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LM1894M 데이터시트(PDF) 5 Page - National Semiconductor (TI) |
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LM1894M 데이터시트(HTML) 5 Page - National Semiconductor (TI) |
5 / 10 page External Component Guide ( Figure 1) Component Value Purpose C1 0.1 µF– 100 µF May be part of power supply, or may be added to suppress power supply oscillation. C2, C13 1 µF Blocks DC, pin 2 and pin 13 are at DC potential of V S/2. C2, C13 form a low frequency pole with 20k R IN. C14 25 µF– 100 µF Improves power supply rejection. C3, C12 0.0033 µF Forms integrator with internal gm block and op amp. Sets bandwidth conversion gain of 33 Hz/µA of gm current. C4, C11 1 µF Output coupling capacitor. Output is at DC potential of V S/2. C5 0.1 µF Works with R1 and R2 to attenuate low frequency transients which could disturb control path operation. C6 0.001 µF Works with input resistance of pin 6 to form part of control path frequency weighting. C8 0.1 µF Combined with L8 and C L forms 19 kHz filter for FM pilot. This is only required in FM applications (Note 9). L8, C L 4.7 mH, 0.015 µF Forms 19 kHz filter for FM pilot. L8 is Toko coil CAN-1A185HM (Notes 8, 9). C9 0.047 µF Works with input resistance of pin 9 to form part of control path frequency weighting. C10 1 µF Set attack and decay time of peak detector. R1, R2 1 k Ω Sensitivity resistors set the noise threshold. Reducing attentuation causes larger signals to be peak detected and larger bandwidth in main signal path. Total value of R1 + R2 should equal 1 k Ω. R8 100 Ω Forms RC roll-off with C8. This is only required in FM applications. Note 8: Toko America Inc., 1250 Feehanville Drive, Mt. Prospect IL 60056 Note 9: When FM applications are not required, pin 8 and pin 9 hook-up as follows: Circuit Operation The LM1894 has two signal paths, a main signal path and a bandwidth control path. The main path is an audio low pass filter comprised of a gm block with a variable current, and an op amp configured as an integrator. As seen in Figure 2,DC feedback constrains the low frequency gain to A V = −1. Above the cutoff frequency of the filter, the output decreases at −6 dB/oct due to the action of the 0.0033 µF capacitor. The purpose of the control paths is to generate a bandwidth control signal which replicates the ear’s sensitivity to noise in the presence of a tone. A single control path is used for both channels to keep the stereo image from wandering. This is done by adding the right and left channels together in the summing amplifier of Figure 2. The R1, R2 resistor divider adjusts the incoming noise level to open slightly the band- width of the low pass filter. Control path gain is about 60 dB and is set by the gain amplifier and peak detector gain. This large gain is needed to ensure the low pass filter bandwidth can be opened by very low noise floors. The capacitors be- tween the summing amplifier output and the peak detector input determine the frequency weighting as shown in the typical performance curves. The 1 µF capacitor at pin 10, in conjunction with internal resistors, sets the attack and decay times. The voltage is converted into a proportional current which is fed into the gm blocks. The bandwidth sensitivity to gm current is 33 Hz/µA. In FM stereo applications at 19 kHz pilot filter is inserted between pin 8 and pin 9 as shown in Figure 1. Figure 3 is an interesting curve and deserves some discus- sion. Although the output of the DNR system is a linear func- tion of input signal, the −3 dB bandwidth is not. This is due to the non-linear nature of the control path. The DNR system has a uniform frequency response, but looking at the −3 dB bandwidth on a steady state basis with a single frequency in- put can be misleading. It must be remembered that a single input frequency can only give a single −3 dB bandwidth and the roll-off from this point must be a smooth −6 dB/oct. A more accurate evaluation of the frequency response can be seen in Figure 4. In this case the main signal path is fre- quency swept, while the control path has a constant fre- quency applied. It can be seen that different control path fre- quencies each give a distinctive gain roll-off. Psychoacoustic Basics The dynamic noise reduction system is a low pass filter that has a variable bandwidth of 1 kHz to 30 kHz, dependent on music spectrum. The DNR system operates on three prin- ciples of psychoacoustics. 1. White noise can mask pure tones. The total noise energy required to mask a pure tone must equal the energy of the tone itself. Within certain limits, the wider the band of mask- ing noise about the tone, the lower the noise amplitude need DS007918-6 www.national.com 5 |
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