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G8MNY  > ATV      14.05.18 21:46l 43 Lines 2004 Bytes #190 (0) @ WW
BID : 12702_GB7CIP
Subj: Radar Clipper for 23cms
Sent: 180316/1000Z @:GB7CIP.#32.GBR.EURO #:12702 [Caterham Surrey GBR]
To  : ATV@WW

By G8MNY                      (CQTV 1994, New to packet Feb 06)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

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
330 & 220k to the other power rails) will give a peak white clipper circuit.

                 +11V Reg 
                2k2            22k                  
                 Ĵ´Ĵ        10n   SYNC  _     
 Centre_          u1u1        Ĵÿ   CLIP  /|    
 Freq |\    Ŀ   LEVEL 220k    
  1-10pڴĴ13  2       1      10      RV1 /       
       Ĵ12  NE564 FM DEMOD    14             /
 IF>ĴĴ6 7    3  11 8  4   5 1k)ĴT1
38MHz 1n 1k       __      \e____\ Video De-emphisis
         1kô    4n7      D1/_\   R1       / & Sound
           n47===    1n   ===          +10kĴ
                            ===       ===C1      330
               2      470      68p      10u       

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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