LM1894MX/NOPB National Semiconductor, LM1894MX/NOPB Datasheet - Page 8

• Manufacturer Part Number: LM1894MX/NOPB
• Description: IC NOISE REDUCT SYS DYNAM 14SOIC
• Manufacturer: National Semiconductor
• Type: Dynamic Noise Reductionr

Specifications of LM1894MX/NOPB

• Applications: Automotive Systems, Players, TV
• Mounting Type: Surface Mount
• Package / Case: 14-SOIC (3.9mm Width), 14-SOL
• For Use With: LM1894MTBD - BOARD EVALUATION LM1894
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM1894MX; *LM1894MX/NOPB; LM1894MX

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 5. 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 kΩ) to light the LEDs of pin
1 and pin 18. The LED bar graph does not indicate signal
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level, but rather instantaneous bandwidth of the two filters; it
should not be used as a signal-level indicator. For greater
FIGURE 3. Output vs Frequency
flexibility in setting the bandwidth sensitivity, R1 and R2
could be replaced by a 1 kΩ 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 maxi-
mum 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
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flat to 10 kHz. The CCIR weighting network has substantial
gain to 8 kHz and any additional roll-off in the cassette player
FIGURE 4. −3 dB Bandwidth vs
will reduce the benefits of DNR noise reduction. A typical
Frequency and Control Signal
signal-to-noise measurement circuit is shown in Figure 6.
The DNR system should be switched from maximum band-
width to nominal bandwidth with tape noise as a signal
source. The reduction in measured noise is the signal-to-
noise ratio improvement.
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