TDA7546 datasheet(22/68 Pages) STMICROELECTRONICS

22 / 68 page

Functional description

TDA7546

22/68

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.

5.1.4

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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