LA3370 View Datasheet(PDF) - SANYO -> Panasonic

Part Name

Description

Manufacturer

LA3370

PLL FM Multiplex Stereo Demodulator for Car Stereo

SANYO -> Panasonic 

LA3370

(1) SNC (stereo noise control)

Stereo S/N ratios deteriorate 21.7dB below monaural but can be improved by varying stereo separation. S/N

improvement becomes apparent, however, only when the separation is 20dB or worse. In that case, the relation

between separation and S/N improvement is shown in Fig.3.

SNC in the LA3370 improves S/N ratios in weak radio fields by varying separation. It varies subsignal

demodulation level and controls separation. By using the IF stage signal meter level output as the source of the

control signal, S/N ratios in region A of Fig.2 can be maintained at about 40dB or better. Ideal S/N enhancements

should provide gradual switching over from stereo to monaural to maintain constant S/N ratios, starting from a point

in region A for 40dB stereo S/N toward a point for 40dB monaural S/N. Methods to set the control level will be

described later.

Fig.4 shows voltages applied to pin 8 (SNC terminal) of LA3370 versus separation characteristics (SNC

characteristics). Pin 8 is also the base of a PNP transistor, so stereo mode is set when pin 8 is open and monaural

mode is set when it is grounded. SNC terminal control is effective only when locked with pilot signals and when

stereo indicator is lit. External circuit parameters can be chosen in large values that do not affect the IF stage meter

output circuit because SNC control currents are small. This makes designing easy. (See Fig.5)

(2) Designing external circuits for SNC characteristics (characteristic setting by drawing)

We recommend the following as a way to adjust SNC characteristics to have smooth transition of separation from

stereo monaural in region A of Fig.2.

Separation vs. S/N enhancement relation……………………… Refer to Fig.3.

SNC pin voltage vs.separation characteristics………………… Refer to Fig.4.

Antenna inputs vs. S/N improvement characteristics can be obtained from the drawing if the graph for IF stage

signal meter output vs. antenna input and the graph for stereo S/N ratio vs. antenna input are known. From desired

S/N characteristics, SNC terminal voltage characteristics can also be obtained. Sample drawings are shown in Fig.6,

where for simplicity's sake, SNC, IF meter, and stereo S/N characteristics have been approximated with straight

lines.

For instance :

To obtain stereo S/N improvement characteristics from SNC characteristics, when (a) in the second quadrant

of the chart represents bare SNC characteristics, point 1 projected to the third quadrant shows a 20dB

separation and a 1dB S/N improvement. When projected from the first to the fourth quadrant, a point

improved by 1dB in S/N over the stereo S/N line in the fourth quadrant corresponds to point 1. Similarly,

point 2 on the SNC characteristics in the second quadrant corresponds to point 2 in the fourth quadrant. Point

3 in the second quadrant corresponds to point 3 in the fourth quadrant. Stereo S/N improvement characteristics

for each point are obtainable. Similarly, (b) characteristics in the second quadrant are projected to form (b)

characteristics in the fourth quadrant, and (c) in the second quadrant to form (c) in the fourth quadrant, thus

providing a way to diagram improvement characteristics.

In the resulting drawings, ideal S/N improvement characteristics are similar to (b) in the fourth quadrant, but

corresponding SNC characteristics have to be (b) characteristics in the second quadrant which are difficult to realize.

Among realistic characteristics, something like (c) appears to be satisfactory. The (c) SNC characteristics are

obtained with a shift by two diodes together with a 1/2 bleeder.

No.619-4/11

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