SI4720-B20-GMR Datasheet, PDF (29/48 Pages) Silicon Laboratories – BROADCAST FM RADIO TRANSCEIVER FOR PORTABLE APPLICATIONS

Si4720/21-B20
5.14. FM Transmitter
The transmitter (TX) integrates a stereo audio ADC to
convert analog audio signals to high fidelity digital
signals. Alternatively, digital audio signals can be
applied to the Si4720/21 directly to reduce power
consumption by eliminating the need to convert audio
baseband signals to analog and back again to digital.
Digital signal processing is used to perform the stereo
MPX encoding and FM modulation to a low digital IF.
Transmit baseband filters suppress out-of-channel
noise and images from the digital low-IF signal. A
quadrature single-sideband mixer up-converts the
digital IF signal to RF, and internal RF filters suppress
noise and harmonics to support the harmonic emission
requirements of cellular phones, GPS, WLAN, and other
wireless standards.
The TXO output has over 10 dB of output level control,
programmable in approximately 1 dB steps. This large
output range enables a variety of antennas to be used
for transmit, such as a monopole stub antenna or a loop
antenna. The 1 dB step size provides fine adjustment of
the output voltage.
The TXO output requires only one external 120 nH
inductor. The inductor is used to resonate the antenna
and is automatically calibrated within the integrated
circuit to provide the optimum output level and
frequency response for supported transmit frequencies.
Users are responsible for adjusting their system’s
radiated power levels to comply with local regulations
on RF transmission (FCC, ETSI, ARIB, etc.).
5.15. Receive Power Scan
The Si4720/21 is the industry’s first FM transmitter with
integrated receive functionality to measure received
signal strength. This has been designed to specifically
handle various antenna lengths including integrated
PCB antennas, wire antennas, and loop antennas,
allowing it to share the same antenna as the transmitter.
The receive function reuses the on-chip varactor from
the transmitter to optimize the receive signal power
applied to the front-end amplifier. Auto-calibration of the
varactor occurs with each tune command for consistent
performance across the FM band.
5.15.1. Stereo Encoder
Figure 19 shows an example modulation level
breakdown for the various components of a typical MPX
signal.
The total modulation level for the MPX signal shown in
Figure 19, assuming no correlation, is equal to the
arithmetic sum of each of the subchannel levels
resulting in 102.67 percent modulation or a peak
frequency deviation of 77.0025 kHz (an instantaneous
frequency deviation of 75 kHz corresponds to 100
percent modulation). Frequency deviation is related to
the amplitude of the MPX signal by a gain constant,
K_VCO, as given by the following equation:
f = K_VCOA_m
where f is the frequency deviation; K_VCO is the
voltage-to-frequency gain constant, and A_m is the
amplitude of the MPX message signal. For a fixed
K_VCO, the amplitude of all the subchannel signals within
the MPX message signal must be scaled to give the
appropriate total frequency deviation.
RDS(t)
L(t)
R(t)
MPX Encoder
57 kHz
C2
m(t)
C0
38 kHz
Frequency
Tripler
Frequency
Doubler
C1
19 kHz
C0
Figure 20. MPX Encoder
Figure 20 shows a conceptual block diagram of an MPX
encoder used to generate the MPX signal. L(t) and R(t)
denote the time domain waveforms from the left and
right audio channels, and RDS(t) denotes the time
domain waveform of the RDS/RBDS signal.
The MPX message signal can be expressed as follows:
m(t) = C₀[L(t) + R(t)] + C₁ cos(219 kHz)
+ C₀[L(t) – R(t)] cos(238 kHz)
+ C₂RDS(t) cos(257 kHz)
where C₀, C₁, and C₂ are gains used to scale the
amplitudes of the audio signals (L(t) ± R(t)), the 19 kHz
pilot tone, and the RDS subcarrier respectively, to
generate the appropriate modulation level. To achieve
the modulation levels of Figure 20 with
K_VCO = 75 kHz/V, C₀ would be set to 0.45; C₁ would be
set to 0.1, and C₂ would be set to 0.0267 giving a peak
audio frequency deviation of 0.9 x 75 kHz = 67.5 kHz, a
peak
pilot
frequency
deviation
of
0.1 x 75 kHz = 7.5 kHz, and a peak RDS frequency
deviation of 0.0267 x 75 kHz = 2.0025 kHz for a total
peak frequency deviation of 77.0025 kHz.
In the Si4720/21, the peak audio, pilot, and RDS
frequency deviations can be programmed directly with
the Transmit Audio, Pilot, and RDS Deviation
commands with an accuracy of 10 Hz. For the example
in Figure 20, the Transmit Audio Deviation is
Rev. 1.0
29