Design and development of signal conditioning electronics for SST-1 microwave interferometer system

Self-biased, square-law based Schottky barrier diodes will be used in homodyne microwave interferometer system (MIS) of Steady State Tokamak (SST-1) for the line average plasma density measurement. Proposed signal conditioning electronics (SCE) improves the strength of the weak time varying signal in the range of 5–6 mV by imparting it with a total gain of 1000 (100 × 10) in two steps and a bandwidth of 9.5–10 kHz. In this way, SCE improves the signal strength and also removes the unwanted spurious noise component picked up by the signal from source to its final destination of data analysis. The front end of the SCE comprises of an instrumentation amplifier AD524 having very low input noise level of 0.3 μV and very low input offset voltage of about 50 μV. The remaining portion of the SCE comprises of filter, optocoupler and differential driver for further conditioning of the signal. The final signal is then acquired and stored using PCI extension for instrumentation (PXI) based data acquisition system (DAQ) having a fast sampling rate of 100 kHz. The phase shift undergone in the conditioned signal while passing through plasma, in reference to a known microwave signal operating at a frequency of 130 GHz, is then measured. A detailed description of the designed signal conditioning electronics for the same is presented.

► Signal conditioning electronics (SCE) designed for the microwave interferometer system of SST-1 Tokamak is presented.
► Self-biased, square-law based Schottky barrier diodes are used for detecting the interferenced microwave signal.
► SCE comprises of plug-in functional modules for imparting various conditioning parameters like gain, bandwidth, optoisolation to the received signal.
► After passing the signal through SCE, noise free fringes can be obtained corresponding to the line integrated density of the plasma.