Electronic – About impedance matching for high frequency and divider effect

impedance

—– 2013/11/08 modify my question

I have an 'A' source has 5 MHz signal (0-5 V square wave) without knowing input impedance and a receiver 'B' has 75 ohm input impedance.

As I know I have to match the impedance match to avoid the reflection effect, but I will have the divider effect if I parallel a resistor to the 'B' input end or series connect a resistor to the 'A' output (means not 0-5 V input for 'B' anymore).

Should I do a 2x amplifier circuit to make it 0-10 V first to avoid the divider effect and use this op as a buffer and series connected to a 75 ohm to make the output impedance as nearly 75 ohm?

If there is any information I can study, please let me know.

Thanks for the help.

—–2013/11/08 upload the 'A' spec
enter image description here

—–2013/11/08 upload the 'A' output via Oscilloscope

  1. sync output with 50 ohm parallel
    enter image description here

  2. sync output without 50 ohm parallel
    enter image description here

Best Answer

Your oscilloscope measurements show that your source is not impedance-matched.

To avoid reflections due to impedance mismatch at the load, I recommend to simply design this as a 75-Ohm system:

schematic

simulate this circuit – Schematic created using CircuitLab

If no (or very little) reflection comes back from the destination end, it won't cause any issues to have the source not impedance-matched, and you will get your full 5-V signal at the destination end (provided the source is able to drive a 75-Ohm load, which your oscilloscope measurements show it is).

You will need choose a coaxial cable with 75-Ohm characteristic impedance, and it will be connectorized with 75 Ohm BNC connectors. According to Wikipedia these connectors "can be made to" intermate with 50-Ohm BNC, but to avoid damaging things you might prefer to use an adapter at the source end. Since your load has a 75-Ohm termination, its connector ought to be the 75-Ohm type also, but it would be wise to double-check.

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