Electronics > RF, Microwave, Ham Radio

Measuring output power with oscilloscope & attenuator

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Hi, I'm trying to measure the power output of an RF power amplifier by connecting a 20 dB attenuator to the amplifier output and then connecting the output of the attenuator straight to the oscilloscope input via a BNC cable.

My question: should the oscilloscope input be set to 50 Ohm mode or to high impedance for correct measurements? Those options give pretty different results. Thanks!

50 Ohms.

Thanks for the fast answer! Do you mind to explain why 50 Ohm should be chosen? I mean the attenuator already acts like 50 Ohm load for the PA, why do we need the oscilliscope input to be 50 Ohm too? Thanks

The cable should be terminated with 50 Ω pass-through dummy load at oscilloscope connector. If your oscilloscope has integrated pass-through 50 Ω dummy load, you can use it. Otherwise you will needs to use external pass-through dummy load on oscilloscope connector. But integrated pass-through dummy load has pretty weak limit.

It is limited with about 5 Vrms (0.5 W or 26.9 dBm). So, it is more safe to disable internal pass-through dummy load and use external one on oscilloscope connector. In such way you can avoid burning out of of your oscilloscope input.

If your oscilloscope has 5 Vrms for internal pass-through dummy load, you can still burn it if you put 50 W through 20 dB attenuator.

Thanks for the explanation! One more question - what if I use the standard oscilloscope probe (as opposed to BNC cable) that gets connected at the attenuator output? Should the oscilloscope input still be set at 50 Ohm? Thanks


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