[time-nuts] Need info on HP 1 MHz ovenized XO

[David]

On Sun, 13 Jan 2013 19:00:46 -0500, "Charles P. Steinmetz"
<charles_steinmetz at lavabit.com> wrote:

>Ed wrote:
>
>>On opening it up I found that the circuitry includes a 74LS73 dual 
>>JK FF, and a 74LS140 - very obscure - apparently a dual 4-input gate 
>>of some sort.
>
>AFAIK, the '140 was only supplied in the "S" series (74S140).  It's a 
>dual 4-input NAND 50 ohm line driver.

That is my understanding.  Driving 50 ohms is going to take so much
power that producing a slower and lower power version would hardly be
worthwhile.  There are a couple of other TTL 50 ohm drivers, the 74128
quad 2-input NOR comes to mind, but I think the 74S140 was the most
popular.  Both of those are still produced by Texas Instruments.

If you want a lower power then the other dual 4-input NAND gates
(7420, etc) use the same pinout.  Cleverly, and I have seen at least
one design that did this, the 74S140 pinout can also be compatible
with the quad 2-input NAND gates (7400, 7437 buffer, etc) if you treat
them as 2-input devices.  The extra 74S140 inputs can either be tied
high or the extra '00 gate inputs and outputs can be tied in parallel
for extra drive.