[time-nuts] PRS-10 PPS output

[Joseph Gwinn]

On Sun, 05 Apr 2020 12:00:02 -0400, time-nuts-request at lists.febo.com 
wrote:
Re: time-nuts Digest, Vol 189, Issue 9
[snip]
> ----------------------------------------------------------------------
> 
> Message: 2
> Date: Sat, 4 Apr 2020 14:08:02 -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
> Message-ID: <1131AF5E-3444-4E48-B5D9-ECB36457D014 at n1k.org>
> Content-Type: text/plain;	charset=utf-8
> 
> 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. 

Well, the coax 1PPS outputs I've had to deal with are all claim in 
their datasheets to be able to drive a 50-ohm load, so I didn't worry 
about overloading the output circuitry with a standard attenuator.  
Their source impedance seemed to be closer to 200 ohms, as I recall.

The PRS-10 datasheet (page 59) says:  "The LOCK/1PPS function may be 
configured via RS-232. The factory default is a low level to indicate 
lock, with a 10μs pulse to +5V at 1PPS, with the leading edge being 
defined as the 1PPS timing reference. This BNC output is a CMOS logic 
output via a 1kΩ resistor."

So, a 50-ohm load (or even a dead short) should not hurt anything.  And 
we can predict the peak voltage over a 50-ohm load driving a 50-ohm 
coax is 5(25/1000)= 0.125 Vpeak.

For the TTL outputs in the days of yore, the series 50-ohm resistor was 
standard practice.    

Joe Gwinn


Context:
> 
>> I've [JMG] 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.
>>