Yes, but the point is to not use end-termination for all the reasons mentioned by others in this thread, such as massive spike in power consumption once per second, over-voltage spikes if the termination is faulty or missing, higher ADEV due to power supply modulation, etc etc.. Your test clearly shows the ringing when the transmission line is left open, and it shows the massive current (5V into 50 Ohms) when end-terminating the cable. It also shows that the cable is ringing up to 7V or more! Which could actually kill your driver circuit by overvoltage if you forget to enable the 50 Ohms termination on your counter or scope for example. The voltage could theoretically spike all the way up to 10V as long as the pulse is traveling back on the coax. All more reasons why this is an undesirable mode of operation. To make this work without the unnecessary power consumption simply remove the end-termination resistor, and use it as the series termination resistor (R1 in your schematic)! Done. Attached are two plots of a series terminated (~55 Ohms) high-speed 1PPS transmission from our CSAC GPSDO board zoomed-in and zoomed-out to show the actual rise-time, and a longer time frame view. The 1PPS pulse was run through about 30 feet of LMR-195 cable, directly connected to the CSAC GPSDO 1PPS CMOS 5V output. There is no massive voltage over-shoot, the output is short-circuit protected, and no matching resistor is required, just a 50 Ohms coax cable. Use an additional 25 Ohm series resistance for 75 Ohms cables. The output rise time is 1.25ns at the end of the 30 foot cable, and the signal fully settles within 800ns, and never goes below 4V after the initial 1.25ns rise. The current spike on the power supply is only there during the time that the cable is being charged up, which is about 30 feet * 2 * 1.5ns/foot = ~100ns. That is short enough for the power supply caps to filter the current spike. In short, one could easily modify the Thunderbolt 1PPS output circuit which is probably a bunch of parallel AC240 gates with some low value series resistors, and modify these resistors to have the equivalent of 50 Ohms impedance. That would alleviate the need for end-termination on the coax, and provide very clean rise time, fall time, and no ringing. BTW: one advantage of this in the lab is that you can connect multiple instruments to one 1PPS output. The signal will take slightly longer to settle as it has to traverse and charge more cable hubs, but in the end there will be 5V on the cable with no DC current flowing, and there won't be any positive ringing above 5V. You cannot drive more than one input if you are using 50 Ohm end-termination without possibly over-loading the driver, and causing massive impedance mismatch, and getting the associated cable ringing etc. bye, Said In a message dated 5/15/2012 11:49:14 Pacific Daylight Time, shali...@gmail.com writes:
The Thunderbolt's output impedance is much less than 10 ohms. However, it is only necessary to filter the end of the line for a clean pulse. See http://www.ko4bb.com/Test_Equipment/CoaxCableMatching.php I used the Thunderbolt's PPS output as a source in those measurements.
<<CSAC_risetime.GIF>>
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