Bridging enhances filter close-in selectivity
Richard M Kurzrok, RMK Consultants, Queens Village, NY -- EDN, March 2, 2000
General filters are bandpass filters that usually employ bridging couplings between nonadjacent interstage couplings (Reference 1). This class of filters also includes bridging coupling across the filter input and output ports. The implementation of input-to-output bridging already exists for a one-pole filter (Reference 2). For a two-pole filter, dielectric resonators help achieve input-to-output bridging coupling (Reference 3).
You can also implement a two-pole LC-bandpass filter using input-to-output bridging (Figure 1). This general filter provides enhanced close-in selectivity by adding a single bridging inductor across the input and output of a conventional bandpass filter. This relatively obscure passive filter may be useful in some applications, and you can probably realize similar filter performance using an active circuit.
You can compare the performance of this general filter to the performance of a conventional two-pole bandpass filter that has a convenient center frequency of 20 MHz. The design parameters are for a lossless 0.01-dB Chebyshev response with a 3-dB bandwidth of 2 MHz and input and output impedances of 50?. Table 1 shows the relative measured amplitude response data. Center-frequency insertion loss was 0.8 dB, corresponding to inductor unloaded Q's of about 150. The measured relative 3-dB bandwidth was 2.1 MHz. The filter response is asymmetrical due to the frequency sensitivity of the capacitive input, output, and interstage couplings.
The general filter adds a bridging inductor from the input to the output of the conventional bandpass filter (Figure 1). The measured center-frequency insertion loss was 0.7 dB, and Table 2 shows the relative amplitude-response data. The general two-pole filter provides nearby rejection peaks with degraded far-out selectivity. The filter exhibits this type of behavior for upper and lower stopbands. You adjust the variable capacitors with the enclosure cover removed. As filter bandwidth becomes smaller, alignment-tool access holes in the cover become necessary. (DI #2491)
REFERENCE1.Kurzrok, RM, "General three-resonator filters," EDN, May 1966, pg 92.
2. Kurzrok, RM, "Single component changes bandpass into general filter," Electronics, April 18, 1966 pg 95.
3. Cohn, SB, "Microwave filters containing high-dielectric resonators," presented at Clearwater, FL, May 5 to 7, 1965, and printed in G-MTT Digest, pg 49.
