Band-reject filter includes compensation
Richard M Kurzrok, RMK Consultants, Queens Village, NY -- EDN, April 13, 2000
Some band-reject LC filters employ a reactance to decouple individual resonators. One such filter uses relatively small shunt inductors to decouple series resonators, known as a top-L coupling. This type of coupling readily achieves narrow filter bandwidths while degrading amplitude response symmetry about the frequency of peak rejection.
Filter peak rejection occurs at a transmission pole that the shunt resonance of the coupling inductor and the effective capacitance (below resonance) of the series resonator create. A transmission zero occurs on the filter's high-frequency skirt due to series resonance. Table 1 shows the amplitude response data of an uncompensated single-resonator band-reject filter with a center, or peak-rejection, frequency, of 14 MHz, a nominal 3-dB bandwidth of 1 MHz, and a 50W source/load impedance. The data in the table reveals appreciable asymmetry.
You can add compensation to the band-reject filter by connecting a grounded shunt capacitor to the mid-point of the coupling inductance, which this circuit realizes using two toroidal inductors in series (Figure 1). The measured 3-dB bandwidth of 0.9 MHz demonstrates good symmetry about the 14-MHz center frequency. Peak rejection of 21.4 dB corresponds to effective resonator unloaded Q of approximately 180. Table 2 shows the measured amplitude response for the compensated one-pole band-reject filter.
The compensating capacitor and the bisected coupling inductor form a tee section. This section is a three-pole lowpass filter with 0.01-dB of passband ripple and a 3-dB cutoff frequency of 47 MHz, which provides supplementary lowpass selectivity superimposed upon the basic band-reject behavior. For multisection band-reject filters using artificial quarter-wave connecting lines, additional lowpass selectivity is available (Reference 1). Also note that the compensated top-L coupled band-reject filter is the LC counterpart of microwave band-reject filters. (Reference 2).
You can avoid the need for compensation by tapping down on shunt resonators connected in series. This technique is useful for band-reject filters that use low-frequency pot-core inductors with many turns. The technique is unattractive for band-reject filters at higher frequencies using toroidal inductors with few turns. The compensated LC band-reject filter (Figure 1) is practical and provides substantial partial correction of filter-response-shape symmetry and can provide additional lowpass filtering. (DI #2514)
REFERENCE1.Kurzrok, R, M, "Band reject filter provides supplementary low pass filtering,"RF Design, October 1999, pg 54, plus errata, November 1999 , pg 16.
2. Kurzrok, R, M "Trimming improves response of waveguide band-reject filter," Electronic Design, Nov 8, 1967, pg 116.
