A measurement instrument for testing a DUT comprises a common port configured to be connectable to a signal output of the DUT for receiving a forward-travelling signal from the DUT. The measurement instrument further comprises a signal line connected to the common port, a signal analysis circuit and a signal generator circuit. The signal analysis circuit receives the forward-travelling signal from the common port. The signal analysis circuit is configured to analyze the forward-travelling signal in order to assess a performance of the DUT. The signal generator circuit is connected to the signal line and is configured to generate a backward-travelling signal that is forwarded to the common port. The signal generator circuit comprises a reference signal input configured to receive a reference signal from a reference signal generator. The signal generator circuit is configured to generate the backward-travelling signal based on the reference signal.