[time-nuts] Hints on PPS Buffer design...

Hal Murray hmurray at megapathdsl.net
Sat Jun 17 05:01:18 EDT 2017

cautery at montac.com said:
> Q3: It's only a 1Hz frequency, but is low inductance a desired trait of the
> chosen resistors? 

It's a 1 Hz repetition rate, but the bandwidth depends upon the rise time.  
If the rise time is ballpark of 1 ns, the bandwidth will be ballpark of 1 
GHz.  So, yes, you want low inductance.  That includes the power to the chip 
as well as the resistors.  Surface mount is your friend.  So are ground/power 

How good is your scope?

> Q:  Why does everyone pick FIVE x 100 Ohm resistors?  That's 20 Ohm out, not
> counting the gate impedance on the hex inverter... 

The FIVE is 6-1.  The one is for isolation.  The 5 is the rest of the 
package.  You might as well use them all as drivers.  You don't want to use 
them for another signal (even if it is supposed to be identical) or you will 
get minor crosstalk when you do things like plug or unplug a cable.

I don't know why you are saying "gate impedance".  That's over on the input 
side.  I would have said "output impedance" or "driver impedance" of the chip 
or section.

I'm not sure why they picked 100 ohms.  Assume the net source impedance is 25 
ohms.  Suppose the far end is terminated with 50 ohms.  There won't be any 
reflections so the source value doesn't matter.  25 ohms will provide a 
higher voltage at the far end than 50.  If you have a CMOS driver and a CMOS 
receiver, 1/2 the voltage at the receiver is nasty.  It might be OK if you 
have HT type receivers.

I would suggest a bit of lab work.  What are you going to use on the far end?

Lots of gear has 1000 ohms rather than 50 so a 50 ohm source impedance takes 
care of the reflections and leaves (almost) the full voltage at the receiver.

cautery at montac.com said:
> thus I can only include that I need to use something slightly more than 250
> Ohms on a 5 gate parallel setup)

More than 250 divided by 5 and rounded up a bit for the output impedance of 
the chip will be more than 50 ohms.  I'd do some experiments.

These are my opinions.  I hate spam.

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