SI4706-D50 Datasheet, PDF (18/36 Pages) Silicon Laboratories – HIGH-PERFORMANCE FM AND RDS/RBDS RECEIVER

Si4706-D50
The Si4706 is feature-rich, providing highly automated
performance with default settings and extensive
programmability and flexibility for customized system
performance.
The Si4706 performs much of the FM demodulation
digitally to achieve high fidelity, optimal performance
versus power consumption, and flexibility of design. The
on-board DSP provides unmatched pilot rejection,
selectivity, and optimum sound quality. The integrated
micro-controller offers both the manufacturer and the
end-user unmatched programmability and flexibility in
the listening experience.
4.2. FM Receiver
The Si4706 FM receiver is based on the proven
Si4700/01/02/03 FM radio receiver. The part leverages
Silicon Laboratories' proven and patented FM broadcast
radio receiver digital architecture, delivering excellent RF
performance and interference rejection. The proven
digital techniques provide good sensitivity in weak signal
environments while allowing superb selectivity and inter-
modulation immunity in strong signal environments.
The part supports the worldwide FM broadcast band (64
to 108 MHz) with channel spacings of 50–200 kHz. The
low-IF architecture utilizes a single converter stage and
digitizes the signal using a high-resolution analog-to-
digital converter. The audio output can be directed
either to an external headphone amplifier via analog
in/out or to other system ICs through digital audio
interface (I²S).
4.3. Stereo Audio Processing
The output of the FM demodulator is a stereo
multiplexed (MPX) signal. The MPX standard was
developed in 1961, and is used worldwide. Today's
MPX signal format consists of left + right (L+R) audio,
left – right (L–R) audio, a 19 kHz pilot tone, and
RDS/RBDS data as shown in Figure 8.
Mono Audio
Left + Right
Stereo
Pilot
Stereo Audio
Left - Right
RDS/
RBDS
0
15 19 23
38
53 57
Frequency (kHz)
Figure 8. MPX Signal Spectrum
4.3.1. Stereo Decoder
The Si4706 integrated stereo decoder automatically
decodes the MPX signal using DSP techniques. The 0–
15 kHz (L+R) signal is the mono output of the FM tuner.
Stereo is generated from the (L+R), (L–R), and a 19 kHz
pilot tone. The pilot tone is used as a reference to
recover the (L–R) signal. Output left and right channels
are obtained by adding and subtracting the (L+R) and
(L–R) signals, respectively.
4.3.2. Stereo-Mono Blending
Adaptive noise suppression is employed to gradually
combine the stereo left and right audio channels to a
mono (L+R) audio signal as the signal quality degrades
to maintain optimum sound fidelity under varying
reception conditions. Three metrics, received signal
strength indicator (RSSI), signal-to-noise ratio (SNR),
and multipath interference, are monitored
simultaneously in forcing a blend from stereo to mono.
The metric which reflects the minimum signal quality
takes precedence and the signal is blended
appropriately.
All three metrics have programmable stereo/mono
thresholds and attack/release rates detailed in “AN322:
Si47xx Programming Guide.” If a metric falls below its
mono threshold, the signal is blended from stereo to full
mono. If all metrics are above their respective stereo
thresholds, then no action is taken to blend the signal. If
a metric falls between its mono and stereo thresholds,
then the signal is blended to the level proportional to the
metric’s value between its mono and stereo thresholds,
with an associated attack and release rate.
Stereo/mono status can be monitored with the
FM_RSQ_STATUS command.
4.4. Received Signal Qualifiers
The quality of a tuned signal can vary depending on
many factors including environmental conditions, time of
day, and position of the antenna. To adequately manage
the audio output and avoid unpleasant audible effects to
the end-user, the Si4706-D50 monitors and provides
indicators of the signal quality, allowing the host
processor to perform additional processing if required
by the customer. The Si4706-D50 monitors signal
quality metrics including RSSI, SNR, and multipath
interference on FM signals. These metrics are used to
optimize signal processing and are also reported to the
host processor. The signal processing algorithms can
use either Silicon Labs' optimized settings
(recommended) or be customized to modify
performance.
18
Rev. 1.0