无线射频识别技术（RFID）是一种非接触的自动识别技术，它是利用射频信号和空间耦合（电感或电磁耦合）或雷达反射的传输特性，实现对被识别物体的自动识别。RFID系统至少包含电子标签和阅读器两部分。RFID阅读器（读写器）通过天线与RFID电子标签进行无线通信，可以实现对标签识别码和内存数据的读出或写入操作。典型的阅读器包含有高频模块(发送器和接收器)、控制单元以及阅读器天线。本文介绍了Linear公司的RFID阅读器解决方案，以及所用的主要元器件的性能和相关的应用电路图，包括800MHz到1500MHz直接变换正交解调器LT5516，700MHz -1050MHz高线性直接正交调制器LT5568，60dB动态范围的50MHz - 3GHz RF功率检测器LT5534，带可编输出范围的12/14/16位SoftSpan DAC  LTC1588以及单/双路4.5ns 3V/5V/±5V轨到轨比较器LT1711。



RFID阅读器方框图。



RFID阅读器所用的主要元件：
1．LT5516：800MHz到1500MHz直接变换正交解调器
LT5516 - 800MHz to 1.5GHz Direct Conversion Quadrature Demodulator
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

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应用电路图：



2．LT5568：700MHz -1050MHz高线性直接正交调制器
LT5568 - 700MHz to 1050MHz High Linearity Direct Quadrature Modulator
The LT5568 is a direct I/Q modulator designed for high performance wireless applications, including wireless infrastructure. It allows direct modulation of an RF signal using differential baseband I and Q signals. It supports PHS, GSM, EDGE, TD-SCDMA, CDMA, CDMA2000, WCDMA, and other systems. It may also be confi gured as an image reject upconverting mixer, by applying 90° phase-shifted signals to the I and Q inputs. The I/Q baseband inputs consist of voltage-to-current converters that in turn drive double-balanced mixers. The outputs of these mixers are summed and applied to an on-chip RF transformer, which converts the differential mixer signals to a 50Ω single-ended output. The four balanced I and Q baseband input ports are intended for DC coupling from a source with a common mode voltage level of about 0.5V. The LO path consists of an LO buffer with single-ended input, and precision quadrature generators that produce the LO drive for the mixers. The supply voltage range is 4.5V to 5.25V。

LT5568主要特性：
Frequency Range: 700MHz to 1050MHz
High OIP3: +22.9dBm at 850MHz
Low Output Noise Floor at 5MHz Offset:
No RF: –160.3dBm/Hz
POUT = 4dBm: –154dBm/Hz
3-Ch CDMA2000 ACPR: –71.4dBc at 850MHz
Integrated LO Buffer and LO Quadrature Phase Generator
50Ω AC-Coupled Single-Ended LO and RF Ports
50Ω DC Interface to Baseband Inputs
Low Carrier Leakage: –43dBm at 850MHz
High Image Rejection: –46dBc at 850MHz
16-Lead 4mm × 4mm QFN Package
LT5568应用电路图：

3．LT5534：60dB动态范围的50MHz - 3GHz RF功率检测器
LT5534 - 50MHz to 3GHz RF Power Detector with 60dB Dynamic Range
The LT5534 is a 50MHz to 3GHz monolithic RF power detector capable of measuring RF signals over a 60dB dynamic range. The RF signal in a decibel scale is precisely converted into DC voltage on a linear scale. The 60dB input dynamic range is achieved using cascaded RF detectors and RF limiters. Their outputs are summed to generate an accurate log-linear DC voltage proportional to the input RF signal in dB. The output is buffered with a low output impedance driver. The LT5534 delivers superior temperature stability (typical output variation within ±1dB over the full temperature range). The output responds in less than 40ns to a large RF input signal。

LT5534主要性能：
RF Frequency Range: 50MHz to 3GHz
Linear Dynamic Range: 60dB
Exceptional Accuracy over Temperature and Power Supply
Fast Transient Response: 38ns Full-Scale Settling Time
Single 2.7V to 5.25V Supply
Low Supply Current: 7mA
Shutdown Current: 0.1µA
Tiny 6-Lead SC70 Package
LT5534应用电路图：



4．LTC1588：带可编输出范围的12/14/16位SoftSpan DAC
LTC1588 - 12-/14-/16-Bit SoftSpan DACs with Programmable Output Range
The LTC®1588/LTC1589/LTC1592 are serial input 12-/14- /16-bit multiplying current output DACs that operates from a single 5V supply. These SoftSpan DACs can be software-programmed for either unipolar or bipolar mode through a 3-wire SPI interface. In either mode, the voltage output range can also be software-programmed. Two output ranges in unipolar mode and four output ranges in bipolar mode are available.

INL and DNL are accurate to 1LSB over the industrial temperature range in both unipolar and bipolar modes. True 16-bit 4-quadrant multiplication is achieved with on-chip four quadrant multiplication resistors. The LTC1588/LTC1589/LTC1592 are available in a 16-lead SSOP package.

These devices include an internal deglitcher circuit that reduces the glitch impulse to less than 2nV-s (typ).
The asynchronous clear pin resets the LTC1588/LTC1589/ LTC1592 to 0V in unipolar or bipolar mode.

LTC1588主要特性：
Six Programmable Output Ranges
Unipolar Mode: 0V to 5V, 0V to 10V
Bipolar Mode: ±5V, ±10V, ±2.5V, –2.5V to 7.5V
1LSB Max DNL and INL Over the Industrial Temperature Range
Glitch Impulse < 2nV-s
16-Lead SSOP Package
Power-On Reset to 0V
Asynchronous Clear to 0V for All Ranges
LTC1588应用电路图：



5．LT1711：单/双路4.5ns 3V/5V/±5V轨到轨比较器
LT1711 - Single/Dual 4.5ns, 3V/5V/±5V, Rail-to-Rail Comparators
The LT®1711/LT1712 are UltraFastTM 4.5ns comparator featuring rail-to-rail inputs, rail-to-rail complementary outputs and an output latch. Optimized for 3V and 5V power supplies, it operates over a single supply voltage range from 2.4V to 12V or from ±2.4V to ±6V dual supplies.

The LT1711/LT1712 is designed for ease of use in a variety of systems. In addition to the wide supply voltage flexibility, the rail-to-rail input common mode range extends 100mV beyond both supply rails, and the outputs are protected against phase reversal for inputs extending further beyond the rails. Also, the rail-to-rail inputs may be taken to opposite rails with no significant increase in input current. The rail-to-rail matched complementary outputs interface directly to TTL or CMOS logic and can sink 10mA to within 0.5V of GND or source 10mA to within 0.7V of V+.

The LT1711/LT1712 has an internal TTL/CMOS compatible latch for retaining data at the outputs. The latch holds data as long as the latch pin is held high. Latch pin hysteresis provides protection against slow moving or noisy latch signals. The LT1711 is available in the 8-pin MSOP package. The LT1712 is available in the 16-pin narrow SSOP package.

LT1711主要特性：

Ultrafast: 4.5ns at 20mV Overdrive
5.5ns at 5mV Overdrive
Rail-to-Rail Inputs
Rail-to-Rail Complementary Outputs (TTL/CMOS Compatible)
Specified at 2.7V, 5V and ±5V Supplies
Output Latch
Inputs Can Exceed Supplies Without Phase Reversal
LT1711: 8-Lead MSOP Package
LT1712: 16-Lead Narrow SSOP Package
LT1711应用电路图：

来源：中电网