R:190707/2047Z @:LW3DBH.1744.BA.ARG.SA #:38930 [MORENO] FBB7.00i $:11628_GB7CIP R:190707/2045Z @:LU4ECL.LP.BA.ARG.SOAM #:4728 [La Plata] FBB7.0.8-beta2 R:190707/2045Z
4393@I0OJJ.ITA.EU [Rome] $:11628_GB7CIP
R:190707/2044Z @:GB7CIP.#32.GBR.EURO #:11628 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
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 330Ω & 220k to the other power rails) will give a peak white clipper circuit.
┌───────┬───────┬─────────────┬──────┬──+11V Reg
2k2 │ 22k │ │
├─┤├┬┤├─┤ │ 10n SYNC │_ │
Centre_ │ u1┴u1 │ ├─┤├┐ CLIP │/| │
Freq |\ ┌────┴───────┴───────┴─┐ ┴ LEVEL 220k │
1-10p┌┤├──┤13 2 1 10 │ RV1 / │ │
└────┤12 NE564 FM DEMOD │14 │ │/
─┤├──┬──┤6 7 3 11 8 4 5 ├───1k──┬───)────┤T1
38MHz 1n 1k └──┬────┬──┬──┬──┬───┬─┘ _│_ │ │\e____\ Video De-emphisis
└─────┼─1k─┴┤├┤ │ │4n7│ D1/_\ │ R1 │ / & Sound
n47=== 1n│ │ === │ +├───┴─10k──┤
│ │ │ │ === ===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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