TDA7546 Datasheet, PDF (22/68 Pages) STMicroelectronics – Multichip module for TMC tuner applications

Functional description
TDA7546
5.1.4
charge current is fixed and the discharge current is controlled by I²C Bus. This level
produces digital signals (ac, ac+) in an additional comparator4. The adjacent channel
information is available as analog output signal after rectifier and +8dB output buffer.
Multipath detector
The input of the multipath detector is AC coupled from internal unweighted fieldstrength. A
programmable bandpass (MPF) and amplifier (MPG) as well as rectifier determines the
influences. This voltage is compared with an adjustable comparator2 thresholds (MPTH).
The output signal of this comparator2 is used for the "Milano" effect. In this case the
adjacent channel detection is switched off. The "Milano" effect is selectable by I²C Bus
(MPOFF). The multipath information is available as analog output signal after rectifier and
+8dB output buffer.
450KHz IF narrow bandpass filter (ISS filter)
The device gets an additional second IF narrow bandpass filter for suppression of noise and
adjacent channel signal influences. This narrow filter has three switchable bandwidthes,
narrow range of 80KHz, mid range of 120KHz and 30KHz for weather band information.
Without ISS filter the IF bandwidth (wide range) is defined only by ceramic filter chain. The
filter is switched in after mixer2 before 450KHz limiter stage. The centre frequency is
matching to the demodulator center frequency.
Deviation detector
In order to avoid distortion in audio output signal the narrow ISS filter is switched OFF for
present overdeviation. Hence the demodulator output signal is detected.
A lowpass filtering and peak rectifier generates a signal that is defined by software
controlled current (TDEV) in an external capacitance. This value is compared with a
programmable comparator3 thresholds (DWTH, DTH) and generates two digital signals
(dev, dev+). For weak signal condition deviation threshold is proportinal to FSU.
ISS switch logic
All digital signals coming from adjacent channel detector, deviation detector and softmute
are acting via switching matrix on ISS filter switch. The IF bandpass switch mode is
controlled by software (ISSON, ISS30, ISS80, CTLOFF).
The switch ON of the IF bandpass is also available by external manipulation of the voltage at
PIN15.
Two application modes are available (APPM). The conditions are described in Table 50.
PLL and IF counter section
PLL frequency synthesizer block
This part contains a frequency synthesizer and a loop filter for the radio tuning system. Only
one VCO is required to build a complete PLL system for FM world tuning . For auto search
stop operation an IF counter system is available.
The counter works in a two stages configuration. The first stage is a swallow counter with a
two modulus (32/33) precounter. The second stage is an 11-bit programmable counter.
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