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Discussion of precise time and frequency measurement

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Buffering a PPS signal

Bob Camp

Fri, May 18, 2012 7:37 PM

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

Bob

-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@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:

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@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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Hi If that's a concern, use a 74ACT part Bob -----Original Message----- From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@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@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 > > >_______________________________________________ >time-nuts mailing list -- time-nuts@febo.com >To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >and follow the instructions there. Jerry Mulchin _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.