SI4704-D60 Datasheet, PDF (24/39 Pages) Silicon Laboratories – BROADCAST FM RADIO RECEIVER WITH RDS/RBDS

Si4704/05-D60
4.11. 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 Si4704/05-
D60 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.
4.12. RDS/RBDS Processor (Si4705-D60
Only)
The Si4705-D60 implements an RDS/RBDS* processor
for symbol decoding, block synchronization, error
detection, and error correction.
The Si4705-D60 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 Si4705-D60 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. The pilot does not have to be present to
decode RDS/RBDS.
*Note: RDS/RBDS is referred to only as RDS throughout the
remainder of this document.
4.13. Tuning
The tuning frequency is directly programmed using the
FM_TUNE_FREQ command. The Si4704/05-D60
supports channel spacing steps of 10 kHz in FM mode.
4.14. Seek
The Si4704/05-D60 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 Si4704/05-D60 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. The Si4704/05-D60 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, SNR, and AFC threshold settings
are adjustable using properties.
4.15. Reference Clock
The Si4704/05-D60 reference clock is programmable,
supporting RCLK frequencies listed in Table 9,
“Reference Clock and Crystal Characteristics,” on
page 15. Refer to Table 2, “DC Characteristics,” on
page 6 for switching voltage levels and Table 9 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 "2. Typical Application
Schematic" on page 17. This mode is enabled using the
POWER_UP command. Refer to “AN332: Si47xx
Programming Guide”.
The Si4704/05-D60 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 Si4704/05-D60 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 Si4704/05-D60 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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Rev. 1.2