When I connect an oscilloscope to a function generator use both oscilloscope cable (one connect to the scope and one connect to the FG), I got 700mVpp when outputing a sine wave of 1Vpp and 10kHz.
But when I use the BNC cable connects to the FG, and the scope cable connect to the scope I got 2Vpp output with the same input.
What is the exact calculation behind this?
If you're saying that the open-circuit output of the function generator is 1VPP and that when you connect it to the scope the scope reads 700mVPP, then if your scope's input impedance looks like 10 megohms paralleled by some few picofarads:
EDIT: 1 megohm paralleled by some few picofarads:
the generator impedance looks like 428k ohms, so clearly something's very wrong.
Do you have your scope's impedance set at 50 ohms, perhaps, and with half of 1VRMS coming from a 50 ohm function generator you're reading 0.7V peak from the scope?