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
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