🚀 go-pugleaf

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