TDA7513T Datasheet, PDF (18/59 Pages) STMicroelectronics – SINGLE-CHIP FM/AM TUNER WITH STEREO DECODER AND AUDIO PROCESSOR

TDA7513T
5.1.5 Multipath Detector
The input of the multipath detector is AC coupled to the internal unfiltered fieldstrength. A programmable
band-pass filter (MPF) and amplifier (MPG) followed by a rectifier measures the multipath content. This
voltage is compared with an adjustable threshold (MPTH) comparator (comparator2). The output signal of
this comparator2 is used to disable the adjacent channel detector control of the ISS filter in case of strong
multipath, which would otherwise result in bandwidth reduction because of the multipath-induced high-fre-
quency content of the fieldstrength signal. The multipath detector influence on the adjacent channel de-
tector is selectable by I²C bus (MPOFF). The multipath information after filtering and rectification is
available as analog output on pin #TUNQUALITY (the gain can be selected via I²C bus) in combination
with the adjacent channel content information. It is possible to enable multipath content information output
only via I²C bus control.
5.1.6 450kHz IF Narrow Bandpass Filter (ISS filter)
The device features an additional automatically selectable IF narrow bandpass filter for suppression noise
and adjacent channel signals. This narrow filter has three switchable bandwidth positions: narrow range
(80kHz), mid range (120kHz) and weather band (30kHz). WHen the ISS filter is not inserted the IF band-
width (wide range) is defined only by the ceramic filter chain. The filter is switched in after mixer2 before
the 450kHz limiter stage. The centre frequency can be finely adjusted (AISS) by software.
5.1.7 Deviation Detector
In order to avoid excessive audio distortion the narrow ISS filter is switched OFF when overdeviation of
the incoming signal is detected. The demodulator output signal is low-pass filtered and rectified to gener-
ate a DC level in an external capacitor through a software-controlled current (TDEV). This level is com-
pared with a programmable threshold (DWTH, DTH) comparator (comparator3) to generate two digital
signals (dev, dev+).
5.1.8 ISS Switch Logic
All digital signals coming from adjacent channel detector, deviation detector and softmute are combined
in a decision matrix to generate the control signals for the ISS filter switch. The IF bandpass switch mode
can be also controlled by software (ISSON, ISS30, ISS80, ISSCTL). The switch-on of the IF bandpass can
be further controlled from the outside by manipulation of the voltage at pin #ISSTC. Two application modes
are available (APPM). The conditions are described in table 1.
5.1.9 Soft Mute Control
The external fieldstrength signal at pin #SMETERTC is the reference for MPX mute control. The start point
and mute depth are programmable over a wide range. The time constant is defined by the external capac-
itor connected to pin #FMMUTETC.
Additionally adjacent channel mute function is supported. A software-configurable highpass / bandpass
filter centered at about 100kHz followed by an amplifier and a peak rectifier generates adjacent noise in-
formation starting from the MPX output; the information is acted upon with the same time constant as the
softmute by the MPX muting circuit. The adjacent channel mute starting point, slope and depth are I²C
bus programmable.
5.1.10Station Detector and Seek Stop
A station detection function is provided for easy seek stop operation. The unfiltered fieldstrength signal is
compared with a programmable threshold and the result (logic '1' if the current station strength is higher
than the threshold) is combined by an AND gate with the IF counter output (logic '1' if the current channel
is centered within a programmable window around the desired frequency). The result is available on pin
#SD for direct connection to the microprocessor. Channel quality assessment for RDS Alternate Frequen-
cy operation makes use of the SD signal in conjunction with analog information on adjacent channel and
multipath content on pin #TUNQUALITY and channel noise (furtherly combined with multipath content in-
formation) on pin #QUALITY.
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