[time-nuts] Re-triggerable Monostable?
grant at ghengineering.co.uk
Thu Oct 25 11:41:58 EDT 2007
I'm working on a synth. project which can take either an external 10MHz
reference input or use an internal TCXO. It would be nice if the
presence (or more importantly, failure) of the external 10MHz ref. could
be detected automatically, and signaled to the user.
There are, of course, a number of ways to do this. One is to use a
level detecting circuit; simply AM detect the ext. ref. input and apply
to a comparator. Lots of circuits to do this, no problem - but requires
a number of components, unless there is some magic IC that will do it
that I'm not aware of - I suppose a dual op-amp is all that's needed,
plus a handful of discretes.
Another option might be to use a re-triggerable monostable, which is
constantly looking for the presence of (say) a falling edge every 100nS
on the ext. ref. input line. It would seem that conceptually, this
would be the easiest way, possibly using less components. Does anybody
have any favourite circuits for this; or are there any caveats that are
not mentioned in the plethora of monostable circuits that are mentioned
on the 'web?
Mnay (older) test equipments have an int/ext switch on the back, with an
indicator on the front, but some newer ones detect the ext. ref. and
switch automatically. I suppose that's not a problem on a $$$$ box,
which this synth. is not.
The output would simply be a high/low digital line, the ext. ref. input
will of course be buffered; I can cope with switching the 10MHz signals.
Any ideas appreciated.
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