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| 型号： | LM1894MX/NOPB |
| 厂商： | National Semiconductor |
| 文件页数： | 2/19页 |
| 文件大小： | 0K |
| 描述： | IC NOISE REDUCT SYS DYNAM 14SOIC |
| 标准包装： | 2,500 |
| 类型： | 动态噪音抑制 |
| 应用： | 车载系统，播放器，电视 |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 14-SOIC（0.154"，3.90mm 宽） |
| 供应商设备封装： | 14-SOICN |
| 包装： | 带卷 (TR) |
| 配用： | LM1894MTBD-ND - BOARD EVALUATION LM1894 |
| 其它名称： | *LM1894MX *LM1894MX/NOPB LM1894MX |
SNAS551C – DECEMBER 1994 – REVISED APRIL 2013
APPLICATION HINTS
The DNR system should always be placed before tone and volume controls as shown in Figure 1. This is
because any adjustment of these controls would alter the noise floor seen by the DNR control path. The
sensitivity resistors R1 and R2 may need to be switched with the input selector, depending on the noise floors of
different sources, i.e., tape, FM, phono. To determine the value of R1 and R2 in a tape system for instance;
apply tape noise (no program material) and adjust the ratio of R1 and R2 to open slightly the bandwidth of the
main signal path. This can easily be done by viewing the capacitor voltage of pin 10 with an oscilloscope, or by
using the circuit of Figure 14. This circuit gives an LED display of the voltage on the peak detector capacitor.
Adjust the values of R1 and R2 (their sum is always 1 k
Ω) to light the LEDs of pin 1 and pin 18. The LED bar
graph does not indicate signal level, but rather instantaneous bandwidth of the two filters; it should not be used
as a signal-level indicator. For greater flexibility in setting the bandwidth sensitivity, R1 and R2 could be replaced
by a 1 k
Ω potentiometer.
To change the minimum and maximum value of bandwidth, the integrating capacitors, C3 and C12, can be
scaled up or down. Since the bandwidth is inversely proportional to the capacitance, changing this 0.0039
μF
capacitor to 0.0033
μF will change the typical bandwidth from 965 Hz–34 kHz to 1.1 kHz–40 kHz. With C3 and
C12 set at 0.0033
μF, the maximum bandwidth is typically 34 kHz. A double pole double throw switch can be
used to completely bypass DNR.
The capacitor on pin 10 in conjunction with internal resistors sets the attack and decay times. The attack time
can be altered by changing the size of C10. Decay times can be decreased by paralleling a resistor with C10,
and increased by increasing the value of C10.
When measuring the amount of noise reduction of the DNR system, the frequency response of the cassette
should be flat to 10 kHz. The CCIR weighting network has substantial gain to 8 kHz and any additional roll-off in
the cassette player will reduce the benefits of DNR noise reduction. A typical signal-to-noise measurement circuit
is shown in Figure 15. The DNR system should be switched from maximum bandwidth to nominal bandwidth with
tape noise as a signal source. The reduction in measured noise is the signal-to-noise ratio improvement.
Figure 14. Bar Graph Display of Peak Detector Voltage
10
Copyright 1994–2013, Texas Instruments Incorporated
Product Folder Links: LM1894
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