Calibration of high-frequency oscilloscopes

The bandwidth of commercial high-frequency circuits is continuously increasing. For the characterisation of corresponding components, ultra-fast sampling oscilloscopes are used, amongst others, which nowadays have a bandwidth of up to 100 GHz. As the time response of the oscilloscopes distorts the measured signal and as this influence becomes stronger with the frequency, the time response of the oscilloscopes must, for precise high-frequency measurements, be traceably calibrated. Thereby, the applied measurement technology requires a bandwidth which is twice or three times as large as the bandwidth of the oscilloscopes. So far, it has been possible to calibrate oscilloscopes with a bandwidth of 70 GHz at PTB. The set-up – having now been extended – also allows the time response of sampling oscilloscopes with a bandwidth of 100 GHz to be calibrated. 

The procedure used for this purpose is based on optoelectronic measurements in the time domain. First of all, ultra-short voltage pulses with a width of less than 2 picoseconds are generated by means of a femtosecond laser in a so-called “photoconductive switch”. The photoconductive switch is integrated in a metallic coplanar waveguide structure on a semiconductor material. The voltage pulses propagate along the waveguide, and their time response can be precisely measured by means of optoelectronic measurement technology. The time axis of the measured waveform is traced back to the SI unit “the second”.

To be able to calibrate instruments with a coaxial input connector by means of these short voltage pulses, the pulses must be transferred from the coplanar to the coaxial waveguide. At PTB, commercial microwave probes are used. After the transfer function of the microwave probe has been determined and further experimental influences have been taken into account – also by means of optoelectronic methods – the shape of a voltage pulse entering the oscilloscope can be calculated very precisely. By comparing the entering pulse with the signal measured by the oscilloscope, the pulse response of the oscilloscope can be determined. During the assessment of the measurement results, the uncertainty is determined by means of a Monte Carlo analysis. This allows the oscilloscope to be completely characterised, as it is not only possible to indicate an uncertainty for every point in time of the pulse response, but also because correlations between different points in time are taken into account.