Electronics > RF, Microwave, Ham Radio

Measuring output power with oscilloscope & attenuator

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isso:
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!

bob91343:
50 Ohms.

isso:
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

radiolistener:
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.

isso:
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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