[time-nuts] Got 60HZ?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Dec 9 14:22:02 EST 2010

If one actually requires a low phase noise/low jitter output using a 
Schmitt trigger is not a good idea.
The effective threshold is modulated by the internal noise of the 
schmitt trigger circuit itself.
This effect can be very significant with low phase noise sources and a 
fast Schmitt trigger.
For noisy input signals the signal phase noise will modulate the phase 
of the output signal.


Chris Albertson wrote:
> What you are doing is "dithering".  That is the "Leap Count".  There is a better
> way that gives you the exact solution.     If you think about it, what is the
> computer doing between counts? nothing really.   Put that time to use.
> Why not measure the the 1Khz period.  Or measure the period of the last
> 1000 counts.  then toggle the output at 60/1000 of that period.  This is a
> "software phase lock loop" .   Another way to think about this is to think what
> would you do if your frqency reference was a one pulse per second "tick".
> About how to "square" a wave.    Some people suggest amplifying the sine wave
> then feeding it to a 74xxx series logic gate.  There is no need to amplify.
> What you do is compare the sine wave to zero.  All sine waves of any amplitude
> even if it is .1 Volt or 100 Volt peak to peak, they all cross zero so
> you flip the square
> wave each the sine wave crosses zero.  But do it with a "Schidt trigger".
> This is a very noise immune circuit.  Place
> the gard bands at -0.1V and +0.1V and it will work with any sine wave,
> no amplifier
> required.   Also you can build it with an 8 pin dip chip, no need for
> a 14 pin chip.
> See here for more on this.
> http://en.wikipedia.org/wiki/Schmitt_trigger
> On Wed, Dec 8, 2010 at 8:13 PM, Michael Poulos<poulosmd at gmail.com>  wrote:
>> Recently I bought a Efratom Ru frequency standard from eBay and a frequency
>> divider chip that makes 1MHZ,100KHZ,25KHZ,10KHZ,100HZ and a 1HZ output.
>> Today I thought of a way to make a nice 60HZ so you can use a mains-powered
>> clock for the display (using amplifier and transformer wired "backwards").

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