[time-nuts] Re-triggerable Monostable?

Brooke Clarke brooke at pacific.net
Thu Oct 25 14:21:14 EDT 2007

Hi Grant:

Your re-triggerable monostable idea has been implemented and is called a 
"missing pulse detector".

Have Fun,

Brooke Clarke
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Cam

Grant Hodgson wrote:
> ); SAEximRunCond expanded to false
> Errors-To: time-nuts-bounces+brooke=pacific.net at febo.com RETRY
> 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.
> regards
> Grant
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