[time-nuts] Hints on PPS Buffer design...
tshoppa at gmail.com
Sat Jun 17 08:19:22 EDT 2017
Clay, as to "why 20 ohms out", there is a long-time-nominal 50 ohm PPS
convention that calls for 5V pulses to be delivered into a 50 ohm load.
If the driving voltage was 5V and source resistance was 50 ohms, then you'd
never get more than 2.5V into the load.
Different references across the net show 5V chips driving parallel 5x100
ohms, or 5x47 ohms. These are all attempts at getting more than 2.5V and
closer to 5V into the load. If you run the driver chip at 6V (which is
actually permissible for some HC-derived logic chips) then you can get even
closer to 5V into a 50 ohm load.
Most devices that consume this 5V-into-50-ohm signal are actually fine with
less than 2.5V but that's very vague.
There are real coaxial cable driver chips as well as arrangements of
bipolar transistors that do a very good driving long 50 ohm coax cables to
5V. These were very popular in the 1960's and 1970's as coax drivers for
mainframe/minicomputer computer peripherals, and are still available today
but not nearly so popular.
A much more popular cable driver standard from the past 20 years would be
LVPECL or LVDS but those are nominally differential and everyone seems to
insist on coax even today.
On Sat, Jun 17, 2017 at 4:15 AM, Clay Autery <cautery at montac.com> wrote:
> Trying to pin down a reasonably optimal buffer design for bringing PPS
> out... I've looked at all the references, like the i3detroit.org site
> et al.
> Of the few schematics and devices I see, most are using a hex inverter
> (1 into the other 5 paralleled with series resistors for "balance" and
> setting output impedance?
> Q: Why does everyone pick FIVE x 100 Ohm resistors? That's 20 Ohm out,
> not counting the gate impedance on the hex inverter...
> Q2: Anyone have a reference to the math for choosing the resistors for
> setting a 50 Ohm nominal out INCLUDING determining and including the
> gate impedance of a particular part.
> (Right now, I am going to use the TI SN74AC04 Hex Inverter) I saw a
> refernence in the archive referring to a 4 gate setup using a different
> part needing 187 Ohm resistors... thus I can only include that I need to
> use something slightly more than 250 Ohms on a 5 gate parallel setup)
> Q3: It's only a 1Hz frequency, but is low inductance a desired trait of
> the chosen resistors?
> I'm sure there are others...
> Clay Autery, KY5G
> MONTAC Enterprises
> (318) 518-1389
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