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參數(shù)資料
| 型號(hào)： | LA3430 |
| 元件分類： | 接收器 |
| 英文描述： | FM, AUDIO DEMODULATOR, PZIP16 |
| 封裝： | 3 MM PITCH, SIP-16 |
| 文件頁(yè)數(shù)： | 11/13頁(yè) |
| 文件大?。?/td> | 336K |
| 代理商： | LA3430 |
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LA3430
No. 1408-7/13
3. SNC (stereo noise control) and HCC (high-cut control)
The LA3430 has SNC and HCC terminals for improved S/N ratios when operating in weak radio fields. By adjusting
the SNC terminal, noises unique to stereo FM in weak fields can be reduced. The HCC terminals permits further
improvement of effective S/N ratios by lowering treble levels of FM noises in weak fields. (See Fig. 2)
STEREO deteriorates approximately 21.7dB (compared to MONO) in weak radio fields (Fig. 2). Generally, when
S/N ratios deteriorate below 30 to 40dB, noises become quite noticeable. Section (1) shows ways to set SNC and
HCC when radio field strengths are divided into 3 regions, A, B, and C, (Fig. 2). SNC is expected to function in
region A, and HCC in region B. In region C, shallow muting is effected in the IF stage.
(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. 5.
SNC in the LA3430 improves S/N ratios in weak radio fields by varying separation. It varies sub-signal
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. 3 shows separation characteristics (SNC characteristics) for voltages applied to pin 8 (SNC terminal) of the
LA3430. 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. 6)
(2) Designing external circuits for SNC characteristics (characteristic setting by drawing)
We recommend the following as a way to designate SNC characteristics to have smooth transition of separation
from stereo to monaural in region A of Fig. 2.
Separation vs S/N-enhancement relationRefer to Fig. 5.
SNC terminal voltages vs separation characteristics Refer to Fig. 3.
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 obtained. Sample drawings are shown
in Fig. 4, 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.
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