Why don't you just experiment? If you start with a cheap 30db attenuator that dissipates 10 watts (about $25 Amazon) and input 5 watts you end up with 0.5 watts output. Now you can string on a bunch of cheap 10 or 20 db 2 watt attenuators (~$15 for 2 Amazon) until you get to say 60 or 80 db total attenuation. Then give it a try. Add or subtract attenuation until you have traces you like
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Protecting the SA is NOT the only objective. You need more than that to get good clean signals. How much more is up to you. Personally I like 80 db on a 1 Watt input (I don't use the internal attenuator). That's one hundred millionth of a watt into the SA! I get great traces! This is a very sensitive instrument! YMMV
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