By G8MNY (CQTV 1994, New to packet Feb 06)
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This circuit was developed to limit the large negative pulses from radar QRM on 23cms ATV that are recovered video from a NE564 demodulator (or symular). These pulses had been effecting the video amp DC levels, resulting in short periods of no sync. The use of negative clipper after the de-emphasis network was not used as that would allow the radar pulses to damage the video syncs, by integrating them with the syncs before the clipper.
Using a complementry circuit (Inverted e.g. a T1=PNP, diode reversed, & the 330ê & 220k to the other power rails) will give a peak white clipper circuit.
ÚÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄÄ+11V Reg
2K2 ³ 22K ³ ³
ÃĴôÃÄ´ ³ 10n SYNC ³_ ³
Centre_ ³ u1Áu1 ³ ÃĴÿ CLIP ³/| ³
Freq |\ ÚÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄ¿ Á LEVEL 220k ³
1-10pÚ´ÃÄÄ´13 2 1 10 ³ RV1 / ³ ³
ÀÄÄÄÄ´12 NE564 FM DEMOD ³14 ³ ³/
Ä´ÃÄÄÂÄÄ´6 7 3 11 8 4 5 ÃÄÄÄ1KÄÄÂÄÄÄ)ÄÄÄÄ´T1
38MHz 1n 1K ÀÄÄÂÄÄÄÄÂÄÄÂÄÄÂÄÄÂÄÄÄÂÄÙ _³_ ³ ³\e____\ Video De-emphisis
ÀÄÄÄÄÄÅÄ1KÄÁ´Ã´ ³ ³4n7³ D1/_\ ³ R1 ³ / & Sound
n47=== 1n³ ³ === ³ +ÃÄÄÄÁÄ10KÄÄ´
³ ³ ³ ³ === ===C1 330R
2 470 ³ ³ ³68p ³10u ³
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Clipping in the buffer before de-emphasis was more effective. The effect of loosing the sound subcarrier during radar clipping was un-noticeable.
The circuit takes the average buffered DC video level with R1 & C1. RV1 is used to just get D1 to clip the high impedance base signal, when a white picture is present by adjusting VR1.
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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