A measurement system for determining a phase and amplitude influence of a device under test, comprising a measurement instrument having a signal generator, a local oscillator, a first mixer and an analysis unit is disclosed. The signal generator is configured to generate a source signal with a predetermined source frequency and a source phase, and to forward the source signal to the device under test, wherein the source signal is altered by the device under test in at least one of amplitude and phase, such that a measurement signal is generated and forwarded to the first mixer. The local oscillator is configured to generate a local oscillator signal with a predetermined local oscillator frequency and a local oscillator phase, and to forward the local oscillator signal to the first mixer. The first mixer is configured to mix the measurement signal and the local oscillator signal, thereby generating a first mixer signal. The analysis unit is located downstream of the first mixer and is configured to analyze the first mixer signal or a processed version of the first mixer signal. The measurement instrument is configured to perform at least two measurements of the phase and amplitude influence of the device under test by analyzing the first mixer signal or the processed version of the first mixer signal, wherein at least one of the source phase and the local oscillator phase is altered between the at least two measurements.