ML6423
10
0
IDEAL SINX/X RESPONSE
–3dB REFERENCE MARKER
–10
–20
–30
–40
–50
–60
A. ML6423 AMPLITUDE RESPONSE
B. SIGNAL DISTORTION SPECTRUM
C. RECONSTRUCTED SIGNAL
SPECTRUM
0
5
10
15
20
25
FREQUENCY (MHz)
Figure 6. ML6423 Reconstruction Performance in the Frequency Domain
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 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.
+5V
DAC
INPUTS
ANALOG
OUTPUTS
ML6423
DAC
Y
Y
CV
C
(CURRENT SOURCING
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%.
75Ω
75Ω
75Ω
+
DAC
C
(CURRENT SOURCING
DAC LOAD
ADJUSTED FOR
1V
P-P
Figure 5b. Typical ML6423 Reconstruction Application
8
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
