A test module (100,200) which is a physically separate module from, but also operable to be connected to and used with a passive intermodulation distortion, PIM, analyser in locating PIM sources in a test medium, said test module (100,200) including: an input port (101,201) operable to connect the test module (100,200) to the PIM analyser and to receive one or more test signals from the PIM analyser; an output port (102,202) operable to connect the test module (100,200) to the test medium and to apply said one or more test signals to the test medium; a monitor port (103,203) operable to receive from the PIM analyser a plurality of PIM signals produced by the test medium in response to the applied test signals; a directional coupler coupled between the input port (101,201) and the output port (102,202); a primary receiver coupled to the directional coupler and operable to receive, via the monitor port (103,203), a plurality of primary return signals produced by the test medium in response to the one or more applied test signals; a reference receiver coupled to the directional coupler and operable to generate a plurality of reference signals from the one or more applied test signals; a phase detector coupled to the primary receiver and reference receiver, said phase detector being operable to measure phase offsets between the plurality of primary return signals and the plurality of reference signals; and a control module coupled to the primary receiver, reference receiver and phase detector, said control module being operable to: convert each signal within the plurality of primary return signals from the primary receiver to a set primary power levels; convert each signal within the plurality of reference signals from reference receiver to a set reference power levels; convert each signal within the plurality of signals from the phase detector to a set of phase offsets; and store the converted power level for the primary return and reference signal levels and the converted set of phase offsets between the primary return and reference signals for further processing. an input port (101,201) operable to connect the test module (100,200) to the PIM analyser and to receive one or more test signals from the PIM analyser; an output port (102,202) operable to connect the test module (100,200) to the test medium and to apply said one or more test signals to the test medium; a monitor port (103,203) operable to receive from the PIM analyser a plurality of PIM signals produced by the test medium in response to the applied test signals; a directional coupler coupled between the input port (101,201) and the output port (102,202); a primary receiver coupled to the directional coupler and operable to receive, via the monitor port (103,203), a plurality of primary return signals produced by the test medium in response to the one or more applied test signals; a reference receiver coupled to the directional coupler and operable to generate a plurality of reference signals from the one or more applied test signals; a phase detector coupled to the primary receiver and reference receiver, said phase detector being operable to measure phase offsets between the plurality of primary return signals and the plurality of reference signals; and a control module coupled to the primary receiver, reference receiver and phase detector, said control module being operable to: convert each signal within the plurality of primary return signals from the primary receiver to a set primary power levels; convert each signal within the plurality of reference signals from reference receiver to a set reference power levels; convert each signal within the plurality of signals from the phase detector to a set of phase offsets; and store the converted power level for the primary return and reference signal levels and the converted set of phase offsets between the primary return and reference signals for further processing. convert each signal within the plurality of primary return signals from the primary receiver to a set primary power levels; convert each signal within the plurality of reference signals from reference receiver to a set reference power levels; convert each signal within the plurality of signals from the phase detector to a set of phase offsets; and store the converted power level for the primary return and reference signal levels and the converted set of phase offsets between the primary return and reference signals for further processing.