TDA7514 datasheet(65/74 Pages) STMICROELECTRONICS

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TDA7514

is available both on the SD pin and in the tuner I2C bus read byte.

5.1.6

QUALITY

The FM quality section of the TDA7514 generates the control signals for all the quality-related functions.

These quality signals are: filtered Smeter, adjacent channel content, multipath content. The controlled

functions are: AGC keying, soft mute, adjacent channel mute, stereo blend, high cut, audio noise blanker.

Externally available quality signals are also generated by this circuit. These signals are: filtered Smeter

(analog and digital), adjacent channel content and multipath content.

External filtering to generate part of the quality signals uses external capacitors: the voltage values on

these capacitors can be kept stored during RDS AF update or fast charged when a new tuned frequency

is selected.

The Smeter filter uses an external capacitor to generate a low-pass time constant of 1 s in reception mode

(10 ms for fast car-radio testing), and an all-internal circuit for a low-pass time constant of 100 us for RDS

AF update. The voltage across the capacitor is stored during RDS AF update; it is fast charged following

the 100 us-filtered signal during jumps to a new frequency. The 1s/100us Smeter is fed to the station de-

tector, to the high cut filter, to the soft mute and to the A-to-D converter.

An internally filtered 1ms-time constant Smeter is further generated to drive the keyed AGC, the ISS and

the weak field filter desensitization circuits.

The stereo blend function is driven by a combination of weak signal strength-, adjacent channel- and mul-

tipath-related information through the SNC (stereo noise control) signal. The SNC signal is a wired-OR of

the three conditions and is generated with a peak holder featuring a 400 ms attack time constant and a 10

s decay time constant. The Smeter contribution to the SNC is programmable in terms of gain and offset;

the same is true for the two other bad quality indicators (filter shape, rectifier offset and gain are program-

mable) keeping in mind that the adjacent channel filter and rectifier are shared with the adjacent channel

mute circuit and that the multipath filter is shared with the ISS multipath detector. The voltage across the

external SNC filtering capacitor is stored during RDS AF update, and is driven by a fast charge circuit dur-

ing jumps to a new frequency. A fast car-radio test mode is also available in which the external capacitor

is disconnected and substituted for by a much smaller internal capacitor.

A faster attack of the stereo blend function in case of sudden onset of adjacent channel or multipath con-

ditions is ensured by actually driving the stereo blend circuit by a further wired-OR circuit sensing the SNC

voltage as well as the unfiltered multipath and adjacent channel rectifier outputs.

The quality signals available to the u-processor are: filtered Smeter (on the dedicated buffered SMETER

pin), the multipath information (on pin QUALMPOUT), a programmable combination of multipath and ad-

jacent channel information (on pin QUALACMPOUT), and 6 bit digitized filtered Smeter (via the tuner I2C

bus read byte).

5.2 AM TUNER SECTION

5.2.1

FRONT END

The AM tuner front end section consists of a high IP3 mixer whose outputs are common with the FM first

mixer.

After going through the first FM IF1 ceramic filter the signal enters the second mixer for conversion to the

second IF of 450 kHz where channel selection takes place. The second mixer exhibits a high IP3 value

too, and has a fixed gain.

The front end AGC detects mainly a wide band signal (RF signal from the input pins of the first mixer) and

a very narrow band signal (Smeter, generated by the signal at the input of the IF2 amplifier after channel

selection has occurred). A third fairly narrow band input (IF1 signal from the input pin of the second mixer)

is also available, although the first mixer-input-referred IP3 figures of the two mixers make this input gen-

erally superfluous. The FE AGC starting points on all three input channels are programmable.

The FE AGC circuit drives the external attenuation P-I-N diodes and the external RF amplifier gain control

terminal.

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