System Auto-calibration

Case Study

During redesign of a medical imaging system, it was necessary to consider how alignment and calibration could be achieved with minimal external equipment and manual operations. An analogue front end included quadrature mixing to baseband and subsequent digitization. Phase and amplitude control was critical to performance. This was to be achieved at various operating frequencies. Some of the sophisticated imaging techniques employed demanded that a low-jitter scheme was employed.

After careful analysis of the alignment problem, it was determined that 4 things were essential:

• appropriate software-controlled adjustments to the forward signal path
• a suitable phase-locked calibration signal and means of injecting it near to front end
• a measurement technique that would determine alignment accuracy
• a suitable algorithm that allowed residual errors to be minimised

Suitable controls were implemented in the analogue signal path to adjust amplitude and phase independently. A means of generating a low-jitter calibration signal was developed; this necessitated provision of an additional FPGA to the original design, and careful specification to ensure the jitter performance could be achieved. This had several system-wide implications. Algorithms were established to ensure accurate phase and amplitude determination, even in the presence of moderate harmonic distortion and noise. Further algorithms covered error minimisation.

Extensive verification was undertaken to ensure accuracy, repeatability, and range coverage were all achieved. These included checks on the gain and phase adjustments to ensure the two parameters were orthogonal.

This resulted in a successful one-button self-calibration scheme with high accuracy and operational simplicity.