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]> 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] 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] > 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. > _______________________________________________ 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.
