TDA7514 datasheet(64/74 Pages) STMICROELECTRONICS

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TDA7514

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VARIABLE-BANDWIDTH SELECTIVITY SYSTEM (ISS)

The TDA7514 is equipped with the Intelligent Selectivity System that automatically performs IF bandwidth

reduction in presence of strong adjacent channels or weak desired signal. A three position filter is available

at IF2; its bandwidth settings are: 120 kHz, 80 kHz and 22 kHz (the latter only for additional on-board WB

filtering). The filter is positioned between the 2nd mixer and the IF2 limiter. The filter is switched among

its OFF, 120 kHz and 80 kHz positions by a dedicated quality detection circuit.

The conditions that are detected in order to drive the filter position are:

– adjacent channel presence;

– peak frequency deviation magnitude;

– field strength.

The adjacent channel detection (programmable filter shape and gain, programmable rectifier gain, pro-

grammable flag generation thresholds, programmable integrator charge/discharge currents) can be par-

tially or totally disabled, if so programmed, when a strong multipath condition is detected (filter shared with

quality circuits with programmable gain, independent rectifier with programmable gain, programmable in-

tervention threshold). The adjacent channel detector is further disabled when the field strength is lower

than a programmable threshold (slope programmable as well). The output of the detector are two flags

signalling a moderate or a strong adjacent channel presence.

The peak frequency deviation is detected by rectifying and peak-detecting the mono part of the MPX signal

(programmable integrator charge/discharge currents, programmable thresholds). The deviation detector

is desensitized at weak field strength (programmable threshold). The output of the detector are two flags

signalling a moderate or high amount of peak deviation.

The field strength is measured against a programmable threshold and generates one flag.

These five flags are combined to control the ISS bandwidth by access to a 3-D matrix that can be pro-

grammed almost completely: it is therefore possible to specify the filter bandwidth for almost all the pos-

sible flag combinations.

The ISS filter may be disabled by setting the tuner SEEK bit to "1".

5.1.4

SOFT MUTE AND MPX OUTPUT

The demodulator output is passed through muting attenuators before becoming available externally for

the connection to the stereo decoder. The muting and output buffering are shared with the AM circuit.

The FM muting circuit is sensitive to weak field strength and to the presence of adjacent channel. The

weak field strength condition (soft mute) is detected by comparing the 1 second-filtered Smeter with a pro-

grammable threshold. The soft mute depth and slope are software-programmable.

The soft mute is followed by the adjacent channel muting circuit. The presence of adjacent channel is de-

tected as high frequency noise in the MPX signal, so that only the adjacent channel that is not suppressed

by the ISS can activate this muting. The filter is described in the Quality section. It is desensitized under

weak field strength conditions. The adjacent channel mute threshold/slope and depth are programmable.

The output audio amplifier gain can be switched to a +23.5 dB setting in WB in order to equalize the FM

and WB 100% modulation audio levels.

ISS-dependent automatic roll-off compensation is implemented before the output buffer.

5.1.5

STATION DETECTION

The station detection function makes use of signal strength measurement and of IF counting. The result

is available on the SD pin as a logic high value when the tuned channel is considered valid. On the SD pin

the two field strength-related and IF counter output signals are also available separately by suitably pro-

gramming the output multiplexer.

The field strength-related digital output is derived by comparison of the filtered Smeter (the time constant

is 1s in reception and 100us in seek mode - see Quality section) with a programmable threshold.

The FM IF counter circuit detects whether the IF signal is centered inside a programmable frequency win-

dow around the nominal frequency value. The measuring time window is programmable as well. The result

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