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The accuracy of microwave measurements is not only critical for applicationsin telecommunication and radar, but also for future quantum computers. Qubittechnologies such as superconducting qubits or spin qubits require detectingminuscule signals, typically achieved by reflecting a microwave tone off aresonator that is coupled to the qubit. Noise from cabling and amplification,e.g. from temperature variations, can be detrimental to readout fidelity. Wepresent an approach to detect phase and amplitude changes of a device undertest based on the differential measurement of microwave tones generated by twofirst-order sidebands of a carrier signal. The two microwave tones are sentthrough the same cable to the measured device that exhibits a narrow-bandresponse for one sideband and leaves the other unaffected. The reflectedsidebands are interfered by down-conversion with the carrier. By choosingamplitude and phases of the sidebands, suppression of either common amplitudeor common phase noise can be achieved, allowing for fast, stable measurementsof frequency shifts and quality factors of resonators. Test measurements wereperformed on NbN superconducting resonators at 25 mK to calibrate andcharacterise the experimental setup, and to study time-dependent fluctuationsof their resonance frequency.

Readout of quantum devices with a sideband microwave interferometer immune to systematic noise