Wideband filter uses image parameters

Richard M Kurzrok, RMK Consultants, Queens Village, NY -- 10/26/2000

You can design various lowpass and highpass filters using image parameters (Reference 1 and Reference 2). By cascading a highpass and a lowpass filter using image-parameter designs, you can also realize a low-cost wideband filter (Figure 1). A different approach uses a composite design with terminating m-derived half-sections and two interior constant-k full sections. This approach results in viable performance with the use of relatively few components. For this approach, however, m=0.5, which is not the preferred value for classic-filter designs.

In Figure 1's filter, the upper and lower cutoff frequencies of this overall bandpass filter can be independent design parameters. For this filter, the nominal highpass cutoff frequency is 3.3 MHz, and the lowpass cutoff frequency is 10 MHz. Impedance levels are 50?. A male-to-male BNC adapter connects the two separate filter units. At higher frequencies, total integration of the two filters in a single enclosure would be desirable. Use of surface-mount components might also be appropriate.

Table 1 shows the measured amplitude responses of the highpass and lowpass sections. Table 2 summarizes the overall bandpass filter's amplitude response. You can see that the highpass and lowpass cutoff frequencies are sufficiently removed from each other to realize a fairly wide passband of low insertion loss. If the cutoff frequencies of the highpass and lowpass filters are too close to each other, you can reduce interactions between the two filters by using a fixed-pass attenuator to connect the two filters.

The design computations are simple, harking back to the days of slide-rule designs. You can gauge the theoretical performance of composite lowpass filters using modern computer-aided analysis (Reference 3).

Kurzrok, Richard M, "Low cost lowpass filter design using image parameters," Applied Microwave & Wireless, pg 72, February 1999, plus correction pg 12, May 1999.

Kurzrok, Richard M, "Filter design uses image parameters," EDN, May 25, 2000, pg 111.

Kurzrok, Richard M, "Update the design of image-parameter filters," Microwaves & RF, pg 119, May 2000.

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