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Question on 75 ohms system with 50 ohms test equipment

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Member level 4
Mar 28, 2002
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When measuring 75 ohms RF system (TV,Radio,telecom,...) by test equipments with input impedance 50 ohms. How do I correct the readings from the 50 ohms test equipment to the number for 75 ohms system ?

I am trying to read the dBm value from a 75 ohms system RF output by using 50 ohms power meter. can I just connect the equipment directly?

the definition of 0dBm is 1mw on 50 ohms, how about for 75 ohms system? do they have the same power level for 0dBm? how about voltage? I am confused.

I went on an interview and the company was doing Cable boxes. They where using a
50 to 75 Ohm converter. It looked like a fancy Pad. I bet you could find one online
or Google. Here's an-type converter
HP 11852B

All the relations and definitions hold except that the characteristic impedance is different (0 dBm, etc.)

To use 50 ohm equipment on 75 ohm DUTs, you (obviously) will have a serious mismatch problem.

Using just attenuator pads is one way because it simply attenuates the reflections away - but you use dynamic range fierce.

The alternative is to use matching filters but these only work over a limited range of frequencies.

The best solution is always to use equipment specifically built for 75 ohm. Agilent and others sell 75 ohm solutions specifically for this reason: the work-arounds pretty much suck and are point solutions rather than general solutions.

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