[time-nuts] Re-triggerable Monostable?

Don Collie donmer at woosh.co.nz
Fri Oct 26 06:01:32 EDT 2007

Hi Grant,
    What`s wrong with a diode and a 
capacitor?.........................................Don C.

----- Original Message ----- 
From: "Grant Hodgson" <grant at ghengineering.co.uk>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Friday, October 26, 2007 4:41 AM
Subject: [time-nuts] Re-triggerable Monostable?

> ); SAEximRunCond expanded to false
> Errors-To: time-nuts-bounces+donmer=woosh.co.nz 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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