IQ Demodulation With Real-Time Transmission Line Compensation for ITER Bolometer Diagnostics

The ITER bolometer diagnostic employs a miniaturized metal resistor bolometer configured in a Wheatstone bridge to detect the incoming plasma radiation. Due to the harsh radiation environment, the acquisition system must be located remotely, requiring a long transmission line between the sensor and the electronics. To avoid low-frequency noise affecting the signal integrity, the bolometer input is modulated and the signal is recovered via synchronous in-phase and quadrature demodulation. However, the transmission line introduces frequency-dependent distortion that affects the demodulation accuracy and power estimation. This article presents a method to compensate for such distortion by exploiting the harmonic content of a square-wave modulation. By extracting the amplitudes of the first and third harmonics and comparing them to their ideal ratio, the frequency response of the transmission line can be identified in real time. This enables online compensation for dynamic changes in the transmission line characteristics and broadens the possible modulation frequency range. The proposed approach is validated through a real-time implementation using a 100-m-long cable, showing improved accuracy in estimation performance.