Alright... Curiosity got the better of me, and since most fortunately I don't have to deal with the incessant nagging of a female, I set up some gear on the kitchen table:
Sig-gen: E4438C
RF power meter: PWR-4GHS
Cable: Some kind of funky 6GHz-rated flexible stuff with Huber-Suhner silver/gold connectors.
Unfortunately I don't have a BNC-female to N-female adaptor, so I had to improvise. (Apparent adapter loss, about 0.1dBm at 50 to about 0.2dBm at 1G, is "calibrated out" of my figures.) Everything "warmed up" for about 90 minutes; ambient temp was pretty stable at 22.5 degrees C.
Using a pure unmodulated sine wave carrier, I "calibrated" the E4438C and cable at 50, 100, 200, 300, 400, 500, 600, 700, 800, 900, and 1000MHz, for almost exactly 0dBm. (The smallest steps the E4438C can do is 0.02dBm, so...) Accuracy stats for the power meter are on MiniCircuits site. (FWIW, according to the power meter, the E4438C is "off" +0.08dBm at 50MHz, which gradually changes to +0.22dBm at 1000MHz.)
Set up the DS4014 for 50-Ohm inputs and no BW limit. Here are the measured
RMS voltages...
50MHz: 223mV
100MHz: 221mV
200MHz: 219mV
300MHz: 214mV
400MHz: 212mV
500MHz: 207mV
600MHz: 179mV
700MHz: 149mV
800MHz: 121mV
900MHz: 91mV
1000MHz: 58mV
So now yous guyz could do da math, I'm...
(Fuhgeddaboudit!)
(-3dB point about 650MHz)
PS- All four channels were within a mV of each other.