TDA7513 datasheet(17/58 Pages) STMICROELECTRONICS

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TDA7513

Quality Detection and ISS (see Figure 3) Fieldstrength

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

strength output signal. This internal unfiltered fieldstrength is used for adjacent channel and multipath de-

tection. The behaviour of this output signal can be corrected for DC offset (SL). The internally generated

unfiltered fieldstrength is filtered at pin #SMETERTC and used for softmute function, FM AGC keying 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 to the internal unfiltered fieldstrength. A pro-

grammable and configurable highpass or bandpass filter (ACF) and amplifier (ACG) followed by a rectifier

measure the adjacent channel content. This voltage is compared with an adjustable threshold (ACWTH,

ACNTH) comparator (comparator1). The output signal of this comparator generates a DC level at PIN15

with a programmable time constant. Time constant control (TISS) for the adjacent channel is made by lin-

early charging and discharging an external capacitor following. The charge current is fixed and the dis-

charge current is controlled by I2C bus. This level produces digital signals (ac, ac+) after comparing by the

following comparator4. The adjacent channel information after filtering and rectification is available as an-

alog output on pin #TUNQUALITY (the gain can be selected via I2C bus) in combination with multipath

content information. It is possible to enable adjacent channel content information output only via I2C bus

control.

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.

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.

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+).

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

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