Silicon Laboratories SI4421 Si4421, Detailed Feature-Leveldescription, RF Power Amplifier PA

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Si4421

Si4421

DETAILED FEATURE-LEVEL DESCRIPTION

The Si4421 FSK transceiver is designed to cover the unlicensed frequency bands at 433, 868 and 915 MHz. The device facilitates compliance with FCC and ETSI requirements.

The receiver block employs the Zero-IF approach with I/Q demodulation, allowing the use of a minimal number of external components in a typical application. The Si4421 incorporates a fully integrated multi-band PLL synthesizer, PA with antenna tuning, an LNA with switchable gain, I/Q down converter mixers, baseband filters and amplifiers, and an I/Q demodulator followed by a data filter.

PLL

The programmable PLL synthesizer determines the operating frequency, while preserving accuracy based on the on-chip crystal- controlled reference oscillator. The PLL’s high resolution allows the usage of multiple channels in any of the bands.

RF Power Amplifier (PA)

The power amplifier has an open-collector differential output and can directly drive different PCB antennas with a programmable output power level. An automatic antenna tuning circuit is built in to avoid costly trimming procedures and the so-called “hand effect”.

LNA

The LNA has approximately 250 Ohm input impedance, which functions well with the proposed antennas (see: Application Notes available from www.silabs.com/integration)

If the RF input of the chip is connected to 50 Ohm devices, an external matching circuit is required to provide the correct matching and to minimize the noise figure of the receiver.

The LNA gain can be selected in four steps (between 0 and -20dB relative to the highest gain) according to RF signal strength. It can be useful in an environment with strong interferers.

Baseband Filters

The receiver bandwidth is selectable by programming the bandwidth (BW) of the baseband filters. This allows setting up the receiver according to the characteristics of the signal to be received.

An appropriate bandwidth can be chosen to accommodate various FSK deviation, data rate and crystal tolerance requirements. The filter structure is 7th order Butterworth low- pass with 40 dB suppression at 2 · BW frequency. Offset cancellation is done by using a high-pass filter with a cut-off frequency below 7 kHz.

Full Baseband Amplifier Transfer Function

BW=67kHz

Data Filtering and Clock Recovery

Output data filtering can be completed by an external capacitor or by using digital filtering according to the final application.

Analog operation: The filter is an RC type low-pass filter followed by a Schmitt-trigger (St). The resistor (10 kOhm) and the St are integrated on the chip. An (external) capacitor can be chosen according to the actual bit rate. In this mode, the receiver can handle up to 256 kbps data rate. The FIFO cannot be used in this mode and clock is not provided for the demodulated data.

Digital operation: A digital filter is used with a clock frequency at 29 times the bit rate. In this mode, there is a clock recovery circuit (CR), which can provide synchronized clock to the data. Using this clock the received data can fill a FIFO. The CR has three operation modes: fast, slow, and automatic. In slow mode, its noise immunity is very high, but it has slower settling time and requires more accurate data timing than in fast mode. In automatic mode, the CR automatically changes between fast and slow mode. The CR starts in fast mode, then after locking, it automatically switches to slow mode

(Only the digital data filter and the clock recovery use the bit rate clock. For analog operation, there is no need for setting the correct bit rate.)

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Contents DESCRIPTION FUNCTIONAL BLOCK DIAGRAMFEATURES TYPICAL APPLICATIONSData Filtering and Clock Recovery DETAILED FEATURE-LEVELDESCRIPTIONRF Power Amplifier PA Si4421Data Validity Blocks Low Battery Voltage DetectorWake-UpTimer Crystal OscillatorType Si4421 PACKAGE PIN DEFINITIONSName FunctionName Si4421Internal Pin Connections Internal connectionNote These pins can be left floating PIN6 Logic Diagram FSK / DATA / nFFSPIN10 Logic Diagram nRES I/O Si4421Recommended supply decoupling capacitor values Si4421Typical Application Pin Function vs. Operation ModeRecommended Operating Range Si4421 GENERAL DEVICE SPECIFICATIONSAbsolute Maximum Ratings non-operating Si4421 ELECTRICAL SPECIFICATIONDC Characteristics Si4421 AC Characteristics PLL parametersAC Characteristics Receiver AC Characteristics Turn-on/Turnaroundtimings Si4421AC Characteristics Transmitter AC Characteristics OthersSi4421 Note 10 By designTiming Diagram Si4421 CONTROL INTERFACETiming Specification Si4421 Control CommandsControl Register Default Values 2. Power Management Command Description of the Control CommandsConfiguration Setting Command Si4421Logic connections between power control bits Si44215. Receiver Control Command Frequency Setting Command4. Data Rate Command Si4421VDI Logic Diagram Si44216. Data Filter Command Si44217.FIFO and Reset Mode Command Si442110. AFC Command 8. Synchron Pattern Command9. Receiver FIFO Read Command Si4421Si4421 Bit 5-4rl1 to rl0Si4421 11. TX Configuration Control CommandFrequency Setting Command Si4421 13. Transmitter Register Write Command12. PLL Setting Command Si4421 14. Wake-UpTimer Command15.Low Duty-CycleCommand Vlb= 2.25 + V · 0.1 V Clock divider configuration Si4421Bit Name 17. Status Read CommandSi4421 Si4421 INTERRUPT HANDLING TX REGISTER BUFFERED DATA TRANSMISSION Si4421Si4421 Typical TX register usageSi4421 RX FIFO BUFFERED DATA READ RECOMMENDED PACKET STRUCTURESBit Rate 9.6 kbps Si4421 CRYSTAL SELECTION GUIDELINESBit Rate 2.4 kbps Bit Rate 38.4 kbpsSi4421 RX-TXALIGNMENT PROCEDURES Si4421 RESET MODES SW Reset Command Sensitive Reset Enabled, Ripple on VddFIFO and Reset Mode Command page Si4421Si4421 TYPICAL PERFORMANCE CHARACTERISTICS Channel Selectivity and BlockingBER Curves in 868 MHz Band Si4421BER Curves in 433 MHz Band Si4421 434 MHzEvaluation Board with 50 Ohm Matching Network Si4421 REFERENCE DESIGNSSchematics Frequency Dependent Component ValuesSi4421 PCB Layout Top View Bottom ViewEvaluation Board with Resonant PCB Antenna BIFA SchematicsSi4421 Frequency Dependent Component ValuesTop View Bottom View Si4421PCB Layout Antenna designed for 868/915 MHz band See Detail “A” Section B-B Si4421 PACKAGE INFORMATION16-pinTSSOP Demo Boards and Development Kits RELATED PRODUCTS AND DOCUMENTSSi4421 Universal ISM Band FSK Transceiver Related ResourcesSi4421