Calculating the minimum power-supply requirements for the PA, the desired load voltage, load impedance, and available power-supply voltage or desired transformer ratio are all the parameters that must be known. For this FSK power-line communication example, similar to PRIME, the goal is to drive a 1-VRMS signal into a 2-Ω load. The minimum power-supply voltage required was calculated by adding the peak-to-peak load voltage; the voltage dropped across the HV Cap and inductor, V2; the voltage dropped across the LV Cap, V1; and twice the output swing to rail limit of the PA, VSWING. For FSK and SFSK systems, the peak-to-average ratio is √2, while this ratio is approximately 3:1 for OFDM systems.

Figure 6-12 Typical Line Coupling Circuit

These ratios must be considered when performing calculations that relate the RMS voltages and peak voltages during an analysis. Choosing a large value for the LV Cap results in the voltage drop (V1 ) becoming negligible in most circumstances. The losses in the transformer are also negligible, even at high load currents, if the proper transformer with a low DCR is used. For FSK and SFSK systems, the voltage drop across the HV Cap and inductor, V2, is also usually negligible; in OFDM systems, because of the wider operating bandwidth, voltage drop V2 can be ignored and accounted for by using a 1.5x multiplier on the load voltage as an approximation.

For the AFE031 BoosterPack an FSK signal with a 2-Ω load and 1-VRMS load voltage:

Equation 6.

Equation 7.

For OFDM, the following equation can be used:

Equation 8.

For more information on Power dissipation of the AFE031, see Analog Front-End Design for a Narrowband Power-Line Communications Modem Using the AFE031.