[time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.

[SAIDJACK at aol.com]

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Hi guys,

Tried to bring my point  across, but I guess I failed to do so  properly.

What happens after the edge is very  important because what happens after 
the edge settles is up to 100mA DC current  is flowing through all the coaxes 
AND your building  ground.

Pumping ~5V into 50 Ohms (Thunderbolt)  results in up to 100mA DC current 
flowing. This current flows out into the  center conductor then through the 
50 Ohms termination resistor at the sink and  then back through ALL your 
grounds due to the finite resistance of your  coax.

This includes the instruments' AC power  cord, as well as any 10MHz coax 
you have  connected!

This DC ground current now does many  bad things:

1) it can corrode the connectors  over time in humid environments (eg  
shipboard)

2) it causes measurable and  significant  (~0.5W!) heating in the 
termination resistor ( I have IR video  that shows the termination resistor blink 
like a christmas tree once a  second)

3) it causes significant dips in the  source power supply and heating of 
the driver ICs in the  source

4) it causes a high voltage drop across  all coax connections which results 
in a corresponding shift in the ground  potential of the 10MHz signal and 
thus results in amplitude modulation of the  10MHz signal (CMOS). RG-142 
shield has 0.0075 ohms per meter, so the AM  modulation of the 10MHz signal over 
several meters could be in the millivolts -  not conducive for measuring 
stability in ppt

5)  if the termination fails or you leave the coax end-termination 
unconnected then  your driver (a number of standard AC gates in parallel in case of 
the  Thunderbolt) will get the full brunt of the reflected pulse which will 
be up to  10V for a significant amount of time so you are over-stressing 
that gate. If the  termination fails or is disabled, your counter input or 
scope input may also be  overstressed by the double amplitude. On the falling 
edge it gets even worse:  the reflections generate negative voltages far below 
ground level and can also  cause driver over-stress. 

In  summary:

End-termination is designed for  maximum power transfer for RF signals. It 
should not be used for transmitting DC  signals such as 1PPS signals (the 
1PPS pulse is a very high frequency AC signal  until the reflections settle in 
some 10's of nanoseconds, then it is a DC  signal)

Series termination such as used for  reflected wave switching (ie PCI) is 
the way to go for 1PPS signals and has  essentially no drawbacks for fast 
rising edges other than that a resistor must  be inserted at the output of the 
driver.

Hope I  made the advantages of series rather than end termination clear. I 
understand  that we all were taught in school that a coax needs to be 
terminated, and series  termination is just that - but at the other end of the 
cable which is somewhat  counter intuitive.

The above except item 1) is  easy to verify and a lot if fun to do. All you 
have to do is insert that single  series resistor after the driving gates 
and remove the end-termination and your  system will be updated to 21st 
century  standards.

Btw I have extensive scope plots  comparing series- to end-termination over 
10+ feet of coax if anyone is  interested.

Bye,
Said



In a message dated 9/15/2014 06:44:21 Pacific Daylight Time,  
tvb at LeapSecond.com writes:

How  important are all these cable / termination / impedance issues for 
1PPS  signals? I know ringing and reflections are undesirable in many 
applications.  But for 1PPS?

I often use pick whatever cable, termination, and trigger  level gives the 
cleanest edge, the best risetime. What happens to the signal  tens or 
hundreds of nanoseconds after the edge seems irrelevant to me. Could  one of you 
RF experts  comment?

/tvb

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