Figure 2. Three-section filter response, S21 and S11, 1.6MHz and 800kHz channel separation.

Classic combiners, as stated above, will have approximately 30dB of rejection at the channel to be combined. Illustrated in Figure 2 is the typical frequency response (S21 and S11) of a three-section filter used for combining channels with 1.6MHz separation and the same filter with 800kHz separation. The limited rejection (approx. 7.5dB minimum) for 800kHz spacing results in higher loss (approx. 1.2dB) at the broadband input.

Filter rejection vs. module insertion loss at broadband input to output is plotted in Figure 3 and illustrates the need for high rejection for low insertion loss at the broadband port.

Figure 3. Rejection vs. broadband port loss

The use of filters to combine FM and digital sidebands was discussed early in the rollout of HD Radio, but quickly ignored due to filter order (required for 30dB rejection) and related cost, size and loss issues. A six-section sharp-tuned filter with multiple cross couplings would be required to achieve 30dB rejection at the IBOC MP1 digital sidebands. Six-section pseudo-elliptic function filters are not difficult to design. However, due to narrow bandwidth compared to classic-tuned FM filters, and the higher order, significant losses are incurred.

Higher losses result in low-power handling capability for a given size filter. The six-section sharp-tuned filter and table summarizing loss results is illustrated in Figure 4. The integrated, or average loss of this narrow sharp-tuned filter using high Q, 20" to 24" square cavities providing 30dB rejection at the digital sidebands is approximately 1.31dB. Note however, that due to the finite cavity Q there is still significant loss at the sideband inner edges. Using this size cavity in a classic four-section filter with wider bandwidth (400kHz plus) would have approximately 0.25dB loss and handle 30kW. The sharp-tuned filter in Figure 4 using the same cavity geometry can only handle 6.5kW without a lot of additional cooling. It's easy to see why sharp-tuned filters were quickly disregarded as a solution to combine FM and digital sidebands.

Figure 4. Sharp-tuned filter response for combining digital sidebands to FM.

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