Wideband filters with bandwidths in excess of an octave can be created by cascading a high-pass filter and a low-pass filter. In this design idea, nine-pole high-pass and low-pass filters have been constructed using a modified equal-element design. A new, low-cost wideband filter has been created using a minimum number of different components.
The new wideband filter is realized using a high-pass filter with 3-dB cutoffs of 3.56 MHz. The high-pass filter is followed by a low-pass filter with 3-dB cutoffs of 12.88 MHz. Both of these filters have a design reference frequency of 6.77 MHz. The wideband filter uses only a single value of capacitance and a single value of inductance (see the figure).
This is achieved by judicious use of series and parallel combinations. While all filter capacitors have a value of 470 pF ±5%, all inductors feature a value of 1.175 µH. The inductors are formed using 15 turns of #26 magnet wire on a Micro Metals T37-2 toroid. The high-pass and low-pass filters are designed for 50-Ω impedance. Type BNC connectors are used at the inputs and outputs. The two filters are interconnected using a male-male BNC adapter.
The measured amplitude response data for the wideband filter is shown in the table. The modified, low-pass, equal-element filter features reasonably good responses below the design reference frequency. By using a common design reference frequency for both filters, the pass-band regions with non-optimal responses are staggered in frequency. This avoids excessive interactions between the two filters.
With modified equal-element units, the wideband filter provides design simplicity that reduces manufacturing costs. Wideband-filter-response shapes are not optimal. They may suit situations where cost reduction is necessary and filter-response shape specifications are not too stringent. This design methodology can be extended readily to eleven-pole and thirteen-pole filters.