturing costs vs. silver-based inks.
Projects like BGT Materials’ graphene RFID tags already demonstrate these advantages.
4.10 How do standards like ISO/IEC 18000-6 address power challenges?
The standard optimizes:
Duty cycling: Tags communicate in short bursts to conserve energy.
Asymmetric protocols: More power is allocated to reader-to-tag commands.
Error recovery: Tags reset if power drops, avoiding corrupted data.