New Method for Displaying the RF Cavity Impedance on a Smith Chart

A new technique for the Smith Chart display of RF cavity impedance is described. The system was developed for the compact synchrotron HELIOS 2 currently being built by Oxford Instruments. There are three major improvements compared with the standard four probe method: firstly t,his technique uses two probes on the transmission line instead of four, secondly AGC (Automatic Gain Control) on the forward signal (signal going from the source t,o the cavity) requires only 10 dB dynamic range of amplitude detection, the precision being limited by the linearity of the detector. (With the four probe method, t,he linearity over the wide dynamic range of amplitude det,ection (more than 6OdB) severely limits the precision). Finally in contrast to the standard four probe method, this technique does not require a complicated calibration procedure. The Smith Chart is displayed via a C program on an IBM compatible PC wit,11 a 80286 processor, under Windows, saving the need for a dedicated Network Analyscr.