TDA7540N Datasheet, PDF (27/76 Pages) STMicroelectronics – AM/FM car radio tuner IC with stereo decoder and intelligent selectivity system (ISS)

TDA7540N
4
Functional description
Functional description
4.1
4.1.1
4.1.2
4.1.3
FM section
Mixer1, AGC and 1.IF
FM quadrature 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.
The mixer1 tank can be adjusted by software (IF1T). 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, 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 pin diode attenuator.
A 10.7MHz programmable amplifier (IFG1) correct the IF ceramic insertion loss and the
costumer level plan application.
Mixer2, limiter and demodulator
In this 2nd 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 including spike
cancellation (DNB). MPX output DC offset versus noise DC level is correctable by software
(DEM), if tuner softmute is activated.
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 PIN22 (FSU) after +6dB buffer
stage. It is possible to combinate the IF counter result with this FSU via programmable
comparator (SSTH). The behaviour of FSU signal can be corrected for DC offset (SL) and
slope (SMSL). The generated unweighted fieldstrength is externally filtered 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
PIN27 by programmable time constant. Time control (TISS) for a present adjacent channel
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