The I-Q modulator is a key component in a Low Level Radio-Frequency (LLRF) system for the amplitude and phase control of the electromagnetic wave inside the accelerating cavities. Its amplitude and phase unbalance have a strong impact on the dynamic and accuracy of the feedback loops. For some frequencies, commercial I-Q modulators are available. Nevertheless, these components are usually designed for broadband communication purposes, and their unbalances may be incompatible with the amplitude and phase stability requirements for demanding accelerators. Since LLRF systems are typically narrow-banded, i.e. limited to a few Megahertz, an customised I-Q modulator could be designed by optimising each subcomponent at the desired frequency in term of coupling, matching or harmonics. This approach that guarantees high accuracies with a full-custom design is presented in this paper. |
