[time-nuts] PRS-10 PPS output

[Joseph Gwinn]

On Fri, 03 Apr 2020 20:00:59 -0400, time-nuts-request at lists.febo.com 
wrote:

Re: time-nuts Digest, Vol 189, Issue 7
-------------------------------------------------------------
> 
> Message: 1
> Date: Fri, 3 Apr 2020 15:48:50 -0400
> From: Bob kb8tq <kb8tq at n1k.org>
> To: Discussion of precise time and frequency measurement
> 	<time-nuts at lists.febo.com>
> Subject: Re: [time-nuts] PRS-10 PPS output
> 
> Hi
> 
> At least according to page 20 in:
> 
> https://www.thinksrs.com/downloads/pdfs/manuals/PRS10m.pdf
> 
> the ?raw? PPS output from the PRS-10 is just a CMOS gate output. 
> There is nothing suggesting it is designed to drive a 50 ohm load 
> or a coax cable. 
> 
[snip]
> 
> Best guess: It's doing what it's supposed to, but not quite what 
> you expected it to do.
> 
> 
> Message: 2
> Date: Fri, 3 Apr 2020 20:27:25 +0000 (UTC)
> From: Taka Kamiya <tkamiya9 at yahoo.com>
> To: Discussion of precise time and frequency measurement
> 	<time-nuts at lists.febo.com>
> Subject: Re: [time-nuts] PRS-10 PPS output
> 
> I've seen this with my own.? You are catching the leading edge of 
> ringing of the signal.? 
> 
> Since the output is CMOS, and high impedance, it really doesn't have 
> enough current to push through low impedance cables.? Try shorter and 
> different cable.? Terminate it with 50 ohms.? Set the trigger level 
> little higher.? In my setup, the cable is actually a very short 
> twisted pair, and only 2" long into a buffer board.

I've had this issue with coax 1PPS outputs across the board.  What I 
generally do is to attach a coaxial 50-ohm attenuator between 
instrument coax output connector and the 50-ohm cable it will drive.  
The attenuator matches the output impedance to the coax impedance.  I 
use attenuators between 3 dB and 10 db.  The actual attenuation is not 
as marked, because of the mismatch between the ~1K output impedance and 
the 50-ohm input of the attenuator.  I suppose that a 75-ohm attenuator 
may work as well or better to drive 50-ohm cable, but have not tried 
it.  All the ringing et al are suppressed because the length of the 
mismatched part of the path is maybe an inch or two.

Joe Gwinn

> 
> End of time-nuts Digest, Vol 189, Issue 7
> *****************************************