ML6421CS-4 MICRO-LINEAR [Micro Linear Corporation], ML6421CS-4 Datasheet - Page 10

• Manufacturer Part Number: ML6421CS-4
• Description: Triple Phase and Sinx/x Equalized, Low-Pass Video Filter
• Manufacturer: MICRO-LINEAR [Micro Linear Corporation]

ML6421
ML6421 VIDEO LOW PASS FILTER
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
Resistance values should be adjusted to provide 2V
the input of the ML6421.
The ML6421 will drive 75 source termination resistors
(making the total load 150 ) 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
10
Figure 12. ML6421 Reconstruction Performance in the Frequency Domain
or 150 ).
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(CONTINUIED
at
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%.