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EDN Access -- 05.11.95 sync separator includes AG

-May 11, 1995

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Design Ideas:May 11, 1995

Sync separator includes AGC


Dana Romero,
Salt Lake City, UT

Proper sync separation in video-signal processing requires that the incoming signal be relatively stable. When substantial amplitude variation exists, a pair of ICs (Fig 1) combine to form a gain-control circuit that helps to stabilize the sync-separation function. IC1 is a sync separator that has the features of the standard LM1881, plus horizontal sync decoding and a LEVEL-OUT pin to indicate the overall signal level at the input. Using this feature to control IC2's variable-gain amplifier, the circuit provides automatic gain control for sync separation. Most of the components surrounding IC1 are close to the manufacturer's suggested values, and the data sheet explains their functions.

IC3, a single-supply op amp, is the only other IC needed to complete the circuit. A single-supply op amp guarantees that the gain-control voltage at IC2 remains positive while also performing the necessary translation of the LEVEL-OUT voltage from IC1. Specifically, the dc voltage from IC1's pin 9 is nominally equal to twice the amplitude of the sync pulse of the video signal at pin 4. But, the negative feedback the entire loop requires means that the gain of IC2 has to increase as the amplitude at IC1's pin 4 decreases. Connecting IC3 as Fig 1 shows accomplishes this function by inverting only the voltage changes from IC1. IC3's overall output remains positive. In addition, potentiometer R1 provides some auxiliary gain control. Note that a video-speed amplifier is not required for IC3 because the overall level doesn't change at that rate.

IC2 serves as a preamplifier for IC1, and you must choose the feedback divider resistors, R2 and R3, so that their parallel resistance is low. (The data sheet for the EL4451 recommends a resistance of 360(ohm) or less.) Potentiometer R4 sets VREF and adjusts the dc offset of the video signal out of pin 14. The equation for VOUT is

xVG+VREF)/H,

where VIN is the voltage difference between pins 4 and 5, VG is the voltage difference between pins 7 and 8 (all pins of IC2), and H is the feedback ratio R3/(R2+R3). Although IC2 is designed to accept differential inputs for VIN and VG, this design grounds pins 5 and 8. (DI #1700)


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