[time-nuts] Buffering a PPS signal

Bob Camp lists at rtty.us
Fri May 18 19:37:57 UTC 2012


Hi

If that's a concern, use a 74ACT part

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Jerry Mulchin
Sent: Friday, May 18, 2012 3:20 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Buffering a PPS signal

Be careful that you use the proper logic device for the GPS 1PPS output.
An 74AC device has a minimum HIGH input logic level of 2.1 volts when
running with
a 3.0 volt VCC, and 3.65 when running at 5 volt VCC. A GPS receiver running
at 3.3
volts may be on the edge. Consider using a 74F part instead as it has a
lower HIGH
input requirement when running a 5 volt VCC than a 74AC.

Jerry

At 11:50 AM 5/18/2012, you wrote:
>On 05/18/2012 07:24 AM, Bob Camp wrote:
>> Hi
>> 
>> For a 50 ohm buffer, you probably want something like 200 ohms in series
>> with each output (4 buffers) or 400 ohms (8 buffers).
>
>It may be worthwhile considering the gate output impedance as well.  For
>example, for 5V AC logic driving a 50 ohm line the initial edge from the
gate is
>about 4V - which implies a Zo of about 13 ohms.  A 50 ohm buffer with 4
gates
>would therefore use 187 ohms series resistors.
>
>There is an app note attached here that discusses this:
>
>  http://c-c-i.com/node/107
>
>-- 
>Bob Smither                                              Smither at c-c-i.com
>==========================================================================
>A well regulated Militia, being necessary to the security of a free State,
>the right of the people to keep and bear Arms, shall not be infringed.
>       -- Second amendment to the Constitution of the united States
>==========================================================================
>Circuit Concepts, Inc.                                        281-331-2744
>
>
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Jerry Mulchin



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