Thanks for all the replies. There is no clock in my circuit so I can't
use the time sampling approach.
I heard about delay generator but still not sure how they could replace
the "functional" nature of monostable. i.e triggering on edges of
incoming signal with fix period (t).
Any more light on that?
Please post your replies below the previous posts (bottom post) rather
than above them (top posting). I've cut and pasted your reply to look
as if it was bottom posted.
If you want a counter-based digital monostable - which gives you the
maximum control of your pulse width - you need to introduce a clock
into your system. Farnell list a number of 50MHz clock modules, most of
them costing around $5.
The alternative is to used tapped lumped constant delay lines, which
cost at least twice as much. Farnell list parts from C&D Technologies -
which are real lumed constant delay lines, and ostensibly similar parts
from Maxim/Dallas which - IIRR - are actually a number of monostables
hooked up in series, which is a option that you might consider, since
the recovery time for either one of a pair of monostables in series is
going to be more than long enough to allow you to produce very nearly
overlapping pulses.
There is the problem that you have got to arrange to cover the gap when
the first monostable has timed out and triggered the second, but the
output of the second monstable hasn't yet reacted to the trigger. Three
monostables would make this easy.
The mechanism for capturing your trigger edge and turning it into a
constant width pulse could be as simple as an RS flip-flop. The
incoming edge flips the bistable from the Reset to to the Set state,
and the Set output is fed into a delay line, and in due course
propagates around to flip the bistable back to the Reset state.
With an untapped delay line, the RS flip-flop is then held in the
Reset state for the whole period of the delay, which isn't what you
want.
With a tapped delay line, you can gate the signal going into the delay
line with the outputs from all the subsequent taps, which holds the
duration of the pulse going through the delay line, and the recovery
time, down to the tap-spacing.
The details of the implementation are going to depend on what you are
actually doing, and on what the triggerig pulse actually looks like -
if it s a well-defined narrow pulse, you might be better off feeding
that straignt into the delay line.
Hope this helps. E-mail me - my address is real - if you don't want to
publish the details of your application.
