[time-nuts] Opening an Isotemp OCXO

David davidwhess at gmail.com
Fri Oct 28 16:36:14 EDT 2016

I do not think the designer was considering noise at all because tying
the inputs together would not do anything useful.  Emitter resistance
is inversely proportional to emitter current (26/mA) but putting them
in parallel lowers the current through each emitter so the total
emitter resistance stays the same.

Supply current is separate for each TTL gate so by using a single
8-input part, total power it is about half that of a dual 4-input part
and a quarter of a quad 2-input part depending on the exact operating
conditions.  Unused *outputs* should be high for lowest power.

74S30 Single 8-Input 5mA 10mA
74S20 Dual 4-Input 8mA 18mA
74S00 Quad 2-Input 16mA 36mA

One clever design I ran across used the 7420 dual 4-input NAND pinout
but wired the inputs which are pin compatible so the 7400 quad 2-input
pinout would also work.  This allows using a 74x00, 74x20, or 74S120
dual 4-input NAND buffer but it draws even more power. 

74S120 Dual 4-Input 18mA 44mA

On Fri, 28 Oct 2016 13:31:38 -0400, you wrote:

>I wounder if originally the designer was hoping to use all 8 wire or'd
>inputs to lower the input referred noise during midscale transition. Then
>backed out later for some reason.
>On Fri, Oct 28, 2016 at 1:03 PM, Scott Stobbe <scott.j.stobbe at gmail.com>
>> Could also be a quirk about the 74S30 that gives it better phase noise
>> over a basic buffer.
>> On Friday, 28 October 2016, jimlux <jimlux at earthlink.net> wrote:
>>> On 10/28/16 9:13 AM, Scott Stobbe wrote:
>>>> The OCXO82-59 datasheet lists 12V supply, 5V clock out, could also be a
>>>> blown regulator in your ocxo, if it is indeed a 12v model.
>>>> There you go..the design could use a 74S30 as a driver - it's fast,
>>> fairly good drive, but runs off 5V.  If the regulator is shorted, and you
>>> put 12V on it, it will cook.

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