[time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
SAIDJACK at aol.com
SAIDJACK at aol.com
Mon Sep 15 16:53:20 UTC 2014
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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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