First, in order to be liberated from the constraint of using any particular
attenuator to make particular equations apply, I think it's best to pass
through the dBm unit and add or subtract according to whatever attenuation
is used. (By the way, a 20W attenuator is only needed when sampling CW. The
little attenuators are fine for sampling Vpp of pulses.)
So, power in watts is indeed Pw=Vpp^2/400 for 50 ohm termination
and dBm = (10*log Pw)+30
so, if you have sampled through a 30 ohm attenuator, add another 30 to get
your real dBm.
Also keep in mind that RG58U 50-ohm coax has a nominal attenuation of 17.13
dB per 100 ft at 1 GHz, so my 22 ft test cable probably is knocking down
about 4 dB.
Actually, I don't use the equations. Phil Louthan faxed me a power
conversion chart years ago and I keep it near my scope. I miss Phil! Has
anyone heard from him?
Bill
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William C. Stevens, Ph.D. Nuclear Magnetic Resonance Facility
Director Southern Illinois University
Carbondale, IL 62901-4405
wstevens@siu.edu
http://www.siu.edu/departments/shops/bill.html
wstevens@intrnet.net voice: 618-453-6498 fax: -6408
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