[time-nuts] Hints on PPS Buffer design...
cautery at montac.com
Sat Jun 17 09:14:20 EDT 2017
On 6/17/2017 4:01 AM, Hal Murray wrote:
>> 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?
Not nearly good enough, as I don't have one. Have to borrow. Thank you
for pointing me in the right direction on bandwidth...
>> 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.
Yes, I got this part... At least I generally get something. ;-)
> 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.
Apologize, was parroting the term used in the archived Time Nuts post I
found referencing the need to address the "driver impedance" in
selecting series resistors when trying to set the output impedance.
> 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.
OK... I'll have to try and study this some more, because for a simpleton
like me, and impedance mismatch between the PPS out port on the Nortel
and the device to which it is connected of 25:50 ohms, reduces to 1:2,
flipped around is a 2:1 VSWR, which means a fairly significant power
loss... Maybe it doesn't matter much with a short cable.. but the cable
is also tiny/lossy.
> 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.
Yes, I intend to do extensive testing. I was just trying to trim away
some of the waste up front... time, materials, et al. :-)
>> 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.
Yep... gonna need some more instruments asap... I may just have to wing
it and accept "good enough" until I can round up the rest of the
Thanks for your assistance and not brutalizing me for my lack of
knowledge. I'm trying to learn.
- C. Autery
More information about the time-nuts