I have a source with 5V 10Mhz Squarewave output, then I have an OR logic gate then 20m coaxial cable and then I have another OR logic gate. I guess I will need a matched load (now it is 100Ohm (random resistance)) for this system. Should I add the termination load in series or using voltage divider?
I must note - I do not need any numbers, I am just curious about pros and cons in the two following systems.
Here is the updated picture. Coaxial cable: 20m, 50Ohm Voltage Source: 5V, 10Mhz, Square Wave
Answer
I would series terminate the line assuming the receiving end is a single receiver, if its hitting multiple devices you basically have to end terminate.
Your source resistor should be output_impedance + R = Transmission_line_impedance. You may need to fiddle with this as your going to have to compensate for impedance changes caused by the connectors, etc. Side note, are you sure your logic gate can drive this load? Most can't, you may need a line driver on the sending side.
As for your pictorial end termination, its not correct. That resistor should be as close to the receiver as possible, you should also consider Thevenin termination as it uses much less power than the parallel termination.
In general (this can vary a bit by application):
Source Termination: Lowest power, slowest rise/fall times, single destination only.
Parallel Termination: Highest power, only 1 resistor
Thevenin Termination: Middle power, 2 resistors
Parallel AC Termination: Middle-low power, fastest rise/fall times, 1 resistor, 1 capacitor
For most signals what i would say is this:
If its point to point and rise/fall times are not required to be especially fast (most point to point applications): source terminate
If its point to multiple point: Thevenin terminate AFTER the last device.
If rise/fall times are absolutely critical (this is fairly rarely needed): Parallel AC terminate.
If your boss is bitching about cost and your not running on batteries: Parallel terminate
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