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

Graham / KE9H ke9h.graham at gmail.com
Sat Jun 17 08:43:26 EDT 2017

Read the spec sheet on the part you are using for a driver.
The size of the resistor on the paralleled driver side will be set by the
maximum current of the devices.

Decide how conservative a design you want.
Do you want it to drive into a shorted load and survive?
[If each output is rated at 20 mA, in a 5 Volt system, then R=E/I, = 250

Or do you only want it to work into a 50 Ohm load (to ground)?

Or perhaps only a Thevenin load? 50 Ohm load, but 100 Ohms up to +V, and
100 Ohms to ground.

Remember that 50 Ohms in a 5 Volt system will draw 100 mA on the 'High'
which can be a lot of current
for a modern IC.

--- Graham


On Sat, Jun 17, 2017 at 4:01 AM, Hal Murray <hmurray at megapathdsl.net> wrote:

> 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
> planes.
> 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
> 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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