[time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
saidjack at aol.com
Mon Sep 15 12:11:08 EDT 2014
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.
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.
Sent From iPhone
> On Sep 15, 2014, at 6:43, "Tom Van Baak" <tvb at LeapSecond.com> wrote:
> 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?
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