Optimum Pre-Emphasis for Frequency Modulation Applications

Frequency modulated communication receiving systems that operate under varying RF input power conditions use a generalized preemphasis schedule that provides improved communication reception. In many instances, a receiving system operates with a stable RF input power. The optimization of FM systems, operating with a stable RF input power, requires the use of a particular pre-emphasis schedule. Typical FM systems are not frequency modulated with a constant modulation index for all baseband frequencies. Instead, they use a signal pre-emphasis circuit at the transmitter and a complementary de-emphasis circuit at the receiver to optimize system operation. Preemphasis and de-emphasis are required whenever the noise amplitude varies significantly for different frequencies in the baseband (Figure la). FM pre-emphasis at the transmitter accentuates certain baseband frequencies with respect to other frequencies in the baseband (Figure Ib). The complementary FM deemphasis at the receiver de- accentuates those frequencies in the baseband that were originally pre-emphasized by the transmitter, and maintains a constant output signal to noise ratio density throughout the baseband (Figure Ic)