Joined: 12 Dec 2006 Posts: 1316 Helped: 252 Location: Iraq
29 Dec 2008 17:17 Re: How does passive tag receive energy
I think this may help u -------------------------- 2. RFID Transponder System Architecture: Data carrying devices that are fixed on the items to be identified in an RFID system are called transponders or tags. The main purpose of a transponder is to carry ID information of the object. The block diagram of a passive RFID transponder is shown in Figure 2. It is composed of 1) an antenna; 2) a radio frequency (RF) transceiver section; 3) an analog detection and/or rectification section, which detects, and in passive tags, rectifies RF power into an equivalent dc voltage; and 4) a digital control section that is either a microprocessor or some other digital system. 2.1 Detection Section: 
The detection/rectification section converts RF energy received by the transponder’s antenna and converts it to a baseband signal or an equivalent dc voltage. In a fully passive tag, a voltage doubler or quadrupler circuit is used to pump up the voltage for efficient operation of the digital section. Therefore, the RF section receives the signal from the reader, which is used to provide dc power supply for the control section and modulate and transmit the RF signal so that the ID data can be retransmitted to the RFID reader.
Depending on the tag’s ability to be read only (RO) or read/write (R/W), the RF section can be modified in such a way that it also extracts the interrogation code from the reader. The extracted code is transformed from an RF signal to a baseband signal using demodulation circuitry and sent to the control section for further processing. It is important to note that the RF signal from the reader has to provide sufficient power to the tag for efficient communication between the two entities. Passive and semipassive RFID systems that use amplitude shift-keying (ASK) modulations for reader-tag communication are especially affected by this RF signal’s power budget. In this regard, the RF rectifier circuit plays a vital role. The rectifier circuit can be realized in various ways depending on the technology used for the transponder design. In the rectifier circuit, there is a limiter and voltage pump circuit for either limiting or increasing the dc power. Obviously, the closer the transponder is to the RFID reader, the more power it will extract.
2.2 Digital Control Section: 
The block diagram of the transponder control section is shown in Figure 3. The digital section has both analog and digital signal processing subsections. Today’s trend is to integrate the digital control section into a single integrated circuit (IC) so that minimum spacing and package dimensions are achieved. Additionally efficient power budgeting is possible when the functional blocks are in close proximity to each other. That is why very low-powered RFID ICs are available in the market. After the RF signal is received and demodulated in the RF section, the demodulated signal is sent to an analog to- digital converter (ADC), which converts the RF signal to a low frequency baseband signal. The baseband signal is then converted to a digital signal and processed further in the protocol detection circuit and decrypted. The signal is processed by the microcontroller, which generates a response signal. The response signal is sent to the encryption circuitry and then converted to an analog signal by the digital- to-analog converter (DAC). This signal is then modulated by an RF carrier signal and propagated back to the reader via the tag antenna. This operation is done in the RF section of the tag and the antenna.