Texas Instruments THS7303EVM Evaluation Module THS7303EVM THS7303EVM Datenbogen
Produktcode
THS7303EVM
SLOS479B
–
OCTOBER 2005
–
REVISED MARCH 2011
LOW-PASS FILTER AND BYPASS MODES
Each channel of the THS7303 incorporates a fifth-order low-pass filter. These video reconstruction filters
minimize DAC images from being passed onto the video receiver. Depending on the receiver design, failure to
eliminate these DAC images can cause picture quality problems due to aliasing of the ADC. Another benefit of
the filter is smoothing out aberrations in the signal that some DACs produce if their own internal filtering is not
adequate. This smoothing helps with picture quality and insures the signal meets video bandwidth requirements.
minimize DAC images from being passed onto the video receiver. Depending on the receiver design, failure to
eliminate these DAC images can cause picture quality problems due to aliasing of the ADC. Another benefit of
the filter is smoothing out aberrations in the signal that some DACs produce if their own internal filtering is not
adequate. This smoothing helps with picture quality and insures the signal meets video bandwidth requirements.
Each filter has a Butterworth characteristic associated to it. The benefit of the Butterworth response is that the
frequency response is flat, with a relatively steep initial attenuation at the corner frequency. The problem with this
characteristic is that the group delay rises near the corner frequency. Group delay is defined as the change in
phase (radians/second) divided by a change in frequency. An increase in group delay corresponds to a time
domain pulse response that has overshoot and some possible ringing associated with the overshoot.
frequency response is flat, with a relatively steep initial attenuation at the corner frequency. The problem with this
characteristic is that the group delay rises near the corner frequency. Group delay is defined as the change in
phase (radians/second) divided by a change in frequency. An increase in group delay corresponds to a time
domain pulse response that has overshoot and some possible ringing associated with the overshoot.
The use of other type of filters, such as elliptic or chebyshev, are not recommended for video applications
because of their very large group delay variations near the corner frequency, resulting in significant overshoot
and ringing. While these elliptic or chebyshev filters may help meet the video standard specifications with respect
to amplitude attenuation, their group delay is well beyond the standard specifications. Couple this with the fact
that video can go from a white pixel to a black pixel over and over again, ringing can easily occur. Ringing
typically causes a display to have ghosting or fuzziness appear on the edges of a sharp transition. On the other
hand, a Bessel filter has an ideally flat group delay response, but the rate of attenuation is typically too low for
acceptable image rejection. Thus, the Butterworth filter is a respectable compromise for both attenuation and
group delay.
because of their very large group delay variations near the corner frequency, resulting in significant overshoot
and ringing. While these elliptic or chebyshev filters may help meet the video standard specifications with respect
to amplitude attenuation, their group delay is well beyond the standard specifications. Couple this with the fact
that video can go from a white pixel to a black pixel over and over again, ringing can easily occur. Ringing
typically causes a display to have ghosting or fuzziness appear on the edges of a sharp transition. On the other
hand, a Bessel filter has an ideally flat group delay response, but the rate of attenuation is typically too low for
acceptable image rejection. Thus, the Butterworth filter is a respectable compromise for both attenuation and
group delay.
The THS7303 filter has a slightly lower group delay variation near the corner frequency compared to an ideal
Butterworth filter. This results in a time domain pulse response which still has some overshoot, but not as much
as a true Butterworth filter. Additionally, the initial rate of attenuation in the frequency response is not as fast as
an ideal Butterworth response, but it is an acceptable initial rate of attenuation considering the pulse and group
delay characteristic benefits.
Butterworth filter. This results in a time domain pulse response which still has some overshoot, but not as much
as a true Butterworth filter. Additionally, the initial rate of attenuation in the frequency response is not as fast as
an ideal Butterworth response, but it is an acceptable initial rate of attenuation considering the pulse and group
delay characteristic benefits.
The THS7303 filters have a nominal corner (
–
3 dB) frequency selectable at 9 MHz, 16 MHz, and 35 MHz along
with a bypass mode. The 9-MHz filter is ideal for standard definition (SD) NTSC, PAL, and SECAM composite
video (CVBS) signals. It is also useful for S-Video signals (Y
video (CVBS) signals. It is also useful for S-Video signals (Y
’
C
’
), 480i / 576i Y
’
P
’
B
P
’
R
, G'B'R', and Y
’
U
’
V
’
video
signals. The
–
3-dB corner frequency was designed to be 9 MHz to allow a maximally flat video signal while
achieving over 40-dB of attenuation at 27 MHz
—
a common frequency between the ADC second and third
Nyquist zones found in many video receivers. This is important because any signal appearing around this
frequency can appear in the baseband as a result of aliasing effects of an analog-to-digital converter found in a
receiver.
frequency can appear in the baseband as a result of aliasing effects of an analog-to-digital converter found in a
receiver.
The 9-MHz filter frequency was chosen to account for process variations in the THS7303. To ensure the required
video frequencies are the least affected, the filter corner frequency must be high enough to allow for component
variations.
video frequencies are the least affected, the filter corner frequency must be high enough to allow for component
variations.
Another
consideration
is
the
attenuation
must
be
large
enough
to
ensure
the
anti-aliasing/reconstruction filtering meets the system demands. Thus, the selection of the filter frequencies was
not chosen arbitrarily.
not chosen arbitrarily.
The 16-MHz filter was designed to pass 480p and 576p Y
’
P
’
B
P
’
R
and G'B'R' video signals, sometimes referred as
enhanced definition (ED). Additionally, this filter can be used to pass computer VGA signals with flat frequency
response in the video spectrum. The 16-MHz filter can also be used for SD signals to ensure there is no
amplitude aberration, and to have an exceptional low group delay within the SD video frequency range.
response in the video spectrum. The 16-MHz filter can also be used for SD signals to ensure there is no
amplitude aberration, and to have an exceptional low group delay within the SD video frequency range.
The 35-MHz filter is designed to pass high definition (HD) 720p and 1080i Y
’
P
’
B
P
’
R
video signals along with
G
’
B
’
R
’
(R
’
G
’
B
’
) SVGA and XGA signals. If a 4:2:2 system is used, the P
’
B
P
’
R
channels do not require the full
bandwidth as required by the Y
’
channel. However, it is still recommended to use the same filter frequency of the
Y
’
channel to match the group delay and timing of all three signals. Otherwise, extra delay compensation is
required to minimize timing variations. This filter is also useful for passing 480p/576p signals with little amplitude
or group delay variations within the ED frequency range.
or group delay variations within the ED frequency range.
The THS7303 bypass mode has a 190-MHz bandwidth (
–
3 dB) and a 300 V/
μ
s slew rate to pass G
’
B
’
R
’
(R
’
G
’
B
’
)
SXGA and UXGA signals with little degradation. This bypass mode is also useful for HDTV 1080p signals that
require a 60-MHz video signal bandwidth.
require a 60-MHz video signal bandwidth.
34
Copyright
©
2005
–
2011, Texas Instruments Incorporated
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