Detailed Analysis of Split-Level Combining

High level combining is a practical way to upgrade existing FM transmitters to IBOC. Very well understood, with readily available 10 dB couplers, it is a very attractive method with which to upgrade even the highest power levels in the field. Unfortunately, if offers only a combining efficiency of 83%, as 90% of the IBOC power is wasted into a dummy load, along with 10% of the generated FM power. Additionally, the power headroom demanded from the FM transmitter is proving to be an unsurpassable requirement for some existing sites ready to upgrade. This paper presents an innovative combining technique that minimizes the FM and IBOC power loss in high level combining. Instead of combining two totally incoherent signals with its known resulting losses, this method first achieves partial coherence of the signals, and only then combines them at an optimum coupling ratio. A system implemented this way offers two main benefits; a higher overall system efficiency, and the elimination of the need for FM headroom. Though the paper focuses on the particular case of FM HD radio upgrade, this technique can also be used whenever low loss is needed when combining dissimilar signals.