基本的RFID系統由三部分組成∶天線或線圈、帶RFID解碼器的收發器和RFID電子標簽（每個標簽具有唯一的電子識別碼）。RFID的應用環境可能非常惡劣,能受到來自無繩電話、無線耳機、無線數據網絡以及其他臨近讀卡器的干擾。因此,必須將每一讀卡器的RF接收器前端設計為能夠抵御強干擾信號，避免產生可導致詢問錯誤的失真。接收器的噪聲必須保持在較低的水準，以便具備足夠的動態范圍，從而以無錯方式檢測出低電平標簽響應信號。接收器的核心是Linear公司的LT5516，這是一種高度集成化的直接轉換正交解調器" title="正交解調器">正交解調器.本文介紹了RFID接收器的方框圖,LT5516,帶5MHz低通濾波器" title="低通濾波器">低通濾波器非常低噪音" title="低噪音">低噪音差分放大器LT6600-5以及非常低噪音高頻有源RC濾波器LT1568的主要特性" title="主要特性">主要特性以及應用電路圖.

RFID Receiver Front-End / 20kHz to 5MHz

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1.LT5516:800MHz -1.5GHz 直接變換正交解調器
The LT5516 is an 800MHz to 1.5GHz direct conversion quadrature demodulator optimized for high linearity receiver applications. It is suitable for communications receivers where an RF or IF signal is directly converted into I and Q baseband signals with bandwidth up to 260MHz. The LT5516 incorporates balanced I and Q mixers, LO buffer amplifiers and a precision, high frequency quadrature generator.
In an RF receiver, the high linearity of the LT5516 provides excellent spur-free dynamic range, even with fixed gain front end amplification. This direct conversion receiver can eliminate the need for intermediate frequency (IF) signal processing, as well as the corresponding requirements for image filtering and IF filtering. Channel filtering can be performed directly at the outputs of the I and Q channels. These outputs can interface directly to channelselect filters (LPFs) or to a baseband amplifier
LT5516主要特性:

Frequency Range: 800MHz to 1.5GHz
High IIP3: 21.5dBm at 900MHz
High IIP2: 52dBm
Noise Figure: 12.8dB at 900MHz
Conversion Gain: 4.3dB at 900MHz
I/Q Gain Mismatch: 0.2dB
Shutdown Mode
16-Lead QFN 4mm x 4mm Package with Exposed Pad
LT5516典型應用:

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2.LT6600-5:帶5MHz低通濾波器非常低噪音差分放大器
The LT6600-5 combines a fully differential amplifier with a 4th order 5MHz lowpass filter approximating a Chebyshev frequency response. Most differential amplifiers require many precision external components to tailor gain and bandwidth. In contrast, with the LT6600-5, two external resistors program differential gain, and the filters 5MHz cutoff frequency and passband ripple are internally set. The LT6600-5 also provides the necessary level shifting to set its output common mode voltage to accommodate the reference voltage requirements of A/Ds.

Using a proprietary internal architecture, the LT6600-5 integrates an antialiasing filter and a differential amplifier/driver without compromising distortion or low noise performance. At unity gain the measured in band signal-to-noise ratio is an impressive 82dB. At higher gains the input referred noise decreases so the part can process smaller input differential signals without significantly degrading the output signal-to-noise ratio.

The LT6600-5 also features low voltage operation. The differential design provides outstanding performance for a 2VP-P signal level while the part operates with a single 3V supply
LT6600-5主要特性:

Programmable Differential Gain via Two External Resistors
Adjustable Output Common Mode Voltage
Operates and Specified with 3V, 5V, ±5V Supplies
0.5dB Ripple 4th Order Lowpass Filter with 5MHz Cutoff
82dB S/N with 3V Supply and 2VP-P Output
Low Distortion, 2VP-P, 800 Ohm Load 1MHz: 93dBc 2nd, 96dBc 3rd Fully Differential Inputs and Outputs
Compatible with Popular Differential Amplifier Pinouts
Available in an SO-8 Package

LT6600-5典型應用:

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3.LT1568 :非常低噪音高頻有源RC濾波器
The LT®1568 is an easy-to-use, active-RC filter building block with rail-to-rail inputs and outputs. The internal capacitors of the IC and the GBW product of the internal low noise op amps are trimmed such that consistent and repeatable filter responses can be achieved. With a single resistor value, the LT1568 provides a pair of matched 2-pole Butterworth lowpass filters with unity gain suitable for I/Q channels.
By using unequal-valued external resistors, the two 2-pole sections can create different frequency responses or gains. In addition, the two stages may be cascaded to create a single 4-pole filter with a programmable response. Capable of cutoff frequencies up to 10MHz, the LT1568 is ideal for antialiasing or channel filtering in high speed data communications systems. The LT1568 can also be used as a bandpass filter.
The LT1568 features very low noise, supporting signal-to-noise ratios of over 90dB. It also provides single-ended to differential signal conversion for directly driving high speed A/D converters. The LT1568 has a shutdown mode that reduces supply current to approximately 0.5mA on a 5V supply.

The LT1568 is available in a narrow 16-lead SSOP package.
LT1568主要特性:

Up to 10MHz Center Frequency on a Single 3V Supply
Easy to Use—A Single Resistor Value Sets Lowpass Cutoff Frequency (200kHz <= f C <= 5MHz)
Unequal Resistor Values Extend Cutoff Frequency Up to 10MHz
Extremely Flexible—Different Resistor Values Allow Lowpass Transfer Functions with or Without Gain
SNR = 92dB (fC = 2MHz, 2VP-P)
THD = –84dB (fC = 2MHz, 1VP-P)
Internal Capacitors Trimmed to ±0.75%
Single 4-Pole Lowpass Filter or Matched Pair of 2-Pole Lowpass Filters
Can be Connected as a Bandpass Filter
Single-Ended or Differential Output
Operates from Single 3V (2.7V Min) to ±5V Supply
Rail-to-Rail Input and Output Voltages

LT1568應用電路圖: