Figure 5. Mild-tuned, four-section filter response for combining digital sidebands to FM

A directional filter combining solution that purposely exhibits loss was overlooked. Reducing filter order and requiring less rejection at the digital sidebands has less impact on the host FM loss, and digital loss is less than current high-level combining solutions. Figure 5 illustrates one solution using a four-pole filter with a Chebyshev response. Note the integrated loss and variations over the FM (±75kHz) is significantly less than the sharp-tuned filter.

The loss numbers in Figure 5 are calculated using cavity unloaded Qs of 20" to 24" square cavities. The higher losses as compared to classic-tuned, four-section filters are due to the narrow bandwidth. Figure 6 illustrates the effect of bandwidth on losses for a four-pole filter.

Figure 6. Mild-tuned, four-section filter response for combining digital sidebands to FM

Derek Small is director of filter products for Myat.