Hi These are saturated logic signals. When you terminate both source and load you get an interesting issue with compatible logic levels.
For instance: 5V CMOS switches at roughly 2.5V. If you series terminate and load terminate, your destination now sees a 0 to 2.5V signal. Either it’s running 2.5V CMOS and switching at 1.25V or you have a problem. Bob On Sep 15, 2014, at 9:03 AM, Dave Martindale <[email protected]> wrote: > 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.
