TDA7513T_07 Datasheet, PDF (29/84 Pages) STMicroelectronics – Single chip FM/AM tuner with stereo decoder and audio processor

TDA7513T
Functional description
multipath content on pin #TUNQUALITY and channel noise (furtherly combined with
multipath content information) on pin #QUALITY.
3.2
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 programmable 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 American 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 frequency 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 signal 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 fieldstrength 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 strength of the narrow-band signal
is higher than an internally defined threshold. The fieldstrength signal must be low-passed
to remove audio content and this is done by use of the capacitor at pin #SMETERTC with an
I²C bus programmable internal resistor. The value of the capacitor is determined for correct
FM operation; the value of the internal resistor for AM is selectable in order to make the AM
time constant suitable for AM operation.
A station detection function is provided for easy seek stop operation. The 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.
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