AD725-EB datasheet(16/20 Pages) AD

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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

100

–50

1.0

0.5

–0.5

0.0

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

OSC

68 H

STND

YTRAP

18pF

9pF

47k

1N4148

NTSC/

PAL

MOMENTARY

CSYNC

GIN

10 F

0.1 F

10 F

REF

SOURCE

MEASURE

NETWORK ANALYZER

FET

PROBE

COMP

220 F

SOURCE

BIAS

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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