[time-nuts] Why are 1PPS signals so skinny?

[shalimr9 at gmail.com]

The Thunderbolt's output impedance is much less than 10 ohms. However, it is only necessary to filter the end of the line for a clean pulse.

See http://www.ko4bb.com/Test_Equipment/CoaxCableMatching.php

I used the Thunderbolt's PPS output as a source in those measurements.

Didier KO4BB

Sent from my BlackBerry Wireless thingy while I do other things...

-----Original Message-----
From: Said Jackson <saidjack at aol.com>
Sender: time-nuts-bounces at febo.com
Date: Mon, 14 May 2012 19:02:51 
To: Tom Van Baak<tvb at leapsecond.com>; Discussion of precise time and frequency measurement<time-nuts at febo.com>
Reply-To: Discussion of precise time and frequency measurement
	<time-nuts at febo.com>
Cc: Discussion of precise time and frequency measurement<time-nuts at febo.com>
Subject: Re: [time-nuts] Why are 1PPS signals so skinny?

These types of pulses should be routed as open-ended source-terminated reflected wave switched transmission lines. Power will only flow for nanoseconds as the pulse travels over the line. There won't be a drop of 50% of the voltage at the target and no large power spikes in the unit or requirements for proper impedance matching at the receiver side.

Some units like the thunderbolt look quite bad driving a 50 ohms transmission line, others that are designed with proper 50 ohms series impedance create a sharp nice signal.

Bye,
Said




Sent from my iPad

On May 14, 2012, at 17:21, "Tom Van Baak" <tvb at LeapSecond.com> wrote:

> Mark,
> 
> I too once preferred 50% duty cycle 1 Hz signals because they seemed more "natural". But one day during an experiment where I was comparing a large set of clocks I noticed my lab's digital AC power meter was jumping by tens of watts every second.
> 
> When a dozen DUT generate 1PPS along with as many REF pulses (via cascaded pulse distribution amps) and then these all go to both inputs of a TIC and there's also LED's on both TIC channels as well as the dist amps, the net load is enormous. The last thing you want in a precision timing lab is to load your AC line down exactly once a second. Remember 5V into 50R is 0.1 Amps. That was a modest amount of current in the 1950's, but massive overkill today.
> 
> So that's why I now prefer short (e.g., 1 ms or 10 us) pulses.
> 
> /tvb
> 
> 
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