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MICRF004 Datasheet(PDF) 7 Page - Micrel Semiconductor |
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MICRF004 Datasheet(HTML) 7 Page - Micrel Semiconductor |
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7 / 16 page  February 9, 2000 7 MICRF004/RF044 MICRF004/RF044 Micrel Functional Description Refer to “MICRF004 Block Diagram”. Identified in the block diagram are the four sections of the IC: UHF Downconverter, OOK Demodulator, Reference and Control, and Wakeup. Also shown in the figure are two capacitors (C TH, CAGC) and one timing component (CR), usually a ceramic resonator. With the exception of a supply decoupling capacitor, these are the only external components needed by the MICRF004 to assemble a complete UHF receiver. Four control inputs are shown in the block diagram: SEL0, SEL1, SWEN, and SHUT. Using these logic inputs, the user can control the operating mode and selectable features of the IC. These inputs are CMOS compatible, and are pulled-up on the IC. Sweep-Mode Enable Logic-input SWEN selects either fixed-mode or sweep-mode operation. When SWEN is low, the IC is in fixed mode, and functions as a conventional superheterodyne receiver. When SWEN is high, the IC is in sweep mode. Fixed-Mode Operation For applications where the transmit frequency must be accu- rately set (that is, applications where a SAW transmitter is used for its mechanical stability), the MICRF004 may be configured as a standard superheterodyne receiver (fixed mode). Fixed-mode operation receives a narrower band- width making it less susceptable to competing signals. Fixed mode is selected by connecting SWEN to ground which forces the on-chip LO frequency to a fixed value. In fixed mode a crystal (higher frequency tolerance) must be used instead of a ceramic resonator (lower frequency tolerance). Data rates beyond 10kb/s are possible in fixed mode. Functional Diagram Peak Detector AGC Control 2nd Order Programmable Low-Pass Filter 5th Order Band-Pass Filter Programmable Synthesizer Control Logic RSC Resettable Counter Reference Oscillator CR Ceramic Resonator CAGC ANT SEL0 VDD VSS SEL1 SWEN REFOSC 500kHz Switched- Capacitor Resistor WAKEB CTH DO MICRF004 RF Amp IF Amp IF Amp Compa- rator Wakeup Reference and Control VHF Downconverter OOK Demodulator fRX fLO fIF SHUT CAGC CTH Sweep-Mode Operation In sweep mode, while the topology is still superheterodyne, the LO (local oscillator) is deterministically swept over a range of frequencies at rates greater than the data rate. When coupled with a peak-detecting demodulator, this technique effectively increases the RF bandwidth of the MICRF004, allowing the device to operate in applications where signifi- cant transmitter-receiver frequency misalignment may exist. The swept-LO technique does not affect the IF bandwidth, therefore noise performance is not degraded relative to fixed mode. The IF bandwidth is 500kHz whether the device is operating in fixed or sweep mode. Due to limitations imposed by the LO sweeping process, the upper limit on data rate in sweep mode is approximately 2.5kb/s. Examples of sweep-mode operation include applications utilizing low-cost LC-based transmitters, where the transmit frequency may vary up to ±0.5% over initial tolerance, aging, and temperature. In sweep mode, the LO frequency is varied in a defined fashion which results in downconversion of all signals in a band approximately 1.5% around the nominal transmit frequency. The transmitter may drift up to ±0.5% without the need to retune the receiver and without impacting system performance. Similar performance is not currently available with crystal-based superheterodyne receivers which can operate only with SAW- or crystal-based transmitters. In sweep mode only, a range reduction will occur in installa- tions where there is an undesired competing signal of suffi- cient strength within of 2% to 3% around the transmit fre- quency. This is because the process indiscriminately in- |
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