Baseband Circuits for an RFID Receiver

  
Figure 1 shows the block diagram of a direct conversion RF receiver—the receiver demodulates an RF carrier directly into a baseband signal without an intermediate frequency down-conversion (a zero IF receiver). The antenna, shared by both the transmitter and receiver, detects an RF carrier and passes it through a bandpass fi lter to an LT®5516 demodulator’s RF input. The LT5516 direct conversion demodulator frequency range of 800MHz to 1.5GHz includes the UHF range used by RFID readers (860MHz to 960MHz). The excellent linearity of the LT5516 provides for high sensitivity to low level signals, even in the presence of large interfering signals. The LT6231 low noise dual op amp acts as a differential to single-ended amplifi er to drive the single-ended input of the lowpass fi lter.
Baseband Circuits for an RFID Receiver - schematic

Analog baseband fi ltering is performed by the LT1568, a low noise, precision RC fi lter building block. The LT1568 fi lter provides a simple solution for designing lowpass and bandpass fi lters with cutoff frequencies from 100kHz to 10MHz. These cutoff frequencies are suffi cient for the 250kHz to 4MHz signal spectrum typically used in UHF RFID systems. The differential output of an LT1568 drives the inputs of an LTC2298 ADC. The LTC2298 is a 65Msps, low power (400mW), dual 14-bit analog to digital converter with 74dB signal-to-noise ratio (SNR). The digital signal processor (DSP) that follows the ADC analyzes the received signal from multiple tags and provides additional fi ltering. A Low Noise Differential to Single-Ended Amplifi er Figure 2 shows an LT6231 difference amplifi er used to convert the LT5516 differential I or Q output to a singleended output. The addition of external 270pF capacitors across the 60Ω resistors limits the demodulator’s output to 10MHz to prevent any high frequency interference from reaching the LT6231 amplifi er. AC coupling to the baseband amplifi er is used because DC coupling is not necessary for the amplitude shift keying (ASK) RFID signal. The highpass pole provided by the AC coupling capacitors and the amplifi er input resistors is set to 8kHz. The differential amplifi er’s input resistors are set to 140Ω in order to minimize the input referred noise. The noise...




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