SI4420-D1-FT Datasheet, PDF (2/33 Pages) List of Unclassifed Manufacturers – Si4420 Universal ISM Band FSK Transceiver

DETAILED FEATURE-LEVEL DESCRIPTION
The Si4420 FSK transceiver is designed to cover the unlicensed
frequency bands at 315, 433, 868 and 915 MHz. The devices
facilitate 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 Si4420 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.
The RF VCO in the PLL performs automatic calibration, which
requires only a few microseconds. Calibration always occurs
when the synthesizer starts. If temperature or supply voltage
changes significantly or operational band has changed, VCO
recalibration is recommended.. Recalibration can be initiated at
any time by switching the synthesizer off and back on again.
RF Power Amplifier (PA)
The power amplifier has an open-collector differential output and
can directly drive a loop antenna 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 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 (0, –6, –14, –20 dB 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.
Si4420
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 can not 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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