ML6423

8

Figure 6. ML6423 Reconstruction Performance in the Frequency Domain

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 6 shows the problem in the frequency domain.

Curve A shows the amplitude response of the ML6423

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 ML6423. 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 7 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

ML6423. 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 8 was generated from a swept

frequency waveform. Curves A, B, and C are generated as

in Figure 7, but additional curves D and E help quantify

the effect of filtering in the time domain. Curves D and 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 ML6423 removes these

artifacts and generates an analog output signal that rivals

the oversampled (and more ideal) signal waveforms. The

ML6423 reduces the amplitude variation from over 6% to

less than 1%.

Figure 5b. Typical ML6423 Reconstruction Application

Y

DAC

INPUTS

DAC

(CURRENT SOURCING

C

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ANALOG

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C

DAC

(CURRENT SOURCING

ML6423

DAC LOAD

ADJUSTED FOR

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Ω

75

Ω

75

Ω

+5V

+

FILTER PERFORMANCE

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0

–10

–20

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–60

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FREQUENCY (MHz)

IDEAL SINX/X RESPONSE

–3dB REFERENCE MARKER

A. ML6423 AMPLITUDE RESPONSE

B. SIGNAL DISTORTION SPECTRUM

C. RECONSTRUCTED SIGNAL

SPECTRUM