TDA7546 datasheet(21/68 Pages) STMICROELECTRONICS

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TDA7546

Functional description

21/68

Functional description

5.1

Tuner

5.1.1

Mixer1, AGC and 1.IF

The FM quadrature I/Q-mixer converts RF to IF1 of 10.7MHz. The mixer provides inherent

image rejection and wide dynamic range with low noise and large input signal performance.

The mixer1 tank can be adjusted by software (IF1T). For accurate image rejection the gain-

and phase-error generated as well in mixer as VCO stage can be compensated by software

(G,PH)

It is capable of tuning the US FM, US weather, Europe FM, Japan FM and East Europe FM

bands

–

US FM = 87.9 to 107.9 MHz

–

US weather = 162.4 to 162.55 MHz

–

Europe FM = 87.5 to 108 MHz

–

Japan FM = 76 to 91 MHz

–

East Europe FM = 65.8 to 74 MHz

The AGC operates on different sensitivities and bandwidths in order to improve the input

sensitivity and dynamic range. AGC thresholds are programmable by software

(RFAGC,IFAGC,KAGC). The output signal is a controlled current for double pin diode

attenuator. Two 10.7MHz programmable amplifiers (IFG1, IFG2) correct the IF ceramic

insertion loss and the costumer level plan application.

5.1.2

Mixer2, limiter and demodulator

In this 2. mixer stage the first 10.7MHz IF is converted into the second 450KHz IF. A multi-

stage limiter generates signals for the complete integrated demodulator without external

tank. MPX output DC offset versus noise DC level is correctable by software (DEM).

5.1.3

Quality detection and ISS

Fieldstrength

Parallel to mixer2 input a 10.7MHz limiter generates a signal for digital IF counter and a

fieldstrength output signal. This internal unweighted fieldstrength is used for keying AGC,

adjacent channel and multipath detection and is available at PIN14 (FSU) after +6dB buffer

stage. The behaviour of this output signal can be corrected for DC offset (SL) and slope

(SMSL). The internal generated unweighted fieldstrength is filtered at PIN35 and used for

softmute function and generation of ISS filter switching signal for weak input level (sm).

Adjacent channel detector

The input of the adjacent channel detector is AC coupled from internal unweighted

fieldstrength. A programmable highpass or bandpass (ACF) and amplifier (ACG) as well as

rectifier determines the influences. This voltage is compared with adjustable comparator1

thresholds (ACWTH, ACNTH). The output signal of this comparator generates a DC level at

PIN15 by programmable time constant. Time control (TISS) for a present adjacent channel

is made by charge and discharge current after comparator1 in an external capacitance. The

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