SI4736-B20 Datasheet, PDF (19/38 Pages) Silicon Laboratories – BROADCAST WEATHER BAND RADIO RECEIVER

Si4736/37/38/39-B20
4.2. Operating Modes
The Si4736/37/38/39 operates in an FM receive, an AM
receive, or a weather band receive mode. In FM mode
and WB mode, radio signals are received on FMI (pin 2)
and processed by the FM front-end circuitry. In AM
mode, radio signals are received on AMI (pin 4) and
processed by the AM front-end circuitry. In addition to
the receiver mode, there is a clocking mode to choose
to clock the Si473x from a reference clock or crystal. On
the Si4737/39, there is an audio output mode to choose
between an analog and/or digital audio output. In the
analog audio output mode, pin 13 is ROUT, pin 14 is
LOUT, and pin 17 is GPO3. In the digital audio mode,
pin 15 is DOUT, pin 16 is DFS, and pin 17 is DCLK.
Concurrent analog/digital audio output mode requires
pins 13, 14, 15, 16, and 17. The receiver mode and the
audio output mode are set by the POWER_UP
command listed in Table 12. Si473x Command
Summary.
4.3. FM Receiver
The Si4736/37/38/39 FM receiver is based on the
proven Si4700/01 FM tuner. The receiver uses a digital
low-IF architecture, allowing the elimination of external
components and factory adjustments. The
Si4736/37/38/39 integrates a low noise amplifier (LNA)
supporting the worldwide FM broadcast band (76 to 108
MHz). An AGC circuit controls the gain of the LNA to
optimize sensitivity and rejection of strong interferers.
For testing purposes, the AGC can be disabled. Refer to
Section "5. Commands and Properties" on page 25 for
additional programming and configuration information.
An image-reject mixer downconverts the RF signal to
low-IF. The quadrature mixer output is amplified,
filtered, and digitized with high resolution analog-to-
digital converters (ADCs). This advanced architecture
allows the Si4736/37/38/39 to perform channel
selection, FM demodulation, and stereo audio
processing to achieve superior performance compared
to traditional analog architectures.
4.4. AM Receiver (Si4736/37)
The highly integrated Si4736/37 supports worldwide AM
band reception from 520 to 1710 kHz using a digital
low-IF architecture with a minimum number of external
components and no manual alignment required. This
digital low-IF architecture allows for high-precision
filtering and offers excellent selectivity and noise
suppression. The DSP also provides 9 or 10 kHz
channel selection, AM demodulation, soft mute, and
additional features such as adjustable channel
bandwidth settings. Similar to the FM receiver, the
integrated LNA and AGC optimize sensitivity and
rejection of strong interferers allowing better reception
of weak stations.
The Si4736/37 provides highly accurate digital AM
tuning without factory adjustments. To offer maximum
flexibility, the receiver supports a wide range of ferrite
loop sticks from 180–450 µH. An air loop antenna is
supported by using a transformer to increase the
effective inductance from the air loop. Using a 1:5 turn
ratio inductor, the inductance is increased by 25 times
and easily supports all typical AM air loop antennas,
which generally vary between 10 and 20 µH.
4.5. Weather Band Receiver
The Si4736/37/38/39 supports weather band reception
from 162.4 to 162.55 MHz. The highly integrated
Si4736/37/38/39 meets NOAA specification, receives all
seven NOAA specified frequencies, implements narrow-
band FM de-emphasis, and supports 1050 Hz alert tone
detection. In addition, the Si4736/37/38/39 provides
advanced features not available on conventional radios,
such as an AFC and a dynamic channel bandwidth filter.
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.
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