TDA7541 Datasheet, PDF (27/76 Pages) STMicroelectronics – AM/FM car radio tuner IC with stereo decoder and intelligent selectivity system

TDA7541
4
Functional description
Functional description
4.1
4.1.1
4.1.2
4.1.3
FM part
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.
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.
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 V_FSUint. This internal voltage V_FSUint 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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