AN332 Datasheet, PDF (233/302 Pages) Silicon Laboratories – Si47XX PROGRAMMING GUIDE

AN332
11.3. Audio Limiter for FM Transmitter
A limiter is available to prevent overmodulation by dynamically attenuating the audio level such that the maximum
audio deviation does not exceed the level set by the TX_AUDIO_DEVIATION property. The limiter is useful when
trying to maximize the audio volume, minimize receiver-generated distortion and prevent overmodulation that may
result in violating FCC and ETSI modulation limits. The OVERMOD bit is set by the device when the peak voltage
prior to the limiter exceeds the level set by the TX_AUDIO_DEVIATION property. When the limiter is enabled, the
OVERMOD bit is an indication that the limiter has dynamically attenuated the audio level. The limiter attack time is
instantaneous (within on sample period) and the release time is adjustable with the TX_LIMITER_RELEASE_TIME
property.
Note: Limiter is enabled by default.
11.4. Maximizing Audio Volume for FM Transmitter
The audio input chain is shown in Figure 33:
LILEVEL
LIATTEN
ACEN
ACTHRESH
ACATTACK
ACRELEASE
ACGAIN
PREEMPH
LIMITEN
FROM
INPUT
PGA
ADC
COMPRESSOR
PRE-EMPHASIS
LIMITER
TO
MODULATOR
INLEVEL
IALDH
IALDL
OVERMOD
Figure 33. Audio Input Chain
To maximize audio volume:
1. Set the input line attenuation, line level and audio deviation.
The input line attenuation should be set to the lowest setting that is above the maximum level provided by the
audio source, either 190, 301, 416 or 636 mV_PK.
The line level should be set to the maximum source audio level plus headroom. When the limiter is enabled,
2 dB of headroom is recommended. 2 dB of headroom is recommended so that the limiter will not be engaged
the entire time it is enabled. When the limiter is disabled and 50 µs pre-emphasis is selected,
13.5 dB of headroom is required. When the limiter is disabled and 75 µS pre-emphasis is selected, 17 dB of
headroom is required. Table 50 summarizes these settings:
Table 50. Line Input Headroom
Pre-emphasis Limiter On (dB) Limiter Off (dB)
Off
0
0
50 µs
0
13.5
75 µs
0
17
Rev. 1.0
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