SI4712-B30 Datasheet, PDF (22/42 Pages) Silicon Laboratories – FM RADIO TRANSMITTER WITH RECEIVE POWER SCAN

Si4712/13-B30
5.3.1. Stereo Encoder
Figure 14 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 14, 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 14. MPX Encoder
Figure 14 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 14 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 Si4712/13, 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 14, the Transmit Audio Deviation is
programmed with the value 6750, the Transmit Pilot
Deviation with 750, and the Transmit RDS Deviation
with 200, generating peak audio frequency deviations of
67.5 kHz, peak pilot deviations of 7.5 kHz, and peak
RDS deviations of 2.0 kHz for a total peak frequency
deviation of 77 kHz. The total peak transmit frequency
deviation of the Si4712/13 can range from 0 to 100 kHz
and is equal to the arithmetic sum of the Transmit
Audio, Pilot, and RDS deviations. Users must comply
with local regulations on radio frequency transmissions.
Each of the individual deviations (transmit audio, pilot,
and RDS) can be independently programmed; however,
the total peak frequency deviation cannot exceed
100 kHz.
The Si4712/13 provides an overmodulation indicator to
allow the user to dynamically set the maximum
deviation level. If the instantaneous frequency exceeds
the deviation level specified by the
TX_AUDIO_DEVIATION property, the SQINT interrupt
bit (and optional interrupt) will be set.
22
Rev. 1.1