SI4720-B20-GMR Datasheet, PDF (28/48 Pages) Silicon Laboratories – BROADCAST FM RADIO TRANSCEIVER FOR PORTABLE APPLICATIONS

Si4720/21-B20
5.7. De-Emphasis
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 Si4720/21
incorporates a de-emphasis filter which 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.
5.8. 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.
5.9. Soft Mute
The soft mute feature is available to attenuate the audio
outputs and minimize audible noise in very weak signal
conditions. The softmute attenuation level is adjustable
using the FM_SOFT_MUTE_MAX_ATTENUATION
property.
5.10. RDS/RBDS Processor (Si4721 Only)
The Si4721 implements an RDS/RBDS* processor for
symbol decoding, block synchronization, error
detection, and error correction.
The Si4721 device is user configurable and provides an
optional interrupt when RDS is synchronized, loses
synchronization, and/or the user configurable RDS
FIFO threshold has been met.
The Si4721 reports RDS decoder synchronization
status and detailed bit errors in the information word for
each RDS block with the FM_RDS_STATUS command.
The range of reportable block errors is 0, 1–2, 3–5, or
6+. More than six errors indicates that the
corresponding block information word contains six or
more non-correctable errors or that the block checkword
contains errors.
*Note: RDS/RBDS is referred to only as RDS throughout the
remainder of this document.
5.11. 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 and
command. The Si4720/21 supports channel spacing of
50, 100, or 200 kHz in FM receiver mode.
5.12. Seek
Seek tuning will search up or down for a valid channel.
Valid channels are found when the receive signal
strength indicator (RSSI) and the signal-to-noise ratio
(SNR) values exceed the set threshold. Using the SNR
qualifier rather than solely relying on the more
traditional RSSI qualifier can reduce false stops and
increase the number of valid stations detected. Seek is
initiated using the FM_SEEK_START command. The
RSSI and SNR threshold settings are adjustable using
properties (see Table 15).
Two seek options are available. The device will either
wrap or stop at the band limits. If the seek operation is
unable to find a channel, the device will indicate failure
and return to the channel selected before the seek
operation began.
5.13. Reference Clock
The Si4720/21 reference clock is programmable,
supporting RCLK frequencies in Table 13. Refer to
Table 3, “DC Characteristics,” on page 6 for switching
voltage levels and Table 9, “FM Receiver
Characteristics,” on page 13 for frequency tolerance
information.
An onboard crystal oscillator is available to generate the
32.768 kHz reference when an external crystal and load
capacitors are provided. Refer to "3. Typical Application
Schematic" on page 20. This mode is enabled using the
POWER_UP command. Refer to Table 21 "Si472x
Property Summary".
The Si4720/21 performance may be affected by data
activity on the SDIO bus when using the integrated
internal oscillator. SDIO activity results from polling the
tuner for status or communicating with other devices
that share the SDIO bus. If there is SDIO bus activity
while the Si4720/21 is performing the seek/tune
function, the crystal oscillator may experience jitter,
which may result in mistunes, false stops, and/or lower
SNR.
For best seek/tune results, Silicon Laboratories
recommends that all SDIO data traffic be suspended
during Si4720/21 seek and tune operations. This is
achieved by keeping the bus quiet for all other devices
on the bus, and delaying tuner polling until the tune or
seek operation is complete. The seek/tune complete
(STC) interrupt should be used instead of polling to
determine when a seek/tune operation is complete.
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