Characterisation of stable optical and microwave signals from ultra-stable oscillators or atomic clocks using PTB’s high-performance optical clocks and oscillators. Characterisation of signals with frequency instabilities as low as 1E-16 at 1 second down to 1E-17 at 1e4 seconds of averaging time. Test of operational robustness, and determination of absolute frequencies with measurement uncertainties as low as 2E-16. Scientific and technical support for industry in the related field of quantum technology.