SI4707-B20 Datasheet, PDF (15/30 Pages) Silicon Laboratories – WEATHER BAND AND SAME DATA RECEIVER

Si4707-B20
4.2. Block Diagram Description
4.3. Weather Band Receiver
The Si4707 IC integrates the voltage-controlled
oscillator (VCO) and frequency synthesizer and accepts
a wide-range of programmable reference clocks
(RCLK). The IC also supports a dedicated external
crystal with an integrated crystal oscillator. The
frequency synthesizer generates the quadrature local
oscillator signal used to downconvert the RF input to a
low intermediate frequency. The VCO frequency is
locked to the RCLK and adjusted with an automatic
frequency control (AFC) servo loop during reception.
The VCO frequency is modified according to the target
frequency and varies according to the tuned channel.
The Si4707 uses a digital low-IF architecture,
integrating the entire analog receive chain for WB. The
IC also integrates the functionality of most external
components typically found in competing solutions and
performs all processing in an on-chip digital signal
processor (DSP) and microcontroller (MCU) core. The
analog chain includes a dedicated low-noise amplifier
(LNA), automatic gain control (AGC), image-reject
quadrature mixer, programmable gain amplifier (PGA),
and a set of delta-sigma high-performance ADCs. The
LNA block receives wide-band frequency input at the
FMI input pin. The on-chip resistor blocks control the
gain of the external WB antenna network. The LNA gain
is dynamically controlled by the AGC loop, contingent
on the RF peak detectors and signal strength. The
receive path continues to dedicated quadrature mixers
that downconvert the received signal from RF to low-IF,
filter for out-of-band interferers, and perform a transfer
function to shift the tuned frequency to DC. A single pair
of PGAs filters the multiplexed (MPX) mixer output from
interferers and amplifies the signal again before
delivering it to two high-resolution analog-to-digital
converters (ADC). The digital core performs channel
selection and filtering, digital calibrated tuning, FM
demodulation, and SAME demodulation/decoding. The
core also performs signal quality processing including
received signal strength indicators (RSSI) and SNR.
The Si4707 supports weather band reception from
162.4 to 162.55 MHz. The highly-integrated Si4707
meets NOAA and Weather Radio Canada
specifications, receives all seven specified frequencies,
implements narrow-band FM de-emphasis, and
supports 1050 Hz alert tone detection. In addition, the
Si4707 provides advanced features not available on
conventional radios, such as an AFC, a dynamic
channel bandwidth filter, and RSSI and SNR receive
signal quality indicators.
The AFC locks on to the strongest signal within a
narrow, adjustable frequency range to compensate for
any potential frequency errors such as crystal tolerance
or transmit frequency errors. The AFC ensures the
channel filter is always centered on the desired channel
providing optimal reception. The dynamic channel
bandwidth feature utilizes a wide filter in strong signal
conditions to provide best sound quality and a narrower
filter in weak conditions to provide best sensitivity.
4.4. DAC
High-fidelity stereo digital-to-analog converters (DACs)
drive analog audio signals onto the LOUT and ROUT
pins. Weather band stations broadcast in mono only, so
LOUT and ROUT audio will be the same. The audio
output may be muted. Volume is adjusted digitally with
the RX_VOLUME property.
4.5. SAME Processor
The Si4707 implements a high-performance SAME
processor for demodulation, byte-wise data quality
metrics, and advanced error correction beyond the
commonly used polling method. The SAME decoder
draws on soft decision decoding techniques to provide
robust performance and delivers reception in
environments where signal power is very low or
compromised.
The Si4707 device stores the entire SAME message
and provides a user programmable interrupt at the end
of message detected, start of message detected,
preamble detected, and/or header buffer ready. It also
reports on the confidence level of each byte of data with
a value of 0–3, with 3 representing the highest
confidence level. This feature, unique to the Si4707,
provides extra visibility into the message accuracy,
which helps limit the number of false messages in poor
reception areas.
Rev. 0.8
15