[time-nuts] Re-triggerable Monostable?

Grant Hodgson 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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