Measuring an electrical response to test signals of a device under test (DUT), such as a component or components of a telecommunication (telecom) network, can provide information about how the component or components of the DUT affect signals transmitted through the DUT. A DUT can affect both the amplitude and the phase of a signal. Power measurements can communicate how a DUT attenuates different frequency components of a signal. Phase measurements can communicate how a DUT affects the phase of different frequency components of a signal. For example, a phase measurement of the response of a DUT to a test signal can determine the group delay for each of the frequency components of the test signal, and can be used to determine how linear the response of the DUT is to the test signal. Phase measurements can also help track the path of a test signal or determine causality, e.g. locating a source of passive intermodulation (PIM) within a DUT.
Phase measurements using a transmitter and a receiver separate from each other requires the transmitter and the receiver to be synchronized, for example using a synchronization signal transmitted from the transmitter to the receiver. However, if the distance of a transmission path through the DUT requires the transmitter and the receiver to be separated by a large distance, phase information about the DUT can be difficult to obtain using a synchronization signal transmitted between the transmitter and the receiver based on methods and instruments in accordance with the prior art because a large signal path introduces phase to the synchronization signal and attenuates the synchronization signal. Embodiments of the present invention are intended to address this difficulty.