TDA7513 datasheet(18/58 Pages) STMICROELECTRONICS

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TDA7513

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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.

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.

Station 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.

4.2 AM Section

The upconversion mixer1 is combined with a gain control circuit 1 sensing three input signals: ultra-narrow

band information (from the IF2 amplifier input - pin #AMIF2AMPIN), narrow-band information (from the

mixer2 input - pin #AMMIX2IN) and wide band information (from the mixer1 input - pins #AMMIX1IN+ and

#AMMIX1IN-). This gain control circuit generates two output signals: a current for P-I-N diode attenuation

and a voltage for the external preamplifier cascode upper base. It is possible to put in a separate narrow

bandpass filter before mixer2 at PIN 58. The intervention point for first AGC on all three bands is program-

mable by software.

The oscillator frequency for mixer1 is generated by dividing the FM VCO frequency (AMD) by 6, 8 and 10

(6 for Japan applications, 8 for Eastern European applications, 10 for Western European and North Amer-

ican operation).

In mixer2 the IF1 is downconverted into the 450kHz IF2. The gain of mixer2 is reduced by the 2nd AGC

after the gain of the subsequent IF2 amplifier has been reduced by 30dB. The mixer2 tank center frequen-

cy is software-adjustable (IF2T).

After channel selection is done by the ceramic filter, a 450kHz amplifier with a gain control is included. The

gain is controlled by the AGC2 loop over a 30dB range; the full gain with no AGC applied is programmable.

The AM demodulation is made by multiplication of the IF2 amplifier output by the amplified and limited sig-

nal coming from the IF2 amplifier input, thus making the demodulation process inherently linear.

The demodulated audio signal is low-passed by the capacitor at pin #AMAGC2TC to produce the DC

AGC2 voltage. The low-pass time constant is switchable by a ratio of 30 in order to reduce the settling

time of the AGC2 in 'seek' mode (AMSEEK).

The FM 450kHz limiter is used to generate the square wave needed by the AM demodulator, a field-

strength indication and to feed the AM IF counter. The fieldstrength information is generated mainly from

the narrow-band signal at the input of the IF2 amplifier; since the dynamic range at that input is limited by

the AGC2 action, a fieldstrength extension is made adding the contribution of the signal at the input of

mixer2. Since the bandwidth there is very large, though, the latter contribution is enabled only if the

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