AD723 datasheet(18/20 Pages) AD

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REV. 0

AD723

–18–

Implementing a Luma Trap

The AD723 implementation of a luma trap uses an on-chip

resistor along with an off-chip inductor and capacitor to create

an RLC notch filter. The filter must be tuned to the center

frequency of the video standard being output by the AD723,

3.58 MHz for NTSC or 4.43 MHz for PAL.

The circuit is shown in Figure 11. The 1.4 k

Ω series resistor in

the composite video luma path on the AD723 works against the

impedance of the off-chip series LC to form a notch filter. The

frequency of the filter is given by:

f C

π LC

1.4k

14.318180MHz

YTRAP

LUMA

CSET

4FSC

17.734475MHz

STND

68 H

18pF

47k

1N4148

NTSC/

PAL

AD723

9pF

375

300

375

300

375

300

CVSET

YSET

CHROMA

Figure 11. Luma Trap Circuit for NTSC and PAL Video

Dual-Standard Luma Trap

For a filter that will work for both PAL and NTSC, a means is

required to switch the tuning of the filter between the two

subcarrier frequencies. The PAL standard requires a higher

frequency than NTSC. A basic filter can be made that is tuned

to the PAL subcarrier and a simple diode circuit can then be

used to switch in an extra parallel capacitor that will lower the

filter’s frequency for NTSC operation.

Figure 11 shows how the logic signal that drives STND (Pin 1)

can also be used to drive the circuit that selects the tuning of

the luma trap circuit. When the signal applied to STND is

low (ground), the PAL mode is selected. This results in a bias

of 0 V across D1, which is an Off condition. As a result, C2 is

out of the filter circuit and only C1 tunes the notch filter to the

PAL subcarrier frequency, 4.43 MHz.

On the other hand, when STND is high (3 V), NTSC is selected

and there is a forward bias across D1. This turns the diode on

and adds C2 in parallel with C1. The notch filter is now tuned

to the NTSC subcarrier frequency, 3.58 MHz.

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 AD723 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 12. Luminance Sweep with Trap, CV Pin

–24

0.1

10.0

1.0

–3

–6

–9

–12

–15

–18

–21

FREQUENCY – MHz

Y (LUMA)

C (COMP)

Figure 13. Luminance Frequency Response with NTSC Trap

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