TDA7340G datasheet(17/27 Pages) STMICROELECTRONICS

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1.1 Trigger Path

The incoming MPX signal is highpass-filtered,

amplified and rectified (block RECT-PEAK).

The second order highpass-filter has a corner-fre-

quency of 140KHz.

The rectifier signal, RECT, is used to generate by

peak-rectification a signal called PEAK, which is

available at the PEAK pin.

Also noise with a frequency >100KHz increases

the PEAK voltage. The value of the PEAK voltage

influences the trigger threshold voltage Vth (block

ATC).

Both signals, RECT and PEAK+Vth are fed to a

comparator (block PEAK-COMP) which outputs a

sawtooth-shaped waveform at the TBLANK pin. A

second comparator (block BLANK-COMP) forms

the internal blanking duration of 40

µs.

The noise blanker is supplied by his own biasing

circuit (block BIAS-MONO).

1.2 Automatic Noise Controlled Threshold

Control (ATC)

The are two independent possibilities for pro-

gramming the trigger threshold:

a)the minimum threshold in 8 steps (bits D6, D5

of the STD-byte and bit D5 of the SPKR_LF

byte)

b)and the noise adjusted threshold in 4 steps

(bits D6, D5of the SPKR_LF byte, (see fig.5)

The minimum threshold is used in combination

with a good MPX signal without any noise.

The sensitivity in this operation is high, depending

only on the programmed ”minimum trigger thresh-

old”, bits NTB of the noise blanker byte 1.

It is independent of the PEAK voltage.

If the MPX signal is noisy (low fieldstrength) the

PEAK signal increases due to the higher noise,

which is also rectified (see part 1.1).

With increasing of the PEAK voltage the trigger

threshold voltage increases, too. This particular

gain is programmable in 4 steps (see fig.2).

1.3 Automatic Threshold Control by the

Stereoblend voltage (ATC-SB)

Besides the noise controlled threshold adjustment

there is an additional possibility for influencing the

trigger. It is controlled by the difference between

Vsb and Vr, similar to the Stereoblend. The rea-

son for implementing such a second control will

be explained in the following:

The point where the MPX signal starts to become

noisy is fixed by the RF part. Therefore also the

starting point of the normal noise controlled trig-

ger adjustment is fixed (fig.6).

But in some cases the behaviour of the noise-

blanker can be improved by increasing the

threshold even in a region of higher fieldstrength,

for the MPX signal often shows distortion in this

range.

Because of the overlap of this range and the

range of the stereo/mono transition it can be con-

trolled by Vsb and Vr.

This threshold increase is programmable in 3

steps or switched off (see fig.6).

1.4 Over Deviation Detector (MPX-RECT)

Sometimes when listening to stations with a

higher deviation than 75KHz the noiseblanker

triggers on the high frequency modulation.

To avoid this blanking, which causes noise in the

output signal, the noiseblanker offers a deviation-

dependent threshold adjustment.

By rectifying the MPX signal a further signal rep-

resenting the actual deviation is obtained. It is

used to increase the PEAK voltage.

Offset and gain of this circuit are programmable in

3 steps (the first step turns off the detector, see

fig.7).

1.5 Blend Mode

Another possibility to avoid a disturbing triggering

on modulation is to use the spikes on the field-

strength signal (LEVEL pin).

But in the range of higher fieldstrength the signal

saturates and no more spike detection is possi-

ble. For this reason the TDA7340G offers the

”BLEND MODE”. When ”BLEND MODE” is acti-

vated a smooth transition between the LEVEL-

and the MPX-signal is used to detect the spikes

either on LEVEL or on MPX.

In the lower fieldstrength range mainly the

LEVEL-signal is used whereas in the higher

range mainly the MPX is used. This switching is

controlled also by the normal Stereoblend signal

to avoid additional pins.

With ”BLEND MODE OFF” both signals are used

to detect spikes in the whole fieldstrength range.

1.6 Input Mode

The NB of TDA7340G offers two input modes.

The first one uses the internal trigger path and

optional the LEVEL input. But the TDA7340G of-

fers also an external trigger mode.

During this mode the internal MPX trigger path is

disabled whereas the high pass at the LEVEL pin

is bypassed.

By using an external highpass at the LEVEL-pin

one can adjust the NB’s behaviour to the desired

one.

TDA7340G

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