Why does the measured voltage change when I use a 50 Ohm termination?

Many signal generators and Koheron inputs or outputs use 50 Ohm impedance.

If a source has a 50 Ohm output impedance and is connected to a 50 Ohm load, the two impedances form a voltage divider. The voltage measured at the load is therefore half the source open-circuit voltage.

This commonly explains a factor-of-two difference between an expected and measured signal amplitude.

When comparing voltage levels, always check whether the instrument specifies its amplitude for a 50 Ohm load or for a high-impedance load.