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

Said Jackson saidjack at aol.com
Tue May 15 02:02:51 UTC 2012


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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