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

shalimr9 at gmail.com shalimr9 at gmail.com
Wed May 16 01:19:17 UTC 2012


Absolutely, that's what the pictures I linked to earlier show, particularly the last one. It is sufficient to properly terminate one end to eliminate ringing, as long as you have no tees in line. There may be specific reasons why it is preferable to terminate one end rather than the other, like protecting the driver. 

In any case, if the pulse is narrow, the risk of damage to the driver because of short to ground is also greatly reduced.

In my experience, I prefer to terminate the end because it tends to reduce the noise sensitivity at the receiver (not just for timing, but for anything where noise is critical). If the amplitude is too great, I put a matched divider at the end. I always try to have as much signal amplitude on the cable as possible to improve the S/N ratio.

Didier KO4BB

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

-----Original Message-----
From: Hal Murray <hmurray at megapathdsl.net>
Sender: time-nuts-bounces at febo.com
Date: Tue, 15 May 2012 15:02:36 
To: 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>
Subject: Re: [time-nuts] Why are 1PPS signals so skinny?


SAIDJACK at aol.com said:
> Also, the Thunderbolt has less than 5 Ohms output impedance, so you  get a
> reflection going back from the 50 Ohms end-termination anyway because the
> impedance is mismatched! 

I think that's a different problem.

If the far end termination matches the cable there won't be any reflection.

If the far end isn't terminated correctly, there will be reflections from the 
far end.  There may also be reflections from joints in cables or a Tee and 
input load if you are daisy chaining multiple instruments.  When those 
reflections get back to the typical low impedance driver, they will get 
reflected back again.

It's not uncommon to use both source/series and end/parallel terminations.  The series terminator drops the signal level by 2 but minimizes reflections if you are working in a less than ideal setup.  It also provides a current limit on the driver in case something gets shorted.


-- 
These are my opinions.  I hate spam.




_______________________________________________
time-nuts mailing list -- time-nuts at febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.


More information about the Time-nuts_lists.febo.com mailing list