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Date Posted: 04:31:38 12/17/01 Mon
Author: Potter
Subject: Re: re: Instability
In reply to: lcs 's message, "Re: re: Instability" on 20:17:34 12/14/01 Fri

I initially had some problems with this and what I did was leave out the 2 resistors in question at first and wire in a dual 10k linear pot so I could adjust for the best result, measure the pot setting and replace with the nearest preferred value. My earliest layouts had the resistor and capacitor feeding video to the edge detectors reversed because it made a neater PCB - this arrangement needed a very high resistance in place of the 1k (2k7-4k7)to work reliably (using an LM339). I've corrected the layout now and a much lower value is required (using LM393 which should be identical to LM339 comparators). The best ratio seems to be 100:1 eg 470-4.7-4.7-470 or 1k-10-10-1k etc when feeding video via a 470ohm resistor in series with 220pF from an LM733 video buffer's output. I also used a much larger capacitor on the centre of the divider for Vref ie 47uF instead of 0.33uF.

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