[time-nuts] Hints on PPS Buffer design...
albertson.chris at gmail.com
Sat Jun 17 12:17:32 EDT 2017
Why 100R? If using a 74xxx logic part as the driver is has a maximum
drive current. 100R limits current in case of a short to ground or 5V
supply to only 50mA It is about the lowest value resister i'd want to use
If you need 50 ohm output them go with an small RF amplifier or a
transistors driver. These can drive a 50R load without burning up.
On Sat, Jun 17, 2017 at 1:15 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
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