TDA7541 datasheet(27/76 Pages) STMICROELECTRONICS

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TDA7541

Functional description

27/76

Functional description

4.1

FM part

4.1.1

Mixer 1 AGC and IF amplifier

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

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

For accurate image rejection the phase-error of I/Q can be compensated by software (PH).

It is capable of tuning the US FM, US weather (dedicated WB input), 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, and KAGC). The output signal is a controlled current for pin diode attenuator.

A 10.7MHz programmable amplifier (IFG) correct the IF ceramic insertion loss and the

costumer level plan application.

4.1.2

Mixer2, limiter, FM demodulator and spike cancellation

In the 2nd mixer stage the first 10.7MHz IF is converted into the second 450 kHz IF.

The fully integrated sample and hold FM demodulator including spike cancellation (DSB)

converts the IF signal from the 450KHz limiter (limiter gain 80dB typ.) to the FM multiplex

output signal with very low distortion.

The sensitivity of spike blanking can be set via I2C-bus.

4.1.3

Quality detection and ISS

Field strength

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

strength voltage VFSUint. This internal voltage VFSUint is used for AM IF noise blanker,

adjacent channel and multi path detection and is available at PIN27 (FSU) in a range of 0V

to 5V. The offset and slope of FSU signal can be adjusted via IIC-bus (FSWO and SL) for

application adaptation.

The voltage VFSWO including offset adjust is externally filtered at PIN37 (FSWO) and used

for weak signal mute function and generation of ISS filter control signals in weak signal

condition. It is possible to combine the IF counter result with this voltage VFSWO by

programmable comparator threshold (SSTH).

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