  Superconducting travelling wave parametric amplifiers (TWPAs) are broadband
near-quantum limited microwave amplifiers commonly used for qubit readout and a
wide range of other applications in quantum technologies. The performance of
these amplifiers depends on achieving impedance matching to minimise reflected
signals. Here we apply a microwave calibration technique to extract the
S-parameters of a Josephson junction based TWPA in-operando. This enables
reflections occurring at the TWPA and its extended network of components to be
quantified, and we find that the in-operation performance can be well described
by the off-state measured S-parameters.
