Complex Impedance Measurement with a Network Analyser by Down-Conversion using Unstabilized W-Band-Impatt-Sources

Complex impedance measurements in the frequency range above 27 GHz can be realized by heterodyning the measured signals to the operating frequencies of existing microwave network analysers. The concept works well in Ka-band [1], but in higher frequency bands the inherent phase noise of the solid state sources influences the locking behaviour of the used HP-network analyser seriously. The use of commercially available W-band IMPATT sources for the sweeper and the local oscillator is unsuccessful due to excessive phase noise [2,3]. The problem can be solved at great expense by using synthesized sources [3], but careful design of the IMPATT oscillators together with some modifications of the phase lock loop in the network analyser results in a satisfactory impedance measuring set up for a 3 GHz window at 94.5 GHz. This approach made for a simple low cost set-up uses a small desk top computer to correct for system errors with an algorithm similar to that one proposed by J. Fitzpatrick [4].