This article details a systematic approach for predicting phase noise in large multichannel systems and compares it with measurements on a 16-channel S-band demonstrator. This analytic approach is based on a small set of measurements that can be used to estimate correlated vs. uncorrelated noise contributions. Based on only a few measurements, phase noise under a wide range of conditions can be predicted. The view is that any specific design will need its own system noise analysis, with the 16-channel demonstrator providing a specific design example to use as a basis. Assumptions based on the 16-channel demonstrator are discussed along with limitations about when the assumptions apply and when to add additional noise terms as system complexity grows. The description provided builds on a body of work describing phase noise optimization in RF systems.1–6 Where appropriate, references describing the foundational principles used in this analysis are provided.