nagle@well.sf.ca.us (John Nagle) (01/03/91)
I need to build a filter that will pass only 200ns or shorter pulses found in a video signal. (These pulses appear because the video signal comes from a camera looking at a scene which contains a laser beam.) Any suggestions as to standard ICs which implement a suitable filter with minimal parts count and few if any screwdriver adjustments? John Nagle
whit@milton.u.washington.edu (John Whitmore) (01/08/91)
In article <22397@well.sf.ca.us> nagle@well.sf.ca.us (John Nagle) writes: > > I need to build a filter that will pass only 200ns or shorter >pulses found in a video signal. It's really very simple; you connect a shorted delay line to your signal input (after preamplification, if you prefer). For full 200 ns, you'll need enough coax cable for 100 ns one-way delay; about 60 feet of RG-59 should do it. After 200 ns, the cable is a short-circuit; until then, the pulse passes with only the characteristic impedance of the cable loading it. The cable is equivalent to some horrendous infinite array of inductors/capacitors (any time-selective filter to match its performance would take a LOT of components). A positive-going pulse becomes a positive pulse followed by a negative echo; the large slew from positive pulse to negative echo is easily discriminated from a lot of noise-like artifacts, IF you know that 200 ns pulse width accurately enough to tune for it. Example: --- --- short pulse ----- -------- => ------- -- ---------- --- ----- ----- medium pulse ----- ----- => ------- -------- ----- ------- ----- long pulse ----- ---- => ------- -- -------- ----- As shown above, only the pulse of the correct length (like, 200 ns) has a twice-amplitude fast slew in the as-processed signal. John Whitmore whit@milton.u.washington.edu