HD Radio™ implementation has introduced a great deal of discussion about spectral re-growth problems when digital carriers intermodulate with the primary FM carrier, causing spurious emissions on adjacent channels. Pre-correction systems may be implemented to mitigate the effects of transmitter system nonlinearity giving rise to the out-of-band emissions.
Conventional fixed pre-correction techniques have not provided a sufficient solution to ensure spectral integrity in a changing environment. Changes in VSWR, an adjustment in the output power of the transmitter, a change in amplifier temperature, or aging and failures of RF amplifiers can result in serious transgressions of the HD Radio mask and interference with other stations.
This paper presents theory and measured performance of digital adaptive pre-correction under unstable environmental conditions. Comparison is made between fixed pre-correction curves and adaptive pre-correction under typical conditions at broadcast sites.