TDA7513T datasheet(29/84 Pages) STMICROELECTRONICS

29 / 84 page

TDA7513T

Functional description

29/84

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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