SI4706-D50 Datasheet, PDF (19/36 Pages) Silicon Laboratories – HIGH-PERFORMANCE FM AND RDS/RBDS RECEIVER

Si4706-D50
4.5. De-emphasis
4.10. Seek
Pre-emphasis and de-emphasis is a technique used by
FM broadcasters to improve the signal-to-noise ratio of
FM receivers by reducing the effects of high-frequency
interference and noise. When the FM signal is
transmitted, a pre-emphasis filter is applied to
accentuate the high audio frequencies. The Si4706
incorporates a de-emphasis filter that attenuates high
frequencies to restore a flat frequency response. Two
time constants are used in various regions. The de-
emphasis time constant is programmable to 50 or 75 µs
and is set by the FM_DEEMPHASIS property.
4.6. Stereo DAC
High-fidelity stereo digital-to-analog converters (DACs)
drive analog audio signals onto the LOUT and ROUT
pins. The audio output may be muted. Volume is
adjusted digitally with the RX_VOLUME property.
4.7. Soft Mute
The soft mute feature is available to attenuate the audio
outputs and minimize audible noise in very weak signal
conditions. The soft mute feature is triggered by the
SNR metric. The SNR threshold for activating soft mute
is programmable, as are soft mute attenuation levels
and attack and decay rates.
4.8. FM Hi-Cut Control
Hi-cut control is employed on audio outputs with
degradation of the signal due to low SNR and/or
multipath interference. Two metrics, SNR and multipath
interference, are monitored concurrently in forcing hi-cut
of the audio outputs. Programmable minimum and
maximum thresholds are available for both metrics. The
transition frequency for hi-cut is also programmable with
up to seven hi-cut filter settings. A single set of attack
and release rates for hi-cut are programmable for both
metrics from a range of 2 ms to 64 s. The level of hi-cut
applied can be monitored with the FM_RSQ_STATUS
command. Hi-cut can be disabled by setting the hi-cut
filter to the default audio bandwidth of 15 kHz.
The Si4706 seek functionality is performed completely
on-chip and will search up or down the selected
frequency band for a valid channel. A valid channel is
qualified according to a series of programmable signal
indicators and thresholds. The seek function can be
made to stop at the band edge and provide an interrupt,
or wrap the band and continue seeking until arriving at
the original departure frequency. The device sets
interrupts with found valid stations or, if the seek results
in zero found valid stations, the device indicates failure
and again sets an interrupt. Refer to “AN332: Si47xx
Programming Guide.”
The Si4706-D50 uses RSSI, SNR, and AFC to qualify
stations. Most of these variables have programmable
thresholds for modifying the seek function according to
customer needs.
RSSI is employed first to screen all possible candidate
stations. SNR and AFC are subsequently used in
screening the RSSI qualified stations. The more
thresholds the system engages, the higher the
confidence that any found stations will indeed be valid
broadcast stations; however, the more challenging
levels the thresholds are set to, the longer the overall
seek time as more stations and more qualifiers will be
assessed. The Si4706-D50 defaults set RSSI to a mid-
level threshold and add an SNR threshold set to a level
delivering acceptable audio performance. This trade-off
will eliminate very low RSSI stations while keeping the
seek time to acceptable levels. Generally, the time to
auto-scan and store valid channels for an entire FM
band with all thresholds engaged is very short
depending on the band content.
Seek is initiated using the FM_SEEK_START
command. The RSSI and SNR threshold settings are
adjustable using properties.
4.9. Tuning
The frequency synthesizer uses Silicon Laboratories’
proven technology, including a completely integrated
VCO. 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 reference clock and adjusted
with an automatic frequency control (AFC) servo loop
during reception. The tuning frequency can be directly
programmed using the FM_TUNE_FREQ. The Si4706
supports channel spacing of 50, 100, or 200 kHz in FM
mode.
Rev. 1.0
19