Si4720-A10-GM Silicon Laboratories Inc, Si4720-A10-GM Datasheet - Page 29

• Manufacturer Part Number: Si4720-A10-GM
• Description: RF Transceiver Broadcast FM Radio Transceiver
• Manufacturer: Silicon Laboratories Inc

Specifications of Si4720-A10-GM

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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
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
shows
an
example
modulation
level
Rev. 1.0
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
where Δf is the frequency deviation; K
voltage-to-frequency gain constant, and A
amplitude of the MPX message signal. For a fixed
K
the MPX message signal must be scaled to give the
appropriate total frequency deviation.
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) = C
where C
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
K
set to 0.1, and C
audio frequency deviation of 0.9 x 75 kHz = 67.5 kHz, a
peak
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
commands with an accuracy of 10 Hz. For the example
in
RDS(t)
VCO
VCO
VCO
L(t)
R(t)
Figure 20,
, as given by the following equation:
, the amplitude of all the subchannel signals within
Transmit
= 75 kHz/V, C
+ C
+ C
modulation
0
0
[L(t) + R(t)] + C
0
2
, C
[L(t) – R(t)] cos(2
RDS(t) cos(2
pilot
Figure 20. MPX Encoder
1
, and C
Audio,
38 kHz
2
57 kHz
the
would be set to 0.0267 giving a peak
0
Δf
would be set to 0.45; C
frequency
levels
Transmit
Si4720/21-B20
2
π
=
1
57 kHz)
MPX Encoder
are gains used to scale the
Pilot,
cos(2
K
Frequency
Frequency
Doubler
Tripler
VCO
π
38 kHz)
π
A
of
19 kHz)
m
and
Audio
19 kHz
Figure 20
deviation
RDS
Deviation
C
C
C
C
2
0
1
0
VCO
1
m
Deviation
would be
is the
is the
with
m(t)
29
of
is