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ML6421CS-1 Datasheet(PDF) 10 Page - Micro Linear Corporation |
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ML6421CS-1 Datasheet(HTML) 10 Page - Micro Linear Corporation |
10 / 13 page ML6421 10 Figure 12. ML6421 Reconstruction Performance in the Frequency Domain due to discrete sampling. The fall-off follows a sinx/x response. The ML6421 filters have a complementary boost to provide a flatter overall response. The boost is designed for 13.5MHz Y/C and CV sampling and 6.75MHz U/V sampling. Note: The ML6421 has the same pin-out as the ML6420. In a typical application the ML6421 is used as the final output device in a video processing chain. In this case, inputs to the ML6421 are supplied by DAC outputs with their associated load resistors (typically 75 Ω or 150Ω). Resistance values should be adjusted to provide 2VP-P at the input of the ML6421. The ML6421 will drive 75 Ω source termination resistors (making the total load 150 Ω) so that no external drivers or amplifiers are required. FILTER PERFORMANCE The reconstruction performance of a filter is based on its ability to remove the high band spectral artifacts (that result from the sampling process) without distorting the valid signal spectral contents within the passband. For video signals, the effect of these artifacts is a variation of the amplitude of small detail elements in the picture (such as highlights or fine pattern details) as the elements move relative to the sampling clock. The result is similar to the aliasing problem and causes a “winking” of details as they move in the picture. Figure 12 shows the problem in the frequency domain. Curve A shows the amplitude response of the ML6421 filter, while Curve B shows the signal spectrum as it is distorted by the sampling process. Curve C shows the composite of the two curves which is the result of passing the sampled waveform through the ML6421 filter. It is clear that the distortion artifacts are reduced significantly. Ultimately it is the time domain signal that is viewed on a TV monitor, so the effect of the reconstruction filter on the time domain signal is important. Figure 13 shows the sampling artifacts in the time domain. Curve A is the original signal, Curve B. is the result of CCIR601 sampling, and Curve C. is the same signal filtered through the ML6421. Again the distortions in the signal are essentially removed by the filter. In an effort to measure the time domain effectiveness of a reconstruction filter, Figure 14 was generated from a swept frequency waveform. Curves A, B, and C are generated as in Figure 13, but additional curves D and E help quantify the effect of filtering in the time domain. Curve D and Curve E represent the envelopes (instantaneous amplitudes) of Curves B and C. Again it is evident in Curve D that the envelope varies significantly due to the sampling process. In Curve E, filtering with the ML6421 removes these artifacts and generates an analog output signal that rivals the oversampled (and more ideal) signal waveforms. The ML6421 reduces the amplitude variation from over 6% to less than 1%. ML6421 VIDEO LOW PASS FILTER (CONTINUIED |
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