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AD725 Datasheet(PDF) 16 Page - Analog Devices

Part No. AD725
Description  Low Cost RGB to NTSC/PAL Encoder with Luma Trap Port
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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REV. 0
–16–
AD725
Measuring the Luma Trap Frequency Response
The frequency response of the luma trap can be measured in
two different ways. The first involves using an RGB frequency
sweep input pattern into the AD725 and observing the compos-
ite output on a TV monitor, a TV waveform monitor or on an
oscilloscope.
On a TV monitor, the composite video display will look like
vertical black and white lines that are coarsely spaced (low fre-
quency) on the left side and progress to tightly spaced (high
frequency) on the right side. Somewhere to the right of center,
there will not be discernible stripes, but rather only a gray verti-
cal area. This is the effect of the luma trap, which filters out
luminance detail at a band of frequencies.
At the bottom of the display are markings at each megahertz
that establish a scale of frequency vs. horizontal position. The
location of the center of the gray area along the frequency
marker scale indicates the range of frequencies that are being
filtered out. The gray area should be about halfway between the
3 MHz and 4 MHz markers for NTSC, and about halfway
between the 4 MHz and 5 MHz markers for PAL.
When a horizontal line is viewed on an oscilloscope or video
waveform monitor, the notch in the response will be apparent.
The frequency will have to be interpolated from the location of
the notch position along the H-line.
060
10
20
30
40
50
100
50
0
–50
1.0
0.5
–0.5
0.0
s
Figure 23. Luminance Sweep with Trap, COMP Pin
The second method involves using a network analyzer to mea-
sure the frequency response of the composite signal. In order to
perform this successfully, the AD725 must be given the appro-
priate signals so that it will pass video signals through it. Figure
24 illustrates the setup used for these measurements.
The first requirement is that the part must receive a subcarrier
clock. This will provide clocking to the internal delay line and
enable it to pass the video signal. The subcarrier clock should be
at the 4FSC frequency for either NTSC or PAL.
The second requirement is that the RGB inputs are properly
biased for linear operation, and the timing logic is properly
reset. It is acceptable to ac-couple the RGB inputs and momen-
tarily apply an HSYNC signal to reset the timing and perform
the dc restore. Because the inputs are high-impedance, the
droop during testing will be minimal. It is not desirable to apply
a steady pulse train of HSYNC inputs because the spectrum of
these pulses will show up in the output response.
A more stable, low noise method is shown in Figure 23. The
RGB inputs are biased using a power supply and the source port
bias input of the network analyzer. A momentary sync input is
still applied to the device to reset its internal timing, but droop
during testing will no longer be an issue.
The signal source is applied to the GIN input for largest output
response. This input should be terminated through the appro-
priate termination resistor (matching the output impedance of
the network analyzer). If necessary, calibration inaccuracies can
be flattened out by reading back the input reference using a
FET probe.
4FSC
AGND
DGND
AD725
ENCD
RIN
BIN
HSYNC
VSYNC
CRMA
LUMA
APOS DPOS
0.1 F
10k
5V
5V
14
15
16
8
9
4
5
13
2
OSC
68 H
STND
YTRAP
18pF
9pF
47k
1N4148
NTSC/
PAL
MOMENTARY
CSYNC
5V
GIN
10 F
75
7
6
0.1 F
10 F
11
5V
REF
SOURCE
MEASURE
NETWORK ANALYZER
FET
PROBE
COMP
220 F
75
SOURCE
BIAS
1V
1V
5V
NC
NC
IN
OUT
NC = NO CONNECT
Figure 24. Measurement Setup for Determining Luma
Trap Frequency
The composite output is reverse terminated with a 50
Ω or 75 Ω
resistor and input to the measuring channel of a network analyzer.
Since only the green input is driven, this method does not yield
an absolute measurement of composite signal levels, but the
notch in the composite output will be readily discernible. The
frequency measuring functions of the network analyzer can then
be use to accurately measure the frequency of the luma notch
filter (luma trap).




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