Hi Dave, what you are describing is reflected wave switching, which works perfectly in applications such as the PCI bus. The PCI bus uses it because it lends itself to extremely low power consumption. Your scenario does not work with end-termination either if you have multiple taps, because the taps would have to a) have very high impedance so as not to disturb the signal traveling down the pipe, and b) not have any cable length associated with them so as to prevent the edge going down into the stub and reflecting back from it, and the stub causing a 25 Ohms impedance drop (due to two 50 Ohms transmission lines in parallel). With the rise-times of sub 1ns (the CSAC GPSDO has a rise and fall time of <500ps typically) any cable length becomes an issue, including the couple of inches of wiring inside the counter and through the T-splitter etc. Another issue is that the timing of the instruments would be all off because of the differing propagation delays through the additional cable. So in the end-terminated world having a stub would only work if the stub does not have any cable length associated with it and very high impedance. How many folks distribute their 1PPS signal through a single coax cable to multiple instruments? This should be done through a 1PPS distribution amp. I agree though that in the analog video world you don't want reflections due to even small impedance mismatches because they cause ghosting. In the digital world where all we care about is that one single rising edge per second ghosting is not an issue. My original point was that the video world got it right: use a 75 Ohms output impedance for a 75 Ohms coax. The 1PPS world did not get it right by driving a 50 Ohms coax with a 10 Ohms output impedance. Bye, Said In a message dated 9/15/2014 10:43:24 Pacific Daylight Time, [email protected] writes:
I will agree that the end termination is optional if you are delivering a pulse signal to just one input, which is at the far end of the coax. However, I think there's still a problem with series-only termination when the pulse signal is daisy-chained through multiple inputs. When you apply 5 volts through a 50 ohm terminator to a 50 ohm cable, the instantaneous voltage on the coax is only 2.5 V. A pulse of amplitude 2.5 V travels down the cable, and reflects from the open far end. The reflection travels back along the cable to the source, raising the voltage from 2.5 to 5 V as it passes. A device input located at the far end of the cable sees a single edge of 5 V amplitude, so it's happy. But anything located somewhere along the cable run sees two edges: one from 0 to 2.5 V, then a constant 2.5 V for a period equal to twice the delay of the remaining cable, then another edge from 2.5 to 5 V. Depending on the input threshold, this in-between device might trigger reliably on the first edge, the second edge, or not reliably on either. Having proper far-end termination is critical for analog video, where daisy-chaining is common, and a reflection that's even 1% of the amplitude of the original signal is likely to be visible as a ghost image. With pulse signals, maybe it makes more sense to use one cable per device input, input plus lots of distribution amplifiers and splitters. - Dave On Mon, Sep 15, 2014 at 1:13 PM, S. Jackson via time-nuts <[email protected]_ (mailto:[email protected]) > wrote: Hi Dave, yes there is a reason. The "standard" 1PPS signal termination (Thunderbolt etc) used to be 5 Ohms or less series termination into a 50 Ohms coax (yikes), then end-terminate to get rid of all the undesired reflections. Your example below is properly terminating a 75 Ohms coax with a 75 Ohms series termination. The end-termination then becomes optional and affects the signal level at the sink. So if a higher signal level is desired, simply leave off the 75 Ohms end termination. But in the case of the Thunderbolt they don't use a 50 Ohms output impedance, they use something around 5 Ohms. That is the problem here: the total impedance mismatch from the very low source impedance into the 50 Ohms coax. The reason they do that is so that they can generate a "proper" signal level that is approaching 5V across the 50 Ohms end termination so that the signal remains CMOS compatible. Otherwise if they properly terminated the driver with 50 Ohms they would have a voltage divider and would only generate 2.5V at the sink. bye, Said In a message dated 9/15/2014 06:04:34 Pacific Daylight Time, [email protected]_ (mailto:[email protected]) writes: Is there any reason (other than cost) not to both series-terminate the source and parallel-terminate the sink? When I was dealing with analog video, the standard distribution method was : 1. Buffer amplifier with high input impedance, very low output impedance, and a gain of 2 (so 1 V P-P input becomes 2 V P-P out) 2. A series 75 ohm resistor from the amp output to each individual video output. This formed a 2:1 voltage divider with the 75 ohm coax to give 1 V P-P on the cable. It also isolates the loads from each other. 3. A single video signal could be looped through multiple high impedance loads. 4. 75 ohm parallel termination at the far end of the signal path (usually on the last device). This way, every device along the way saw an undistorted copy of the signal. The buffer amplifier sees a simple resistive load. And any reflections are absorbed at both ends of the cable. - Dave On 15/09/2014 02:04, Fuqua, Bill L wrote: > A lot of devices have a low output impedance so that the signal can be split using a TEE adapter with little loss or need for a distribution amplifier. > However, the cables must be impedance matched at far end, scope input, to prevent reflections which are the source of the ringing. > You can match the impedance at the source and you will get a reflection which will then be absorbed by the source resistance. One way to do this > is to get a small 15 turn pot about 100 Ohms put it, in series with the input source and adjust it until the ringing is gone or you can put it at the far end > ,input of the scope, to ground and do the same. But the best solution is to get a good feed thru 50 Ohm terminator and put it on the input of the scope. > Bill > > _______________________________________________ time-nuts mailing list -- [email protected]_ (mailto:[email protected]) To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected]_ (mailto:[email protected]) To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
