TDA7512 Datasheet, PDF (16/42 Pages) STMicroelectronics – AM/FM CAR RADIO TUNER IC WITH INTELLIGENT SELECTIVITY SYSTEM ISS

TDA7512
1.4.4 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 fre-
quency is matching to the demodulator center frequency.
1.4.5 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 pro-
grammable comparator3 thresholds (DWTH, DTH) and generates two digital signals (dev, dev+). For weak sig-
nal condition deviation threshold is proportinal to FSU.
1.4.6 ISS Switch Logic
All digital signals coming from adjacent channel detector, deviation detector and softmute are acting via switch-
ing 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 34.
1.5 Soft Mute Control
The external fieldstrength signal at PIN 35 is the reference for mute control. The startpoint and mute depth are
programmable (SMTH, SMD) in a wide range. The time constant is defined by external capacitance. Additional
adjacent channel mute function is supported. A highpass filter with -3dB threshold frequency of 100kHz, amplifier
and peak rectifier generates an adjacent noise signal from MPX output with the same time constant for softmute.
This value is compared with comparator5 thresholds (ACM). For present strong adjacent channel the MPX signal
is additional attenuated (ACMD).
1.6 AM Section
The up/down conversion is combined with gain control circuit sensing three input signals, narrow band informa-
tion at PIN 39, upconversion signal (IFAGC) at PIN 58 and wide band information (RFAGC) at PIN 3.This gain
control gives two output signals. The first one is a current for pin diode attenuator and the second one is a volt-
age for preamplifier. Time constant of RF- and IF-AGC is defined by internal 100k resistor and external capacitor
at PIN 62. The intervention points for AGC (DAGC,WAGC) are programmable by software. In order to avoid a
misbehaviour of AGC intervention point it is important to know that the DAGC threshold has to be lower than
WAGC threshold !
The oscillator frequency for upconcersion-mixer1 is generated by dividing the FM VCO frequency after VCO
(VCOD) and AM predivider(AMD). It is possible to put in a separate narrow bandpass filter before mixer2 at PIN
58. In this case input P58 needs the DC-operation point from PIN 53 via resistance matched with filter imped-
ance. Additional it is possible to use second 10,7MHz ceramic filter by internal switch between mixer2 input and
PIN 52. This feature increases 900KHz attenuation.
In mixer2 the IF1 is downconverted into the IF2 450kHz. After filtering by ceramic filter a 450kHz amplifier is
included with an additional gain control of IF2 below DAGC threshold. Time constant is defined by capacitance
at PIN 40
Mixer1 and mixer2 tanks are software controlled adjustable (IF1T, IF2T).
The demodulator is a peak detector to generate the audio output signal.
A separate output is available for AMIF stereo (AMST).
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