[time-nuts] PRS-10 PPS output

[Bob kb8tq]

Hi

Pulling the output of a normal CMOS gate to ground through a small 
resistor is not generally a good idea. That’s what an attenuator or far 
end termination of the coax is doing. It’s not specifically outlawed in the
spec, but it’s still not what they are designed to do. Also the output level
is going to be pretty wimpy run through an attenuator. 

One way to “fix” the problem is with a 50 ohm series resistor at the source
end. That only works to the degree that the output impedance of the gate
is very low when in saturation. How true this is …. that depends. 

A far better approach is to buffer the output through something like a 
set of 2, 3, or 4 NC7SZ125’s (or similar) in parallel. The output from 
something like that has much higher drive capability than a single gate
output. Source termination with a single 50 ohm or with 100, 150, or 200
ohms on each gate works pretty well in this case. 

(If you want to dig further into the details of “why source termination?” 
there are several posts from Said back about 4 or 5 years ago).

You do have the problem of not knowing what the “far end” is expecting. 
If it has a 50 ohm load *and* is looking for a 5V logic signal …. source 
termination isn’t going to work with normal logic gates doing the drive 
end of things. You would need a high speed gate that is happy with a 
10V supply to do that …..

Unfortunately, there *is* gear out there that terminates a PPS with a 
50 ohm load *and* expects some sort of logic level. Is it set up for 1.2V, 
1.65V, 2.5V …. ???? Often this is not very well documented. 

If you *do* set up to “straight drive” a 50 ohm end termination to 5V in order
to do right by a 2.5V logic level: Spend some time looking at the power
distribution approach on your board. That’s a 0.1A 10 us wide pulse you
are thumping onto the supply rail…..

Bob

> On Apr 4, 2020, at 1:20 PM, Joseph Gwinn <joegwinn at comcast.net> wrote:
> 
> 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
>> *****************************************
> 
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