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.
