[time-nuts] 1Mhz to 1 PPS

Hal Murray hmurray at megapathdsl.net
Tue Nov 27 13:53:47 EST 2007

> I would like to convert a 1 MHz sine wave to a 1 pps TTL.

I'm assuming you aren't too concerned about jitter.

There are two issues.  The first is converting your sine wave into a valid 
logic level.  The second is dividing by a million.

If your sine wave has a reasonable amplitude, I'd just feed it into a logic 
gate.  If it's too big, I'd add a resistive divider.  You probably want to AC 
couple the input.  That needs something to bias it at the right level.  A big 
resistor from an inverted output usually works well.  It sets the bias point 
to give you a 50-50 duty cycle.

If you want to get fancy, use a comparator.

For home construction, a row of 74HC390s is the best divider I can think of.  
It gives you 2 divide by 10 stages in each package so you only need 3 chips.

Are you tight for space?  Do you like low level software?  I'd probably do 
the dividing in software on a PIC or AVR.  The 8 pin dips are easy to work 
with but they come in tiny packages too.  They are usually setup to work with 
a raw crystal or external clock.  You can probably find something that will 
work with your sine wave.

Another alternative to a row of '390s is a CPLD or FPGA.  They usually only 
come in packages with tiny pins which are hard for my old eyes to work with.  
They might make sense if you need some logic for something else.

These are my opinions, not necessarily my employer's.  I hate spam.

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